Imported Upstream version 1.21.0

[platform/upstream/grpc.git] / src / core / ext / filters / client_channel / client_channel.cc

/*
 *
 * Copyright 2015 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

#include <grpc/support/port_platform.h>

#include "src/core/ext/filters/client_channel/client_channel.h"

#include <inttypes.h>
#include <limits.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

#include <grpc/support/alloc.h>
#include <grpc/support/log.h>
#include <grpc/support/string_util.h>
#include <grpc/support/sync.h>

#include "src/core/ext/filters/client_channel/backup_poller.h"
#include "src/core/ext/filters/client_channel/global_subchannel_pool.h"
#include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
#include "src/core/ext/filters/client_channel/lb_policy_registry.h"
#include "src/core/ext/filters/client_channel/local_subchannel_pool.h"
#include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
#include "src/core/ext/filters/client_channel/resolver_registry.h"
#include "src/core/ext/filters/client_channel/resolver_result_parsing.h"
#include "src/core/ext/filters/client_channel/resolving_lb_policy.h"
#include "src/core/ext/filters/client_channel/retry_throttle.h"
#include "src/core/ext/filters/client_channel/service_config.h"
#include "src/core/ext/filters/client_channel/subchannel.h"
#include "src/core/ext/filters/deadline/deadline_filter.h"
#include "src/core/lib/backoff/backoff.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/channel/connected_channel.h"
#include "src/core/lib/channel/status_util.h"
#include "src/core/lib/gpr/string.h"
#include "src/core/lib/gprpp/inlined_vector.h"
#include "src/core/lib/gprpp/manual_constructor.h"
#include "src/core/lib/gprpp/sync.h"
#include "src/core/lib/iomgr/combiner.h"
#include "src/core/lib/iomgr/iomgr.h"
#include "src/core/lib/iomgr/polling_entity.h"
#include "src/core/lib/profiling/timers.h"
#include "src/core/lib/slice/slice_internal.h"
#include "src/core/lib/slice/slice_string_helpers.h"
#include "src/core/lib/surface/channel.h"
#include "src/core/lib/transport/connectivity_state.h"
#include "src/core/lib/transport/error_utils.h"
#include "src/core/lib/transport/metadata.h"
#include "src/core/lib/transport/metadata_batch.h"
#include "src/core/lib/transport/static_metadata.h"
#include "src/core/lib/transport/status_metadata.h"

using grpc_core::internal::ClientChannelMethodParsedObject;
using grpc_core::internal::ServerRetryThrottleData;

//
// Client channel filter
//

// By default, we buffer 256 KiB per RPC for retries.
// TODO(roth): Do we have any data to suggest a better value?
#define DEFAULT_PER_RPC_RETRY_BUFFER_SIZE (256 << 10)

// This value was picked arbitrarily.  It can be changed if there is
// any even moderately compelling reason to do so.
#define RETRY_BACKOFF_JITTER 0.2

// Max number of batches that can be pending on a call at any given
// time.  This includes one batch for each of the following ops:
//   recv_initial_metadata
//   send_initial_metadata
//   recv_message
//   send_message
//   recv_trailing_metadata
//   send_trailing_metadata
#define MAX_PENDING_BATCHES 6

namespace grpc_core {

TraceFlag grpc_client_channel_call_trace(false, "client_channel_call");
TraceFlag grpc_client_channel_routing_trace(false, "client_channel_routing");

namespace {

//
// ChannelData definition
//

class ChannelData {
 public:
  struct QueuedPick {
    LoadBalancingPolicy::PickArgs pick;
    grpc_call_element* elem;
    QueuedPick* next = nullptr;
  };

  static grpc_error* Init(grpc_channel_element* elem,
                          grpc_channel_element_args* args);
  static void Destroy(grpc_channel_element* elem);
  static void StartTransportOp(grpc_channel_element* elem,
                               grpc_transport_op* op);
  static void GetChannelInfo(grpc_channel_element* elem,
                             const grpc_channel_info* info);

  void set_channelz_node(channelz::ClientChannelNode* node) {
    channelz_node_ = node;
    resolving_lb_policy_->set_channelz_node(node->Ref());
  }
  void FillChildRefsForChannelz(channelz::ChildRefsList* child_subchannels,
                                channelz::ChildRefsList* child_channels) {
    if (resolving_lb_policy_ != nullptr) {
      resolving_lb_policy_->FillChildRefsForChannelz(child_subchannels,
                                                     child_channels);
    }
  }

  bool deadline_checking_enabled() const { return deadline_checking_enabled_; }
  bool enable_retries() const { return enable_retries_; }
  size_t per_rpc_retry_buffer_size() const {
    return per_rpc_retry_buffer_size_;
  }

  // Note: Does NOT return a new ref.
  grpc_error* disconnect_error() const {
    return disconnect_error_.Load(MemoryOrder::ACQUIRE);
  }

  grpc_combiner* data_plane_combiner() const { return data_plane_combiner_; }

  LoadBalancingPolicy::SubchannelPicker* picker() const {
    return picker_.get();
  }
  void AddQueuedPick(QueuedPick* pick, grpc_polling_entity* pollent);
  void RemoveQueuedPick(QueuedPick* to_remove, grpc_polling_entity* pollent);

  bool received_service_config_data() const {
    return received_service_config_data_;
  }
  RefCountedPtr<ServerRetryThrottleData> retry_throttle_data() const {
    return retry_throttle_data_;
  }
  RefCountedPtr<ServiceConfig> service_config() const {
    return service_config_;
  }

  grpc_connectivity_state CheckConnectivityState(bool try_to_connect);
  void AddExternalConnectivityWatcher(grpc_polling_entity pollent,
                                      grpc_connectivity_state* state,
                                      grpc_closure* on_complete,
                                      grpc_closure* watcher_timer_init) {
    // Will delete itself.
    New<ExternalConnectivityWatcher>(this, pollent, state, on_complete,
                                     watcher_timer_init);
  }
  int NumExternalConnectivityWatchers() const {
    return external_connectivity_watcher_list_.size();
  }

 private:
  class ConnectivityStateAndPickerSetter;
  class ServiceConfigSetter;
  class ClientChannelControlHelper;

  class ExternalConnectivityWatcher {
   public:
    class WatcherList {
     public:
      WatcherList() { gpr_mu_init(&mu_); }
      ~WatcherList() { gpr_mu_destroy(&mu_); }

      int size() const;
      ExternalConnectivityWatcher* Lookup(grpc_closure* on_complete) const;
      void Add(ExternalConnectivityWatcher* watcher);
      void Remove(const ExternalConnectivityWatcher* watcher);

     private:
      // head_ is guarded by a mutex, since the size() method needs to
      // iterate over the list, and it's called from the C-core API
      // function grpc_channel_num_external_connectivity_watchers(), which
      // is synchronous and therefore cannot run in the combiner.
      mutable gpr_mu mu_;
      ExternalConnectivityWatcher* head_ = nullptr;
    };

    ExternalConnectivityWatcher(ChannelData* chand, grpc_polling_entity pollent,
                                grpc_connectivity_state* state,
                                grpc_closure* on_complete,
                                grpc_closure* watcher_timer_init);

    ~ExternalConnectivityWatcher();

   private:
    static void OnWatchCompleteLocked(void* arg, grpc_error* error);
    static void WatchConnectivityStateLocked(void* arg, grpc_error* ignored);

    ChannelData* chand_;
    grpc_polling_entity pollent_;
    grpc_connectivity_state* state_;
    grpc_closure* on_complete_;
    grpc_closure* watcher_timer_init_;
    grpc_closure my_closure_;
    ExternalConnectivityWatcher* next_ = nullptr;
  };

  ChannelData(grpc_channel_element_args* args, grpc_error** error);
  ~ChannelData();

  static bool ProcessResolverResultLocked(
      void* arg, const Resolver::Result& result, const char** lb_policy_name,
      RefCountedPtr<ParsedLoadBalancingConfig>* lb_policy_config,
      grpc_error** service_config_error);

  grpc_error* DoPingLocked(grpc_transport_op* op);

  static void StartTransportOpLocked(void* arg, grpc_error* ignored);

  static void TryToConnectLocked(void* arg, grpc_error* error_ignored);

  void ProcessLbPolicy(
      const Resolver::Result& resolver_result,
      const internal::ClientChannelGlobalParsedObject* parsed_service_config,
      UniquePtr<char>* lb_policy_name,
      RefCountedPtr<ParsedLoadBalancingConfig>* lb_policy_config);

  //
  // Fields set at construction and never modified.
  //
  const bool deadline_checking_enabled_;
  const bool enable_retries_;
  const size_t per_rpc_retry_buffer_size_;
  grpc_channel_stack* owning_stack_;
  ClientChannelFactory* client_channel_factory_;
  UniquePtr<char> server_name_;
  RefCountedPtr<ServiceConfig> default_service_config_;
  // Initialized shortly after construction.
  channelz::ClientChannelNode* channelz_node_ = nullptr;

  //
  // Fields used in the data plane.  Guarded by data_plane_combiner.
  //
  grpc_combiner* data_plane_combiner_;
  UniquePtr<LoadBalancingPolicy::SubchannelPicker> picker_;
  QueuedPick* queued_picks_ = nullptr;  // Linked list of queued picks.
  // Data from service config.
  bool received_service_config_data_ = false;
  RefCountedPtr<ServerRetryThrottleData> retry_throttle_data_;
  RefCountedPtr<ServiceConfig> service_config_;

  //
  // Fields used in the control plane.  Guarded by combiner.
  //
  grpc_combiner* combiner_;
  grpc_pollset_set* interested_parties_;
  RefCountedPtr<SubchannelPoolInterface> subchannel_pool_;
  OrphanablePtr<LoadBalancingPolicy> resolving_lb_policy_;
  grpc_connectivity_state_tracker state_tracker_;
  ExternalConnectivityWatcher::WatcherList external_connectivity_watcher_list_;
  UniquePtr<char> health_check_service_name_;
  RefCountedPtr<ServiceConfig> saved_service_config_;
  bool received_first_resolver_result_ = false;

  //
  // Fields accessed from both data plane and control plane combiners.
  //
  Atomic<grpc_error*> disconnect_error_;

  //
  // Fields guarded by a mutex, since they need to be accessed
  // synchronously via get_channel_info().
  //
  gpr_mu info_mu_;
  UniquePtr<char> info_lb_policy_name_;
  UniquePtr<char> info_service_config_json_;
};

//
// CallData definition
//

class CallData {
 public:
  static grpc_error* Init(grpc_call_element* elem,
                          const grpc_call_element_args* args);
  static void Destroy(grpc_call_element* elem,
                      const grpc_call_final_info* final_info,
                      grpc_closure* then_schedule_closure);
  static void StartTransportStreamOpBatch(
      grpc_call_element* elem, grpc_transport_stream_op_batch* batch);
  static void SetPollent(grpc_call_element* elem, grpc_polling_entity* pollent);

  RefCountedPtr<SubchannelCall> subchannel_call() { return subchannel_call_; }

  // Invoked by channel for queued picks once resolver results are available.
  void MaybeApplyServiceConfigToCallLocked(grpc_call_element* elem);

  // Invoked by channel for queued picks when the picker is updated.
  static void StartPickLocked(void* arg, grpc_error* error);

 private:
  class QueuedPickCanceller;

  // State used for starting a retryable batch on a subchannel call.
  // This provides its own grpc_transport_stream_op_batch and other data
  // structures needed to populate the ops in the batch.
  // We allocate one struct on the arena for each attempt at starting a
  // batch on a given subchannel call.
  struct SubchannelCallBatchData {
    // Creates a SubchannelCallBatchData object on the call's arena with the
    // specified refcount.  If set_on_complete is true, the batch's
    // on_complete callback will be set to point to on_complete();
    // otherwise, the batch's on_complete callback will be null.
    static SubchannelCallBatchData* Create(grpc_call_element* elem,
                                           int refcount, bool set_on_complete);

    void Unref() {
      if (gpr_unref(&refs)) Destroy();
    }

    SubchannelCallBatchData(grpc_call_element* elem, CallData* calld,
                            int refcount, bool set_on_complete);
    // All dtor code must be added in `Destroy()`. This is because we may
    // call closures in `SubchannelCallBatchData` after they are unrefed by
    // `Unref()`, and msan would complain about accessing this class
    // after calling dtor. As a result we cannot call the `dtor` in `Unref()`.
    // TODO(soheil): We should try to call the dtor in `Unref()`.
    ~SubchannelCallBatchData() { Destroy(); }
    void Destroy();

    gpr_refcount refs;
    grpc_call_element* elem;
    RefCountedPtr<SubchannelCall> subchannel_call;
    // The batch to use in the subchannel call.
    // Its payload field points to SubchannelCallRetryState::batch_payload.
    grpc_transport_stream_op_batch batch;
    // For intercepting on_complete.
    grpc_closure on_complete;
  };

  // Retry state associated with a subchannel call.
  // Stored in the parent_data of the subchannel call object.
  struct SubchannelCallRetryState {
    explicit SubchannelCallRetryState(grpc_call_context_element* context)
        : batch_payload(context),
          started_send_initial_metadata(false),
          completed_send_initial_metadata(false),
          started_send_trailing_metadata(false),
          completed_send_trailing_metadata(false),
          started_recv_initial_metadata(false),
          completed_recv_initial_metadata(false),
          started_recv_trailing_metadata(false),
          completed_recv_trailing_metadata(false),
          retry_dispatched(false) {}

    // SubchannelCallBatchData.batch.payload points to this.
    grpc_transport_stream_op_batch_payload batch_payload;
    // For send_initial_metadata.
    // Note that we need to make a copy of the initial metadata for each
    // subchannel call instead of just referring to the copy in call_data,
    // because filters in the subchannel stack will probably add entries,
    // so we need to start in a pristine state for each attempt of the call.
    grpc_linked_mdelem* send_initial_metadata_storage;
    grpc_metadata_batch send_initial_metadata;
    // For send_message.
    // TODO(roth): Restructure this to eliminate use of ManualConstructor.
    ManualConstructor<ByteStreamCache::CachingByteStream> send_message;
    // For send_trailing_metadata.
    grpc_linked_mdelem* send_trailing_metadata_storage;
    grpc_metadata_batch send_trailing_metadata;
    // For intercepting recv_initial_metadata.
    grpc_metadata_batch recv_initial_metadata;
    grpc_closure recv_initial_metadata_ready;
    bool trailing_metadata_available = false;
    // For intercepting recv_message.
    grpc_closure recv_message_ready;
    OrphanablePtr<ByteStream> recv_message;
    // For intercepting recv_trailing_metadata.
    grpc_metadata_batch recv_trailing_metadata;
    grpc_transport_stream_stats collect_stats;
    grpc_closure recv_trailing_metadata_ready;
    // These fields indicate which ops have been started and completed on
    // this subchannel call.
    size_t started_send_message_count = 0;
    size_t completed_send_message_count = 0;
    size_t started_recv_message_count = 0;
    size_t completed_recv_message_count = 0;
    bool started_send_initial_metadata : 1;
    bool completed_send_initial_metadata : 1;
    bool started_send_trailing_metadata : 1;
    bool completed_send_trailing_metadata : 1;
    bool started_recv_initial_metadata : 1;
    bool completed_recv_initial_metadata : 1;
    bool started_recv_trailing_metadata : 1;
    bool completed_recv_trailing_metadata : 1;
    // State for callback processing.
    SubchannelCallBatchData* recv_initial_metadata_ready_deferred_batch =
        nullptr;
    grpc_error* recv_initial_metadata_error = GRPC_ERROR_NONE;
    SubchannelCallBatchData* recv_message_ready_deferred_batch = nullptr;
    grpc_error* recv_message_error = GRPC_ERROR_NONE;
    SubchannelCallBatchData* recv_trailing_metadata_internal_batch = nullptr;
    // NOTE: Do not move this next to the metadata bitfields above. That would
    //       save space but will also result in a data race because compiler
    //       will generate a 2 byte store which overwrites the meta-data
    //       fields upon setting this field.
    bool retry_dispatched : 1;
  };

  // Pending batches stored in call data.
  struct PendingBatch {
    // The pending batch.  If nullptr, this slot is empty.
    grpc_transport_stream_op_batch* batch;
    // Indicates whether payload for send ops has been cached in CallData.
    bool send_ops_cached;
  };

  CallData(grpc_call_element* elem, const ChannelData& chand,
           const grpc_call_element_args& args);
  ~CallData();

  // Caches data for send ops so that it can be retried later, if not
  // already cached.
  void MaybeCacheSendOpsForBatch(PendingBatch* pending);
  void FreeCachedSendInitialMetadata(ChannelData* chand);
  // Frees cached send_message at index idx.
  void FreeCachedSendMessage(ChannelData* chand, size_t idx);
  void FreeCachedSendTrailingMetadata(ChannelData* chand);
  // Frees cached send ops that have already been completed after
  // committing the call.
  void FreeCachedSendOpDataAfterCommit(grpc_call_element* elem,
                                       SubchannelCallRetryState* retry_state);
  // Frees cached send ops that were completed by the completed batch in
  // batch_data.  Used when batches are completed after the call is committed.
  void FreeCachedSendOpDataForCompletedBatch(
      grpc_call_element* elem, SubchannelCallBatchData* batch_data,
      SubchannelCallRetryState* retry_state);

  static void MaybeInjectRecvTrailingMetadataReadyForLoadBalancingPolicy(
      const LoadBalancingPolicy::PickArgs& pick,
      grpc_transport_stream_op_batch* batch);

  // Returns the index into pending_batches_ to be used for batch.
  static size_t GetBatchIndex(grpc_transport_stream_op_batch* batch);
  void PendingBatchesAdd(grpc_call_element* elem,
                         grpc_transport_stream_op_batch* batch);
  void PendingBatchClear(PendingBatch* pending);
  void MaybeClearPendingBatch(grpc_call_element* elem, PendingBatch* pending);
  static void FailPendingBatchInCallCombiner(void* arg, grpc_error* error);
  // A predicate type and some useful implementations for PendingBatchesFail().
  typedef bool (*YieldCallCombinerPredicate)(
      const CallCombinerClosureList& closures);
  static bool YieldCallCombiner(const CallCombinerClosureList& closures) {
    return true;
  }
  static bool NoYieldCallCombiner(const CallCombinerClosureList& closures) {
    return false;
  }
  static bool YieldCallCombinerIfPendingBatchesFound(
      const CallCombinerClosureList& closures) {
    return closures.size() > 0;
  }
  // Fails all pending batches.
  // If yield_call_combiner_predicate returns true, assumes responsibility for
  // yielding the call combiner.
  void PendingBatchesFail(
      grpc_call_element* elem, grpc_error* error,
      YieldCallCombinerPredicate yield_call_combiner_predicate);
  static void ResumePendingBatchInCallCombiner(void* arg, grpc_error* ignored);
  // Resumes all pending batches on subchannel_call_.
  void PendingBatchesResume(grpc_call_element* elem);
  // Returns a pointer to the first pending batch for which predicate(batch)
  // returns true, or null if not found.
  template <typename Predicate>
  PendingBatch* PendingBatchFind(grpc_call_element* elem,
                                 const char* log_message, Predicate predicate);

  // Commits the call so that no further retry attempts will be performed.
  void RetryCommit(grpc_call_element* elem,
                   SubchannelCallRetryState* retry_state);
  // Starts a retry after appropriate back-off.
  void DoRetry(grpc_call_element* elem, SubchannelCallRetryState* retry_state,
               grpc_millis server_pushback_ms);
  // Returns true if the call is being retried.
  bool MaybeRetry(grpc_call_element* elem, SubchannelCallBatchData* batch_data,
                  grpc_status_code status, grpc_mdelem* server_pushback_md);

  // Invokes recv_initial_metadata_ready for a subchannel batch.
  static void InvokeRecvInitialMetadataCallback(void* arg, grpc_error* error);
  // Intercepts recv_initial_metadata_ready callback for retries.
  // Commits the call and returns the initial metadata up the stack.
  static void RecvInitialMetadataReady(void* arg, grpc_error* error);

  // Invokes recv_message_ready for a subchannel batch.
  static void InvokeRecvMessageCallback(void* arg, grpc_error* error);
  // Intercepts recv_message_ready callback for retries.
  // Commits the call and returns the message up the stack.
  static void RecvMessageReady(void* arg, grpc_error* error);

  // Sets *status and *server_pushback_md based on md_batch and error.
  // Only sets *server_pushback_md if server_pushback_md != nullptr.
  void GetCallStatus(grpc_call_element* elem, grpc_metadata_batch* md_batch,
                     grpc_error* error, grpc_status_code* status,
                     grpc_mdelem** server_pushback_md);
  // Adds recv_trailing_metadata_ready closure to closures.
  void AddClosureForRecvTrailingMetadataReady(
      grpc_call_element* elem, SubchannelCallBatchData* batch_data,
      grpc_error* error, CallCombinerClosureList* closures);
  // Adds any necessary closures for deferred recv_initial_metadata and
  // recv_message callbacks to closures.
  static void AddClosuresForDeferredRecvCallbacks(
      SubchannelCallBatchData* batch_data,
      SubchannelCallRetryState* retry_state, CallCombinerClosureList* closures);
  // Returns true if any op in the batch was not yet started.
  // Only looks at send ops, since recv ops are always started immediately.
  bool PendingBatchIsUnstarted(PendingBatch* pending,
                               SubchannelCallRetryState* retry_state);
  // For any pending batch containing an op that has not yet been started,
  // adds the pending batch's completion closures to closures.
  void AddClosuresToFailUnstartedPendingBatches(
      grpc_call_element* elem, SubchannelCallRetryState* retry_state,
      grpc_error* error, CallCombinerClosureList* closures);
  // Runs necessary closures upon completion of a call attempt.
  void RunClosuresForCompletedCall(SubchannelCallBatchData* batch_data,
                                   grpc_error* error);
  // Intercepts recv_trailing_metadata_ready callback for retries.
  // Commits the call and returns the trailing metadata up the stack.
  static void RecvTrailingMetadataReady(void* arg, grpc_error* error);

  // Adds the on_complete closure for the pending batch completed in
  // batch_data to closures.
  void AddClosuresForCompletedPendingBatch(
      grpc_call_element* elem, SubchannelCallBatchData* batch_data,
      SubchannelCallRetryState* retry_state, grpc_error* error,
      CallCombinerClosureList* closures);

  // If there are any cached ops to replay or pending ops to start on the
  // subchannel call, adds a closure to closures to invoke
  // StartRetriableSubchannelBatches().
  void AddClosuresForReplayOrPendingSendOps(
      grpc_call_element* elem, SubchannelCallBatchData* batch_data,
      SubchannelCallRetryState* retry_state, CallCombinerClosureList* closures);

  // Callback used to intercept on_complete from subchannel calls.
  // Called only when retries are enabled.
  static void OnComplete(void* arg, grpc_error* error);

  static void StartBatchInCallCombiner(void* arg, grpc_error* ignored);
  // Adds a closure to closures that will execute batch in the call combiner.
  void AddClosureForSubchannelBatch(grpc_call_element* elem,
                                    grpc_transport_stream_op_batch* batch,
                                    CallCombinerClosureList* closures);
  // Adds retriable send_initial_metadata op to batch_data.
  void AddRetriableSendInitialMetadataOp(SubchannelCallRetryState* retry_state,
                                         SubchannelCallBatchData* batch_data);
  // Adds retriable send_message op to batch_data.
  void AddRetriableSendMessageOp(grpc_call_element* elem,
                                 SubchannelCallRetryState* retry_state,
                                 SubchannelCallBatchData* batch_data);
  // Adds retriable send_trailing_metadata op to batch_data.
  void AddRetriableSendTrailingMetadataOp(SubchannelCallRetryState* retry_state,
                                          SubchannelCallBatchData* batch_data);
  // Adds retriable recv_initial_metadata op to batch_data.
  void AddRetriableRecvInitialMetadataOp(SubchannelCallRetryState* retry_state,
                                         SubchannelCallBatchData* batch_data);
  // Adds retriable recv_message op to batch_data.
  void AddRetriableRecvMessageOp(SubchannelCallRetryState* retry_state,
                                 SubchannelCallBatchData* batch_data);
  // Adds retriable recv_trailing_metadata op to batch_data.
  void AddRetriableRecvTrailingMetadataOp(SubchannelCallRetryState* retry_state,
                                          SubchannelCallBatchData* batch_data);
  // Helper function used to start a recv_trailing_metadata batch.  This
  // is used in the case where a recv_initial_metadata or recv_message
  // op fails in a way that we know the call is over but when the application
  // has not yet started its own recv_trailing_metadata op.
  void StartInternalRecvTrailingMetadata(grpc_call_element* elem);
  // If there are any cached send ops that need to be replayed on the
  // current subchannel call, creates and returns a new subchannel batch
  // to replay those ops.  Otherwise, returns nullptr.
  SubchannelCallBatchData* MaybeCreateSubchannelBatchForReplay(
      grpc_call_element* elem, SubchannelCallRetryState* retry_state);
  // Adds subchannel batches for pending batches to closures.
  void AddSubchannelBatchesForPendingBatches(
      grpc_call_element* elem, SubchannelCallRetryState* retry_state,
      CallCombinerClosureList* closures);
  // Constructs and starts whatever subchannel batches are needed on the
  // subchannel call.
  static void StartRetriableSubchannelBatches(void* arg, grpc_error* ignored);

  void CreateSubchannelCall(grpc_call_element* elem);
  // Invoked when a pick is completed, on both success or failure.
  static void PickDone(void* arg, grpc_error* error);
  // Removes the call from the channel's list of queued picks.
  void RemoveCallFromQueuedPicksLocked(grpc_call_element* elem);
  // Adds the call to the channel's list of queued picks.
  void AddCallToQueuedPicksLocked(grpc_call_element* elem);
  // Applies service config to the call.  Must be invoked once we know
  // that the resolver has returned results to the channel.
  void ApplyServiceConfigToCallLocked(grpc_call_element* elem);

  // State for handling deadlines.
  // The code in deadline_filter.c requires this to be the first field.
  // TODO(roth): This is slightly sub-optimal in that grpc_deadline_state
  // and this struct both independently store pointers to the call stack
  // and call combiner.  If/when we have time, find a way to avoid this
  // without breaking the grpc_deadline_state abstraction.
  grpc_deadline_state deadline_state_;

  grpc_slice path_;  // Request path.
  gpr_timespec call_start_time_;
  grpc_millis deadline_;
  Arena* arena_;
  grpc_call_stack* owning_call_;
  CallCombiner* call_combiner_;
  grpc_call_context_element* call_context_;

  RefCountedPtr<ServerRetryThrottleData> retry_throttle_data_;
  ServiceConfig::CallData service_config_call_data_;
  const ClientChannelMethodParsedObject* method_params_ = nullptr;

  RefCountedPtr<SubchannelCall> subchannel_call_;

  // Set when we get a cancel_stream op.
  grpc_error* cancel_error_ = GRPC_ERROR_NONE;

  ChannelData::QueuedPick pick_;
  bool pick_queued_ = false;
  bool service_config_applied_ = false;
  QueuedPickCanceller* pick_canceller_ = nullptr;
  grpc_closure pick_closure_;

  grpc_polling_entity* pollent_ = nullptr;

  // Batches are added to this list when received from above.
  // They are removed when we are done handling the batch (i.e., when
  // either we have invoked all of the batch's callbacks or we have
  // passed the batch down to the subchannel call and are not
  // intercepting any of its callbacks).
  PendingBatch pending_batches_[MAX_PENDING_BATCHES] = {};
  bool pending_send_initial_metadata_ : 1;
  bool pending_send_message_ : 1;
  bool pending_send_trailing_metadata_ : 1;

  // Retry state.
  bool enable_retries_ : 1;
  bool retry_committed_ : 1;
  bool last_attempt_got_server_pushback_ : 1;
  int num_attempts_completed_ = 0;
  size_t bytes_buffered_for_retry_ = 0;
  // TODO(roth): Restructure this to eliminate use of ManualConstructor.
  ManualConstructor<BackOff> retry_backoff_;
  grpc_timer retry_timer_;

  // The number of pending retriable subchannel batches containing send ops.
  // We hold a ref to the call stack while this is non-zero, since replay
  // batches may not complete until after all callbacks have been returned
  // to the surface, and we need to make sure that the call is not destroyed
  // until all of these batches have completed.
  // Note that we actually only need to track replay batches, but it's
  // easier to track all batches with send ops.
  int num_pending_retriable_subchannel_send_batches_ = 0;

  // Cached data for retrying send ops.
  // send_initial_metadata
  bool seen_send_initial_metadata_ = false;
  grpc_linked_mdelem* send_initial_metadata_storage_ = nullptr;
  grpc_metadata_batch send_initial_metadata_;
  uint32_t send_initial_metadata_flags_;
  gpr_atm* peer_string_;
  // send_message
  // When we get a send_message op, we replace the original byte stream
  // with a CachingByteStream that caches the slices to a local buffer for
  // use in retries.
  // Note: We inline the cache for the first 3 send_message ops and use
  // dynamic allocation after that.  This number was essentially picked
  // at random; it could be changed in the future to tune performance.
  InlinedVector<ByteStreamCache*, 3> send_messages_;
  // send_trailing_metadata
  bool seen_send_trailing_metadata_ = false;
  grpc_linked_mdelem* send_trailing_metadata_storage_ = nullptr;
  grpc_metadata_batch send_trailing_metadata_;
};

//
// ChannelData::ConnectivityStateAndPickerSetter
//

// A fire-and-forget class that sets the channel's connectivity state
// and then hops into the data plane combiner to update the picker.
// Must be instantiated while holding the control plane combiner.
// Deletes itself when done.
class ChannelData::ConnectivityStateAndPickerSetter {
 public:
  ConnectivityStateAndPickerSetter(
      ChannelData* chand, grpc_connectivity_state state, const char* reason,
      UniquePtr<LoadBalancingPolicy::SubchannelPicker> picker)
      : chand_(chand), picker_(std::move(picker)) {
    // Update connectivity state here, while holding control plane combiner.
    grpc_connectivity_state_set(&chand->state_tracker_, state, reason);
    if (chand->channelz_node_ != nullptr) {
      chand->channelz_node_->AddTraceEvent(
          channelz::ChannelTrace::Severity::Info,
          grpc_slice_from_static_string(
              GetChannelConnectivityStateChangeString(state)));
    }
    // Bounce into the data plane combiner to reset the picker.
    GRPC_CHANNEL_STACK_REF(chand->owning_stack_,
                           "ConnectivityStateAndPickerSetter");
    GRPC_CLOSURE_INIT(&closure_, SetPicker, this,
                      grpc_combiner_scheduler(chand->data_plane_combiner_));
    GRPC_CLOSURE_SCHED(&closure_, GRPC_ERROR_NONE);
  }

 private:
  static const char* GetChannelConnectivityStateChangeString(
      grpc_connectivity_state state) {
    switch (state) {
      case GRPC_CHANNEL_IDLE:
        return "Channel state change to IDLE";
      case GRPC_CHANNEL_CONNECTING:
        return "Channel state change to CONNECTING";
      case GRPC_CHANNEL_READY:
        return "Channel state change to READY";
      case GRPC_CHANNEL_TRANSIENT_FAILURE:
        return "Channel state change to TRANSIENT_FAILURE";
      case GRPC_CHANNEL_SHUTDOWN:
        return "Channel state change to SHUTDOWN";
    }
    GPR_UNREACHABLE_CODE(return "UNKNOWN");
  }

  static void SetPicker(void* arg, grpc_error* ignored) {
    auto* self = static_cast<ConnectivityStateAndPickerSetter*>(arg);
    // Update picker.
    self->chand_->picker_ = std::move(self->picker_);
    // Re-process queued picks.
    for (QueuedPick* pick = self->chand_->queued_picks_; pick != nullptr;
         pick = pick->next) {
      CallData::StartPickLocked(pick->elem, GRPC_ERROR_NONE);
    }
    // Clean up.
    GRPC_CHANNEL_STACK_UNREF(self->chand_->owning_stack_,
                             "ConnectivityStateAndPickerSetter");
    Delete(self);
  }

  ChannelData* chand_;
  UniquePtr<LoadBalancingPolicy::SubchannelPicker> picker_;
  grpc_closure closure_;
};

//
// ChannelData::ServiceConfigSetter
//

// A fire-and-forget class that sets the channel's service config data
// in the data plane combiner.  Deletes itself when done.
class ChannelData::ServiceConfigSetter {
 public:
  ServiceConfigSetter(
      ChannelData* chand,
      Optional<internal::ClientChannelGlobalParsedObject::RetryThrottling>
          retry_throttle_data,
      RefCountedPtr<ServiceConfig> service_config)
      : chand_(chand),
        retry_throttle_data_(retry_throttle_data),
        service_config_(std::move(service_config)) {
    GRPC_CHANNEL_STACK_REF(chand->owning_stack_, "ServiceConfigSetter");
    GRPC_CLOSURE_INIT(&closure_, SetServiceConfigData, this,
                      grpc_combiner_scheduler(chand->data_plane_combiner_));
    GRPC_CLOSURE_SCHED(&closure_, GRPC_ERROR_NONE);
  }

 private:
  static void SetServiceConfigData(void* arg, grpc_error* ignored) {
    ServiceConfigSetter* self = static_cast<ServiceConfigSetter*>(arg);
    ChannelData* chand = self->chand_;
    // Update channel state.
    chand->received_service_config_data_ = true;
    if (self->retry_throttle_data_.has_value()) {
      chand->retry_throttle_data_ =
          internal::ServerRetryThrottleMap::GetDataForServer(
              chand->server_name_.get(),
              self->retry_throttle_data_.value().max_milli_tokens,
              self->retry_throttle_data_.value().milli_token_ratio);
    }
    chand->service_config_ = std::move(self->service_config_);
    // Apply service config to queued picks.
    for (QueuedPick* pick = chand->queued_picks_; pick != nullptr;
         pick = pick->next) {
      CallData* calld = static_cast<CallData*>(pick->elem->call_data);
      calld->MaybeApplyServiceConfigToCallLocked(pick->elem);
    }
    // Clean up.
    GRPC_CHANNEL_STACK_UNREF(self->chand_->owning_stack_,
                             "ServiceConfigSetter");
    Delete(self);
  }

  ChannelData* chand_;
  Optional<internal::ClientChannelGlobalParsedObject::RetryThrottling>
      retry_throttle_data_;
  RefCountedPtr<ServiceConfig> service_config_;
  grpc_closure closure_;
};

//
// ChannelData::ExternalConnectivityWatcher::WatcherList
//

int ChannelData::ExternalConnectivityWatcher::WatcherList::size() const {
  MutexLock lock(&mu_);
  int count = 0;
  for (ExternalConnectivityWatcher* w = head_; w != nullptr; w = w->next_) {
    ++count;
  }
  return count;
}

ChannelData::ExternalConnectivityWatcher*
ChannelData::ExternalConnectivityWatcher::WatcherList::Lookup(
    grpc_closure* on_complete) const {
  MutexLock lock(&mu_);
  ExternalConnectivityWatcher* w = head_;
  while (w != nullptr && w->on_complete_ != on_complete) {
    w = w->next_;
  }
  return w;
}

void ChannelData::ExternalConnectivityWatcher::WatcherList::Add(
    ExternalConnectivityWatcher* watcher) {
  GPR_ASSERT(Lookup(watcher->on_complete_) == nullptr);
  MutexLock lock(&mu_);
  GPR_ASSERT(watcher->next_ == nullptr);
  watcher->next_ = head_;
  head_ = watcher;
}

void ChannelData::ExternalConnectivityWatcher::WatcherList::Remove(
    const ExternalConnectivityWatcher* watcher) {
  MutexLock lock(&mu_);
  if (watcher == head_) {
    head_ = watcher->next_;
    return;
  }
  for (ExternalConnectivityWatcher* w = head_; w != nullptr; w = w->next_) {
    if (w->next_ == watcher) {
      w->next_ = w->next_->next_;
      return;
    }
  }
  GPR_UNREACHABLE_CODE(return );
}

//
// ChannelData::ExternalConnectivityWatcher
//

ChannelData::ExternalConnectivityWatcher::ExternalConnectivityWatcher(
    ChannelData* chand, grpc_polling_entity pollent,
    grpc_connectivity_state* state, grpc_closure* on_complete,
    grpc_closure* watcher_timer_init)
    : chand_(chand),
      pollent_(pollent),
      state_(state),
      on_complete_(on_complete),
      watcher_timer_init_(watcher_timer_init) {
  grpc_polling_entity_add_to_pollset_set(&pollent_,
                                         chand_->interested_parties_);
  GRPC_CHANNEL_STACK_REF(chand_->owning_stack_, "ExternalConnectivityWatcher");
  GRPC_CLOSURE_SCHED(
      GRPC_CLOSURE_INIT(&my_closure_, WatchConnectivityStateLocked, this,
                        grpc_combiner_scheduler(chand_->combiner_)),
      GRPC_ERROR_NONE);
}

ChannelData::ExternalConnectivityWatcher::~ExternalConnectivityWatcher() {
  grpc_polling_entity_del_from_pollset_set(&pollent_,
                                           chand_->interested_parties_);
  GRPC_CHANNEL_STACK_UNREF(chand_->owning_stack_,
                           "ExternalConnectivityWatcher");
}

void ChannelData::ExternalConnectivityWatcher::OnWatchCompleteLocked(
    void* arg, grpc_error* error) {
  ExternalConnectivityWatcher* self =
      static_cast<ExternalConnectivityWatcher*>(arg);
  grpc_closure* on_complete = self->on_complete_;
  self->chand_->external_connectivity_watcher_list_.Remove(self);
  Delete(self);
  GRPC_CLOSURE_SCHED(on_complete, GRPC_ERROR_REF(error));
}

void ChannelData::ExternalConnectivityWatcher::WatchConnectivityStateLocked(
    void* arg, grpc_error* ignored) {
  ExternalConnectivityWatcher* self =
      static_cast<ExternalConnectivityWatcher*>(arg);
  if (self->state_ == nullptr) {
    // Handle cancellation.
    GPR_ASSERT(self->watcher_timer_init_ == nullptr);
    ExternalConnectivityWatcher* found =
        self->chand_->external_connectivity_watcher_list_.Lookup(
            self->on_complete_);
    if (found != nullptr) {
      grpc_connectivity_state_notify_on_state_change(
          &found->chand_->state_tracker_, nullptr, &found->my_closure_);
    }
    Delete(self);
    return;
  }
  // New watcher.
  self->chand_->external_connectivity_watcher_list_.Add(self);
  // This assumes that the closure is scheduled on the ExecCtx scheduler
  // and that GRPC_CLOSURE_RUN would run the closure immediately.
  GRPC_CLOSURE_RUN(self->watcher_timer_init_, GRPC_ERROR_NONE);
  GRPC_CLOSURE_INIT(&self->my_closure_, OnWatchCompleteLocked, self,
                    grpc_combiner_scheduler(self->chand_->combiner_));
  grpc_connectivity_state_notify_on_state_change(
      &self->chand_->state_tracker_, self->state_, &self->my_closure_);
}

//
// ChannelData::ClientChannelControlHelper
//

class ChannelData::ClientChannelControlHelper
    : public LoadBalancingPolicy::ChannelControlHelper {
 public:
  explicit ClientChannelControlHelper(ChannelData* chand) : chand_(chand) {
    GRPC_CHANNEL_STACK_REF(chand_->owning_stack_, "ClientChannelControlHelper");
  }

  ~ClientChannelControlHelper() override {
    GRPC_CHANNEL_STACK_UNREF(chand_->owning_stack_,
                             "ClientChannelControlHelper");
  }

  Subchannel* CreateSubchannel(const grpc_channel_args& args) override {
    grpc_arg args_to_add[2];
    int num_args_to_add = 0;
    if (chand_->health_check_service_name_ != nullptr) {
      args_to_add[0] = grpc_channel_arg_string_create(
          const_cast<char*>("grpc.temp.health_check"),
          const_cast<char*>(chand_->health_check_service_name_.get()));
      num_args_to_add++;
    }
    args_to_add[num_args_to_add++] = SubchannelPoolInterface::CreateChannelArg(
        chand_->subchannel_pool_.get());
    grpc_channel_args* new_args =
        grpc_channel_args_copy_and_add(&args, args_to_add, num_args_to_add);
    Subchannel* subchannel =
        chand_->client_channel_factory_->CreateSubchannel(new_args);
    grpc_channel_args_destroy(new_args);
    return subchannel;
  }

  grpc_channel* CreateChannel(const char* target,
                              const grpc_channel_args& args) override {
    return chand_->client_channel_factory_->CreateChannel(target, &args);
  }

  void UpdateState(
      grpc_connectivity_state state,
      UniquePtr<LoadBalancingPolicy::SubchannelPicker> picker) override {
    grpc_error* disconnect_error =
        chand_->disconnect_error_.Load(MemoryOrder::ACQUIRE);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
      const char* extra = disconnect_error == GRPC_ERROR_NONE
                              ? ""
                              : " (ignoring -- channel shutting down)";
      gpr_log(GPR_INFO, "chand=%p: update: state=%s picker=%p%s", chand_,
              grpc_connectivity_state_name(state), picker.get(), extra);
    }
    // Do update only if not shutting down.
    if (disconnect_error == GRPC_ERROR_NONE) {
      // Will delete itself.
      New<ConnectivityStateAndPickerSetter>(chand_, state, "helper",
                                            std::move(picker));
    }
  }

  // No-op -- we should never get this from ResolvingLoadBalancingPolicy.
  void RequestReresolution() override {}

 private:
  ChannelData* chand_;
};

//
// ChannelData implementation
//

grpc_error* ChannelData::Init(grpc_channel_element* elem,
                              grpc_channel_element_args* args) {
  GPR_ASSERT(args->is_last);
  GPR_ASSERT(elem->filter == &grpc_client_channel_filter);
  grpc_error* error = GRPC_ERROR_NONE;
  new (elem->channel_data) ChannelData(args, &error);
  return error;
}

void ChannelData::Destroy(grpc_channel_element* elem) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  chand->~ChannelData();
}

bool GetEnableRetries(const grpc_channel_args* args) {
  return grpc_channel_arg_get_bool(
      grpc_channel_args_find(args, GRPC_ARG_ENABLE_RETRIES), true);
}

size_t GetMaxPerRpcRetryBufferSize(const grpc_channel_args* args) {
  return static_cast<size_t>(grpc_channel_arg_get_integer(
      grpc_channel_args_find(args, GRPC_ARG_PER_RPC_RETRY_BUFFER_SIZE),
      {DEFAULT_PER_RPC_RETRY_BUFFER_SIZE, 0, INT_MAX}));
}

RefCountedPtr<SubchannelPoolInterface> GetSubchannelPool(
    const grpc_channel_args* args) {
  const bool use_local_subchannel_pool = grpc_channel_arg_get_bool(
      grpc_channel_args_find(args, GRPC_ARG_USE_LOCAL_SUBCHANNEL_POOL), false);
  if (use_local_subchannel_pool) {
    return MakeRefCounted<LocalSubchannelPool>();
  }
  return GlobalSubchannelPool::instance();
}

ChannelData::ChannelData(grpc_channel_element_args* args, grpc_error** error)
    : deadline_checking_enabled_(
          grpc_deadline_checking_enabled(args->channel_args)),
      enable_retries_(GetEnableRetries(args->channel_args)),
      per_rpc_retry_buffer_size_(
          GetMaxPerRpcRetryBufferSize(args->channel_args)),
      owning_stack_(args->channel_stack),
      client_channel_factory_(
          ClientChannelFactory::GetFromChannelArgs(args->channel_args)),
      data_plane_combiner_(grpc_combiner_create()),
      combiner_(grpc_combiner_create()),
      interested_parties_(grpc_pollset_set_create()),
      subchannel_pool_(GetSubchannelPool(args->channel_args)),
      disconnect_error_(GRPC_ERROR_NONE) {
  // Initialize data members.
  grpc_connectivity_state_init(&state_tracker_, GRPC_CHANNEL_IDLE,
                               "client_channel");
  gpr_mu_init(&info_mu_);
  // Start backup polling.
  grpc_client_channel_start_backup_polling(interested_parties_);
  // Check client channel factory.
  if (client_channel_factory_ == nullptr) {
    *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
        "Missing client channel factory in args for client channel filter");
    return;
  }
  // Get server name to resolve, using proxy mapper if needed.
  const char* server_uri = grpc_channel_arg_get_string(
      grpc_channel_args_find(args->channel_args, GRPC_ARG_SERVER_URI));
  if (server_uri == nullptr) {
    *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
        "server URI channel arg missing or wrong type in client channel "
        "filter");
    return;
  }
  // Get default service config
  const char* service_config_json = grpc_channel_arg_get_string(
      grpc_channel_args_find(args->channel_args, GRPC_ARG_SERVICE_CONFIG));
  // TODO(yashkt): Make sure we set the channel in TRANSIENT_FAILURE on an
  // invalid default service config
  if (service_config_json != nullptr) {
    *error = GRPC_ERROR_NONE;
    default_service_config_ = ServiceConfig::Create(service_config_json, error);
    if (*error != GRPC_ERROR_NONE) {
      default_service_config_.reset();
      return;
    }
  }
  grpc_uri* uri = grpc_uri_parse(server_uri, true);
  if (uri != nullptr && uri->path[0] != '\0') {
    server_name_.reset(
        gpr_strdup(uri->path[0] == '/' ? uri->path + 1 : uri->path));
  }
  grpc_uri_destroy(uri);
  char* proxy_name = nullptr;
  grpc_channel_args* new_args = nullptr;
  grpc_proxy_mappers_map_name(server_uri, args->channel_args, &proxy_name,
                              &new_args);
  UniquePtr<char> target_uri(proxy_name != nullptr ? proxy_name
                                                   : gpr_strdup(server_uri));
  // Instantiate resolving LB policy.
  LoadBalancingPolicy::Args lb_args;
  lb_args.combiner = combiner_;
  lb_args.channel_control_helper =
      UniquePtr<LoadBalancingPolicy::ChannelControlHelper>(
          New<ClientChannelControlHelper>(this));
  lb_args.args = new_args != nullptr ? new_args : args->channel_args;
  resolving_lb_policy_.reset(New<ResolvingLoadBalancingPolicy>(
      std::move(lb_args), &grpc_client_channel_routing_trace,
      std::move(target_uri), ProcessResolverResultLocked, this, error));
  grpc_channel_args_destroy(new_args);
  if (*error != GRPC_ERROR_NONE) {
    // Orphan the resolving LB policy and flush the exec_ctx to ensure
    // that it finishes shutting down.  This ensures that if we are
    // failing, we destroy the ClientChannelControlHelper (and thus
    // unref the channel stack) before we return.
    // TODO(roth): This is not a complete solution, because it only
    // catches the case where channel stack initialization fails in this
    // particular filter.  If there is a failure in a different filter, we
    // will leave a dangling ref here, which can cause a crash.  Fortunately,
    // in practice, there are no other filters that can cause failures in
    // channel stack initialization, so this works for now.
    resolving_lb_policy_.reset();
    ExecCtx::Get()->Flush();
  } else {
    grpc_pollset_set_add_pollset_set(resolving_lb_policy_->interested_parties(),
                                     interested_parties_);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
      gpr_log(GPR_INFO, "chand=%p: created resolving_lb_policy=%p", this,
              resolving_lb_policy_.get());
    }
  }
}

ChannelData::~ChannelData() {
  if (resolving_lb_policy_ != nullptr) {
    grpc_pollset_set_del_pollset_set(resolving_lb_policy_->interested_parties(),
                                     interested_parties_);
    resolving_lb_policy_.reset();
  }
  // Stop backup polling.
  grpc_client_channel_stop_backup_polling(interested_parties_);
  grpc_pollset_set_destroy(interested_parties_);
  GRPC_COMBINER_UNREF(data_plane_combiner_, "client_channel");
  GRPC_COMBINER_UNREF(combiner_, "client_channel");
  GRPC_ERROR_UNREF(disconnect_error_.Load(MemoryOrder::RELAXED));
  grpc_connectivity_state_destroy(&state_tracker_);
  gpr_mu_destroy(&info_mu_);
}

void ChannelData::ProcessLbPolicy(
    const Resolver::Result& resolver_result,
    const internal::ClientChannelGlobalParsedObject* parsed_service_config,
    UniquePtr<char>* lb_policy_name,
    RefCountedPtr<ParsedLoadBalancingConfig>* lb_policy_config) {
  // Prefer the LB policy name found in the service config.
  if (parsed_service_config != nullptr &&
      parsed_service_config->parsed_lb_config() != nullptr) {
    lb_policy_name->reset(
        gpr_strdup(parsed_service_config->parsed_lb_config()->name()));
    *lb_policy_config = parsed_service_config->parsed_lb_config();
    return;
  }
  const char* local_policy_name = nullptr;
  if (parsed_service_config != nullptr &&
      parsed_service_config->parsed_deprecated_lb_policy() != nullptr) {
    local_policy_name = parsed_service_config->parsed_deprecated_lb_policy();
  } else {
    const grpc_arg* channel_arg =
        grpc_channel_args_find(resolver_result.args, GRPC_ARG_LB_POLICY_NAME);
    local_policy_name = grpc_channel_arg_get_string(channel_arg);
  }
  // Special case: If at least one balancer address is present, we use
  // the grpclb policy, regardless of what the resolver has returned.
  bool found_balancer_address = false;
  for (size_t i = 0; i < resolver_result.addresses.size(); ++i) {
    const ServerAddress& address = resolver_result.addresses[i];
    if (address.IsBalancer()) {
      found_balancer_address = true;
      break;
    }
  }
  if (found_balancer_address) {
    if (local_policy_name != nullptr &&
        strcmp(local_policy_name, "grpclb") != 0) {
      gpr_log(GPR_INFO,
              "resolver requested LB policy %s but provided at least one "
              "balancer address -- forcing use of grpclb LB policy",
              local_policy_name);
    }
    local_policy_name = "grpclb";
  }
  // Use pick_first if nothing was specified and we didn't select grpclb
  // above.
  lb_policy_name->reset(gpr_strdup(
      local_policy_name == nullptr ? "pick_first" : local_policy_name));
}

// Synchronous callback from ResolvingLoadBalancingPolicy to process a
// resolver result update.
bool ChannelData::ProcessResolverResultLocked(
    void* arg, const Resolver::Result& result, const char** lb_policy_name,
    RefCountedPtr<ParsedLoadBalancingConfig>* lb_policy_config,
    grpc_error** service_config_error) {
  ChannelData* chand = static_cast<ChannelData*>(arg);
  RefCountedPtr<ServiceConfig> service_config;
  // If resolver did not return a service config or returned an invalid service
  // config, we need a fallback service config.
  if (result.service_config_error != GRPC_ERROR_NONE) {
    // If the service config was invalid, then fallback to the saved service
    // config. If there is no saved config either, use the default service
    // config.
    if (chand->saved_service_config_ != nullptr) {
      service_config = chand->saved_service_config_;
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p: resolver returned invalid service config. "
                "Continuing to use previous service config.",
                chand);
      }
    } else if (chand->default_service_config_ != nullptr) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p: resolver returned invalid service config. Using "
                "default service config provided by client API.",
                chand);
      }
      service_config = chand->default_service_config_;
    }
  } else if (result.service_config == nullptr) {
    if (chand->default_service_config_ != nullptr) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p: resolver returned no service config. Using default "
                "service config provided by client API.",
                chand);
      }
      service_config = chand->default_service_config_;
    }
  } else {
    service_config = result.service_config;
  }
  *service_config_error = GRPC_ERROR_REF(result.service_config_error);
  if (service_config == nullptr &&
      result.service_config_error != GRPC_ERROR_NONE) {
    return false;
  }
  UniquePtr<char> service_config_json;
  // Process service config.
  const internal::ClientChannelGlobalParsedObject* parsed_service_config =
      nullptr;
  if (service_config != nullptr) {
    parsed_service_config =
        static_cast<const internal::ClientChannelGlobalParsedObject*>(
            service_config->GetParsedGlobalServiceConfigObject(
                internal::ClientChannelServiceConfigParser::ParserIndex()));
  }
  const bool service_config_changed =
      ((service_config == nullptr) !=
       (chand->saved_service_config_ == nullptr)) ||
      (service_config != nullptr &&
       strcmp(service_config->service_config_json(),
              chand->saved_service_config_->service_config_json()) != 0);
  if (service_config_changed) {
    service_config_json.reset(gpr_strdup(
        service_config != nullptr ? service_config->service_config_json()
                                  : ""));
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p: resolver returned updated service config: \"%s\"",
              chand, service_config_json.get());
    }
    chand->saved_service_config_ = std::move(service_config);
    if (parsed_service_config != nullptr) {
      chand->health_check_service_name_.reset(
          gpr_strdup(parsed_service_config->health_check_service_name()));
    } else {
      chand->health_check_service_name_.reset();
    }
  }
  // We want to set the service config at least once. This should not really be
  // needed, but we are doing it as a defensive approach. This can be removed,
  // if we feel it is unnecessary.
  if (service_config_changed || !chand->received_first_resolver_result_) {
    chand->received_first_resolver_result_ = true;
    Optional<internal::ClientChannelGlobalParsedObject::RetryThrottling>
        retry_throttle_data;
    if (parsed_service_config != nullptr) {
      retry_throttle_data = parsed_service_config->retry_throttling();
    }
    // Create service config setter to update channel state in the data
    // plane combiner.  Destroys itself when done.
    New<ServiceConfigSetter>(chand, retry_throttle_data,
                             chand->saved_service_config_);
  }
  UniquePtr<char> processed_lb_policy_name;
  chand->ProcessLbPolicy(result, parsed_service_config,
                         &processed_lb_policy_name, lb_policy_config);
  // Swap out the data used by GetChannelInfo().
  {
    MutexLock lock(&chand->info_mu_);
    chand->info_lb_policy_name_ = std::move(processed_lb_policy_name);
    if (service_config_json != nullptr) {
      chand->info_service_config_json_ = std::move(service_config_json);
    }
  }
  // Return results.
  *lb_policy_name = chand->info_lb_policy_name_.get();
  return service_config_changed;
}

grpc_error* ChannelData::DoPingLocked(grpc_transport_op* op) {
  if (grpc_connectivity_state_check(&state_tracker_) != GRPC_CHANNEL_READY) {
    return GRPC_ERROR_CREATE_FROM_STATIC_STRING("channel not connected");
  }
  LoadBalancingPolicy::PickArgs pick;
  grpc_error* error = GRPC_ERROR_NONE;
  picker_->Pick(&pick, &error);
  if (pick.connected_subchannel != nullptr) {
    pick.connected_subchannel->Ping(op->send_ping.on_initiate,
                                    op->send_ping.on_ack);
  } else {
    if (error == GRPC_ERROR_NONE) {
      error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
          "LB policy dropped call on ping");
    }
  }
  return error;
}

void ChannelData::StartTransportOpLocked(void* arg, grpc_error* ignored) {
  grpc_transport_op* op = static_cast<grpc_transport_op*>(arg);
  grpc_channel_element* elem =
      static_cast<grpc_channel_element*>(op->handler_private.extra_arg);
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  // Connectivity watch.
  if (op->on_connectivity_state_change != nullptr) {
    grpc_connectivity_state_notify_on_state_change(
        &chand->state_tracker_, op->connectivity_state,
        op->on_connectivity_state_change);
    op->on_connectivity_state_change = nullptr;
    op->connectivity_state = nullptr;
  }
  // Ping.
  if (op->send_ping.on_initiate != nullptr || op->send_ping.on_ack != nullptr) {
    grpc_error* error = chand->DoPingLocked(op);
    if (error != GRPC_ERROR_NONE) {
      GRPC_CLOSURE_SCHED(op->send_ping.on_initiate, GRPC_ERROR_REF(error));
      GRPC_CLOSURE_SCHED(op->send_ping.on_ack, error);
    }
    op->bind_pollset = nullptr;
    op->send_ping.on_initiate = nullptr;
    op->send_ping.on_ack = nullptr;
  }
  // Reset backoff.
  if (op->reset_connect_backoff) {
    if (chand->resolving_lb_policy_ != nullptr) {
      chand->resolving_lb_policy_->ResetBackoffLocked();
    }
  }
  // Disconnect.
  if (op->disconnect_with_error != GRPC_ERROR_NONE) {
    grpc_error* error = GRPC_ERROR_NONE;
    GPR_ASSERT(chand->disconnect_error_.CompareExchangeStrong(
        &error, op->disconnect_with_error, MemoryOrder::ACQ_REL,
        MemoryOrder::ACQUIRE));
    grpc_pollset_set_del_pollset_set(
        chand->resolving_lb_policy_->interested_parties(),
        chand->interested_parties_);
    chand->resolving_lb_policy_.reset();
    // Will delete itself.
    New<ConnectivityStateAndPickerSetter>(
        chand, GRPC_CHANNEL_SHUTDOWN, "shutdown from API",
        UniquePtr<LoadBalancingPolicy::SubchannelPicker>(
            New<LoadBalancingPolicy::TransientFailurePicker>(
                GRPC_ERROR_REF(op->disconnect_with_error))));
  }
  GRPC_CHANNEL_STACK_UNREF(chand->owning_stack_, "start_transport_op");
  GRPC_CLOSURE_SCHED(op->on_consumed, GRPC_ERROR_NONE);
}

void ChannelData::StartTransportOp(grpc_channel_element* elem,
                                   grpc_transport_op* op) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  GPR_ASSERT(op->set_accept_stream == false);
  // Handle bind_pollset.
  if (op->bind_pollset != nullptr) {
    grpc_pollset_set_add_pollset(chand->interested_parties_, op->bind_pollset);
  }
  // Pop into control plane combiner for remaining ops.
  op->handler_private.extra_arg = elem;
  GRPC_CHANNEL_STACK_REF(chand->owning_stack_, "start_transport_op");
  GRPC_CLOSURE_SCHED(
      GRPC_CLOSURE_INIT(&op->handler_private.closure,
                        ChannelData::StartTransportOpLocked, op,
                        grpc_combiner_scheduler(chand->combiner_)),
      GRPC_ERROR_NONE);
}

void ChannelData::GetChannelInfo(grpc_channel_element* elem,
                                 const grpc_channel_info* info) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  MutexLock lock(&chand->info_mu_);
  if (info->lb_policy_name != nullptr) {
    *info->lb_policy_name = gpr_strdup(chand->info_lb_policy_name_.get());
  }
  if (info->service_config_json != nullptr) {
    *info->service_config_json =
        gpr_strdup(chand->info_service_config_json_.get());
  }
}

void ChannelData::AddQueuedPick(QueuedPick* pick,
                                grpc_polling_entity* pollent) {
  // Add call to queued picks list.
  pick->next = queued_picks_;
  queued_picks_ = pick;
  // Add call's pollent to channel's interested_parties, so that I/O
  // can be done under the call's CQ.
  grpc_polling_entity_add_to_pollset_set(pollent, interested_parties_);
}

void ChannelData::RemoveQueuedPick(QueuedPick* to_remove,
                                   grpc_polling_entity* pollent) {
  // Remove call's pollent from channel's interested_parties.
  grpc_polling_entity_del_from_pollset_set(pollent, interested_parties_);
  // Remove from queued picks list.
  for (QueuedPick** pick = &queued_picks_; *pick != nullptr;
       pick = &(*pick)->next) {
    if (*pick == to_remove) {
      *pick = to_remove->next;
      return;
    }
  }
}

void ChannelData::TryToConnectLocked(void* arg, grpc_error* error_ignored) {
  auto* chand = static_cast<ChannelData*>(arg);
  if (chand->resolving_lb_policy_ != nullptr) {
    chand->resolving_lb_policy_->ExitIdleLocked();
  }
  GRPC_CHANNEL_STACK_UNREF(chand->owning_stack_, "TryToConnect");
}

grpc_connectivity_state ChannelData::CheckConnectivityState(
    bool try_to_connect) {
  grpc_connectivity_state out = grpc_connectivity_state_check(&state_tracker_);
  if (out == GRPC_CHANNEL_IDLE && try_to_connect) {
    GRPC_CHANNEL_STACK_REF(owning_stack_, "TryToConnect");
    GRPC_CLOSURE_SCHED(GRPC_CLOSURE_CREATE(TryToConnectLocked, this,
                                           grpc_combiner_scheduler(combiner_)),
                       GRPC_ERROR_NONE);
  }
  return out;
}

//
// CallData implementation
//

// Retry support:
//
// In order to support retries, we act as a proxy for stream op batches.
// When we get a batch from the surface, we add it to our list of pending
// batches, and we then use those batches to construct separate "child"
// batches to be started on the subchannel call.  When the child batches
// return, we then decide which pending batches have been completed and
// schedule their callbacks accordingly.  If a subchannel call fails and
// we want to retry it, we do a new pick and start again, constructing
// new "child" batches for the new subchannel call.
//
// Note that retries are committed when receiving data from the server
// (except for Trailers-Only responses).  However, there may be many
// send ops started before receiving any data, so we may have already
// completed some number of send ops (and returned the completions up to
// the surface) by the time we realize that we need to retry.  To deal
// with this, we cache data for send ops, so that we can replay them on a
// different subchannel call even after we have completed the original
// batches.
//
// There are two sets of data to maintain:
// - In call_data (in the parent channel), we maintain a list of pending
//   ops and cached data for send ops.
// - In the subchannel call, we maintain state to indicate what ops have
//   already been sent down to that call.
//
// When constructing the "child" batches, we compare those two sets of
// data to see which batches need to be sent to the subchannel call.

// TODO(roth): In subsequent PRs:
// - add support for transparent retries (including initial metadata)
// - figure out how to record stats in census for retries
//   (census filter is on top of this one)
// - add census stats for retries

CallData::CallData(grpc_call_element* elem, const ChannelData& chand,
                   const grpc_call_element_args& args)
    : deadline_state_(elem, args.call_stack, args.call_combiner,
                      GPR_LIKELY(chand.deadline_checking_enabled())
                          ? args.deadline
                          : GRPC_MILLIS_INF_FUTURE),
      path_(grpc_slice_ref_internal(args.path)),
      call_start_time_(args.start_time),
      deadline_(args.deadline),
      arena_(args.arena),
      owning_call_(args.call_stack),
      call_combiner_(args.call_combiner),
      call_context_(args.context),
      pending_send_initial_metadata_(false),
      pending_send_message_(false),
      pending_send_trailing_metadata_(false),
      enable_retries_(chand.enable_retries()),
      retry_committed_(false),
      last_attempt_got_server_pushback_(false) {}

CallData::~CallData() {
  grpc_slice_unref_internal(path_);
  GRPC_ERROR_UNREF(cancel_error_);
  // Make sure there are no remaining pending batches.
  for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
    GPR_ASSERT(pending_batches_[i].batch == nullptr);
  }
}

grpc_error* CallData::Init(grpc_call_element* elem,
                           const grpc_call_element_args* args) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  new (elem->call_data) CallData(elem, *chand, *args);
  return GRPC_ERROR_NONE;
}

void CallData::Destroy(grpc_call_element* elem,
                       const grpc_call_final_info* final_info,
                       grpc_closure* then_schedule_closure) {
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (GPR_LIKELY(calld->subchannel_call_ != nullptr)) {
    calld->subchannel_call_->SetAfterCallStackDestroy(then_schedule_closure);
    then_schedule_closure = nullptr;
  }
  calld->~CallData();
  GRPC_CLOSURE_SCHED(then_schedule_closure, GRPC_ERROR_NONE);
}

void CallData::StartTransportStreamOpBatch(
    grpc_call_element* elem, grpc_transport_stream_op_batch* batch) {
  GPR_TIMER_SCOPE("cc_start_transport_stream_op_batch", 0);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (GPR_LIKELY(chand->deadline_checking_enabled())) {
    grpc_deadline_state_client_start_transport_stream_op_batch(elem, batch);
  }
  // If we've previously been cancelled, immediately fail any new batches.
  if (GPR_UNLIKELY(calld->cancel_error_ != GRPC_ERROR_NONE)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: failing batch with error: %s",
              chand, calld, grpc_error_string(calld->cancel_error_));
    }
    // Note: This will release the call combiner.
    grpc_transport_stream_op_batch_finish_with_failure(
        batch, GRPC_ERROR_REF(calld->cancel_error_), calld->call_combiner_);
    return;
  }
  // Handle cancellation.
  if (GPR_UNLIKELY(batch->cancel_stream)) {
    // Stash a copy of cancel_error in our call data, so that we can use
    // it for subsequent operations.  This ensures that if the call is
    // cancelled before any batches are passed down (e.g., if the deadline
    // is in the past when the call starts), we can return the right
    // error to the caller when the first batch does get passed down.
    GRPC_ERROR_UNREF(calld->cancel_error_);
    calld->cancel_error_ =
        GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: recording cancel_error=%s", chand,
              calld, grpc_error_string(calld->cancel_error_));
    }
    // If we do not have a subchannel call (i.e., a pick has not yet
    // been started), fail all pending batches.  Otherwise, send the
    // cancellation down to the subchannel call.
    if (calld->subchannel_call_ == nullptr) {
      // TODO(roth): If there is a pending retry callback, do we need to
      // cancel it here?
      calld->PendingBatchesFail(elem, GRPC_ERROR_REF(calld->cancel_error_),
                                NoYieldCallCombiner);
      // Note: This will release the call combiner.
      grpc_transport_stream_op_batch_finish_with_failure(
          batch, GRPC_ERROR_REF(calld->cancel_error_), calld->call_combiner_);
    } else {
      // Note: This will release the call combiner.
      calld->subchannel_call_->StartTransportStreamOpBatch(batch);
    }
    return;
  }
  // Add the batch to the pending list.
  calld->PendingBatchesAdd(elem, batch);
  // Check if we've already gotten a subchannel call.
  // Note that once we have completed the pick, we do not need to enter
  // the channel combiner, which is more efficient (especially for
  // streaming calls).
  if (calld->subchannel_call_ != nullptr) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: starting batch on subchannel_call=%p", chand,
              calld, calld->subchannel_call_.get());
    }
    calld->PendingBatchesResume(elem);
    return;
  }
  // We do not yet have a subchannel call.
  // For batches containing a send_initial_metadata op, enter the channel
  // combiner to start a pick.
  if (GPR_LIKELY(batch->send_initial_metadata)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: entering client_channel combiner",
              chand, calld);
    }
    GRPC_CLOSURE_SCHED(
        GRPC_CLOSURE_INIT(
            &batch->handler_private.closure, StartPickLocked, elem,
            grpc_combiner_scheduler(chand->data_plane_combiner())),
        GRPC_ERROR_NONE);
  } else {
    // For all other batches, release the call combiner.
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: saved batch, yielding call combiner", chand,
              calld);
    }
    GRPC_CALL_COMBINER_STOP(calld->call_combiner_,
                            "batch does not include send_initial_metadata");
  }
}

void CallData::SetPollent(grpc_call_element* elem,
                          grpc_polling_entity* pollent) {
  CallData* calld = static_cast<CallData*>(elem->call_data);
  calld->pollent_ = pollent;
}

//
// send op data caching
//

void CallData::MaybeCacheSendOpsForBatch(PendingBatch* pending) {
  if (pending->send_ops_cached) return;
  pending->send_ops_cached = true;
  grpc_transport_stream_op_batch* batch = pending->batch;
  // Save a copy of metadata for send_initial_metadata ops.
  if (batch->send_initial_metadata) {
    seen_send_initial_metadata_ = true;
    GPR_ASSERT(send_initial_metadata_storage_ == nullptr);
    grpc_metadata_batch* send_initial_metadata =
        batch->payload->send_initial_metadata.send_initial_metadata;
    send_initial_metadata_storage_ = (grpc_linked_mdelem*)arena_->Alloc(
        sizeof(grpc_linked_mdelem) * send_initial_metadata->list.count);
    grpc_metadata_batch_copy(send_initial_metadata, &send_initial_metadata_,
                             send_initial_metadata_storage_);
    send_initial_metadata_flags_ =
        batch->payload->send_initial_metadata.send_initial_metadata_flags;
    peer_string_ = batch->payload->send_initial_metadata.peer_string;
  }
  // Set up cache for send_message ops.
  if (batch->send_message) {
    ByteStreamCache* cache = arena_->New<ByteStreamCache>(
        std::move(batch->payload->send_message.send_message));
    send_messages_.push_back(cache);
  }
  // Save metadata batch for send_trailing_metadata ops.
  if (batch->send_trailing_metadata) {
    seen_send_trailing_metadata_ = true;
    GPR_ASSERT(send_trailing_metadata_storage_ == nullptr);
    grpc_metadata_batch* send_trailing_metadata =
        batch->payload->send_trailing_metadata.send_trailing_metadata;
    send_trailing_metadata_storage_ = (grpc_linked_mdelem*)arena_->Alloc(
        sizeof(grpc_linked_mdelem) * send_trailing_metadata->list.count);
    grpc_metadata_batch_copy(send_trailing_metadata, &send_trailing_metadata_,
                             send_trailing_metadata_storage_);
  }
}

void CallData::FreeCachedSendInitialMetadata(ChannelData* chand) {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: destroying calld->send_initial_metadata", chand,
            this);
  }
  grpc_metadata_batch_destroy(&send_initial_metadata_);
}

void CallData::FreeCachedSendMessage(ChannelData* chand, size_t idx) {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: destroying calld->send_messages[%" PRIuPTR "]",
            chand, this, idx);
  }
  send_messages_[idx]->Destroy();
}

void CallData::FreeCachedSendTrailingMetadata(ChannelData* chand) {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: destroying calld->send_trailing_metadata",
            chand, this);
  }
  grpc_metadata_batch_destroy(&send_trailing_metadata_);
}

void CallData::FreeCachedSendOpDataAfterCommit(
    grpc_call_element* elem, SubchannelCallRetryState* retry_state) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (retry_state->completed_send_initial_metadata) {
    FreeCachedSendInitialMetadata(chand);
  }
  for (size_t i = 0; i < retry_state->completed_send_message_count; ++i) {
    FreeCachedSendMessage(chand, i);
  }
  if (retry_state->completed_send_trailing_metadata) {
    FreeCachedSendTrailingMetadata(chand);
  }
}

void CallData::FreeCachedSendOpDataForCompletedBatch(
    grpc_call_element* elem, SubchannelCallBatchData* batch_data,
    SubchannelCallRetryState* retry_state) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (batch_data->batch.send_initial_metadata) {
    FreeCachedSendInitialMetadata(chand);
  }
  if (batch_data->batch.send_message) {
    FreeCachedSendMessage(chand, retry_state->completed_send_message_count - 1);
  }
  if (batch_data->batch.send_trailing_metadata) {
    FreeCachedSendTrailingMetadata(chand);
  }
}

//
// LB recv_trailing_metadata_ready handling
//

void CallData::MaybeInjectRecvTrailingMetadataReadyForLoadBalancingPolicy(
    const LoadBalancingPolicy::PickArgs& pick,
    grpc_transport_stream_op_batch* batch) {
  if (pick.recv_trailing_metadata_ready != nullptr) {
    *pick.original_recv_trailing_metadata_ready =
        batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready;
    batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready =
        pick.recv_trailing_metadata_ready;
    if (pick.recv_trailing_metadata != nullptr) {
      *pick.recv_trailing_metadata =
          batch->payload->recv_trailing_metadata.recv_trailing_metadata;
    }
  }
}

//
// pending_batches management
//

size_t CallData::GetBatchIndex(grpc_transport_stream_op_batch* batch) {
  // Note: It is important the send_initial_metadata be the first entry
  // here, since the code in pick_subchannel_locked() assumes it will be.
  if (batch->send_initial_metadata) return 0;
  if (batch->send_message) return 1;
  if (batch->send_trailing_metadata) return 2;
  if (batch->recv_initial_metadata) return 3;
  if (batch->recv_message) return 4;
  if (batch->recv_trailing_metadata) return 5;
  GPR_UNREACHABLE_CODE(return (size_t)-1);
}

// This is called via the call combiner, so access to calld is synchronized.
void CallData::PendingBatchesAdd(grpc_call_element* elem,
                                 grpc_transport_stream_op_batch* batch) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  const size_t idx = GetBatchIndex(batch);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: adding pending batch at index %" PRIuPTR, chand,
            this, idx);
  }
  PendingBatch* pending = &pending_batches_[idx];
  GPR_ASSERT(pending->batch == nullptr);
  pending->batch = batch;
  pending->send_ops_cached = false;
  if (enable_retries_) {
    // Update state in calld about pending batches.
    // Also check if the batch takes us over the retry buffer limit.
    // Note: We don't check the size of trailing metadata here, because
    // gRPC clients do not send trailing metadata.
    if (batch->send_initial_metadata) {
      pending_send_initial_metadata_ = true;
      bytes_buffered_for_retry_ += grpc_metadata_batch_size(
          batch->payload->send_initial_metadata.send_initial_metadata);
    }
    if (batch->send_message) {
      pending_send_message_ = true;
      bytes_buffered_for_retry_ +=
          batch->payload->send_message.send_message->length();
    }
    if (batch->send_trailing_metadata) {
      pending_send_trailing_metadata_ = true;
    }
    if (GPR_UNLIKELY(bytes_buffered_for_retry_ >
                     chand->per_rpc_retry_buffer_size())) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p calld=%p: exceeded retry buffer size, committing",
                chand, this);
      }
      SubchannelCallRetryState* retry_state =
          subchannel_call_ == nullptr ? nullptr
                                      : static_cast<SubchannelCallRetryState*>(
                                            subchannel_call_->GetParentData());
      RetryCommit(elem, retry_state);
      // If we are not going to retry and have not yet started, pretend
      // retries are disabled so that we don't bother with retry overhead.
      if (num_attempts_completed_ == 0) {
        if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
          gpr_log(GPR_INFO,
                  "chand=%p calld=%p: disabling retries before first attempt",
                  chand, this);
        }
        enable_retries_ = false;
      }
    }
  }
}

void CallData::PendingBatchClear(PendingBatch* pending) {
  if (enable_retries_) {
    if (pending->batch->send_initial_metadata) {
      pending_send_initial_metadata_ = false;
    }
    if (pending->batch->send_message) {
      pending_send_message_ = false;
    }
    if (pending->batch->send_trailing_metadata) {
      pending_send_trailing_metadata_ = false;
    }
  }
  pending->batch = nullptr;
}

void CallData::MaybeClearPendingBatch(grpc_call_element* elem,
                                      PendingBatch* pending) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  grpc_transport_stream_op_batch* batch = pending->batch;
  // We clear the pending batch if all of its callbacks have been
  // scheduled and reset to nullptr.
  if (batch->on_complete == nullptr &&
      (!batch->recv_initial_metadata ||
       batch->payload->recv_initial_metadata.recv_initial_metadata_ready ==
           nullptr) &&
      (!batch->recv_message ||
       batch->payload->recv_message.recv_message_ready == nullptr) &&
      (!batch->recv_trailing_metadata ||
       batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready ==
           nullptr)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: clearing pending batch", chand,
              this);
    }
    PendingBatchClear(pending);
  }
}

// This is called via the call combiner, so access to calld is synchronized.
void CallData::FailPendingBatchInCallCombiner(void* arg, grpc_error* error) {
  grpc_transport_stream_op_batch* batch =
      static_cast<grpc_transport_stream_op_batch*>(arg);
  CallData* calld = static_cast<CallData*>(batch->handler_private.extra_arg);
  // Note: This will release the call combiner.
  grpc_transport_stream_op_batch_finish_with_failure(
      batch, GRPC_ERROR_REF(error), calld->call_combiner_);
}

// This is called via the call combiner, so access to calld is synchronized.
void CallData::PendingBatchesFail(
    grpc_call_element* elem, grpc_error* error,
    YieldCallCombinerPredicate yield_call_combiner_predicate) {
  GPR_ASSERT(error != GRPC_ERROR_NONE);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    size_t num_batches = 0;
    for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
      if (pending_batches_[i].batch != nullptr) ++num_batches;
    }
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: failing %" PRIuPTR " pending batches: %s",
            elem->channel_data, this, num_batches, grpc_error_string(error));
  }
  CallCombinerClosureList closures;
  for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
    PendingBatch* pending = &pending_batches_[i];
    grpc_transport_stream_op_batch* batch = pending->batch;
    if (batch != nullptr) {
      if (batch->recv_trailing_metadata) {
        MaybeInjectRecvTrailingMetadataReadyForLoadBalancingPolicy(pick_.pick,
                                                                   batch);
      }
      batch->handler_private.extra_arg = this;
      GRPC_CLOSURE_INIT(&batch->handler_private.closure,
                        FailPendingBatchInCallCombiner, batch,
                        grpc_schedule_on_exec_ctx);
      closures.Add(&batch->handler_private.closure, GRPC_ERROR_REF(error),
                   "PendingBatchesFail");
      PendingBatchClear(pending);
    }
  }
  if (yield_call_combiner_predicate(closures)) {
    closures.RunClosures(call_combiner_);
  } else {
    closures.RunClosuresWithoutYielding(call_combiner_);
  }
  GRPC_ERROR_UNREF(error);
}

// This is called via the call combiner, so access to calld is synchronized.
void CallData::ResumePendingBatchInCallCombiner(void* arg,
                                                grpc_error* ignored) {
  grpc_transport_stream_op_batch* batch =
      static_cast<grpc_transport_stream_op_batch*>(arg);
  SubchannelCall* subchannel_call =
      static_cast<SubchannelCall*>(batch->handler_private.extra_arg);
  // Note: This will release the call combiner.
  subchannel_call->StartTransportStreamOpBatch(batch);
}

// This is called via the call combiner, so access to calld is synchronized.
void CallData::PendingBatchesResume(grpc_call_element* elem) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (enable_retries_) {
    StartRetriableSubchannelBatches(elem, GRPC_ERROR_NONE);
    return;
  }
  // Retries not enabled; send down batches as-is.
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    size_t num_batches = 0;
    for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
      if (pending_batches_[i].batch != nullptr) ++num_batches;
    }
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: starting %" PRIuPTR
            " pending batches on subchannel_call=%p",
            chand, this, num_batches, subchannel_call_.get());
  }
  CallCombinerClosureList closures;
  for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
    PendingBatch* pending = &pending_batches_[i];
    grpc_transport_stream_op_batch* batch = pending->batch;
    if (batch != nullptr) {
      if (batch->recv_trailing_metadata) {
        MaybeInjectRecvTrailingMetadataReadyForLoadBalancingPolicy(pick_.pick,
                                                                   batch);
      }
      batch->handler_private.extra_arg = subchannel_call_.get();
      GRPC_CLOSURE_INIT(&batch->handler_private.closure,
                        ResumePendingBatchInCallCombiner, batch,
                        grpc_schedule_on_exec_ctx);
      closures.Add(&batch->handler_private.closure, GRPC_ERROR_NONE,
                   "PendingBatchesResume");
      PendingBatchClear(pending);
    }
  }
  // Note: This will release the call combiner.
  closures.RunClosures(call_combiner_);
}

template <typename Predicate>
CallData::PendingBatch* CallData::PendingBatchFind(grpc_call_element* elem,
                                                   const char* log_message,
                                                   Predicate predicate) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
    PendingBatch* pending = &pending_batches_[i];
    grpc_transport_stream_op_batch* batch = pending->batch;
    if (batch != nullptr && predicate(batch)) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p calld=%p: %s pending batch at index %" PRIuPTR, chand,
                this, log_message, i);
      }
      return pending;
    }
  }
  return nullptr;
}

//
// retry code
//

void CallData::RetryCommit(grpc_call_element* elem,
                           SubchannelCallRetryState* retry_state) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (retry_committed_) return;
  retry_committed_ = true;
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: committing retries", chand, this);
  }
  if (retry_state != nullptr) {
    FreeCachedSendOpDataAfterCommit(elem, retry_state);
  }
}

void CallData::DoRetry(grpc_call_element* elem,
                       SubchannelCallRetryState* retry_state,
                       grpc_millis server_pushback_ms) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  GPR_ASSERT(method_params_ != nullptr);
  const auto* retry_policy = method_params_->retry_policy();
  GPR_ASSERT(retry_policy != nullptr);
  // Reset subchannel call and connected subchannel.
  subchannel_call_.reset();
  pick_.pick.connected_subchannel.reset();
  // Compute backoff delay.
  grpc_millis next_attempt_time;
  if (server_pushback_ms >= 0) {
    next_attempt_time = ExecCtx::Get()->Now() + server_pushback_ms;
    last_attempt_got_server_pushback_ = true;
  } else {
    if (num_attempts_completed_ == 1 || last_attempt_got_server_pushback_) {
      retry_backoff_.Init(
          BackOff::Options()
              .set_initial_backoff(retry_policy->initial_backoff)
              .set_multiplier(retry_policy->backoff_multiplier)
              .set_jitter(RETRY_BACKOFF_JITTER)
              .set_max_backoff(retry_policy->max_backoff));
      last_attempt_got_server_pushback_ = false;
    }
    next_attempt_time = retry_backoff_->NextAttemptTime();
  }
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: retrying failed call in %" PRId64 " ms", chand,
            this, next_attempt_time - ExecCtx::Get()->Now());
  }
  // Schedule retry after computed delay.
  GRPC_CLOSURE_INIT(&pick_closure_, StartPickLocked, elem,
                    grpc_combiner_scheduler(chand->data_plane_combiner()));
  grpc_timer_init(&retry_timer_, next_attempt_time, &pick_closure_);
  // Update bookkeeping.
  if (retry_state != nullptr) retry_state->retry_dispatched = true;
}

bool CallData::MaybeRetry(grpc_call_element* elem,
                          SubchannelCallBatchData* batch_data,
                          grpc_status_code status,
                          grpc_mdelem* server_pushback_md) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  // Get retry policy.
  if (method_params_ == nullptr) return false;
  const auto* retry_policy = method_params_->retry_policy();
  if (retry_policy == nullptr) return false;
  // If we've already dispatched a retry from this call, return true.
  // This catches the case where the batch has multiple callbacks
  // (i.e., it includes either recv_message or recv_initial_metadata).
  SubchannelCallRetryState* retry_state = nullptr;
  if (batch_data != nullptr) {
    retry_state = static_cast<SubchannelCallRetryState*>(
        batch_data->subchannel_call->GetParentData());
    if (retry_state->retry_dispatched) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
        gpr_log(GPR_INFO, "chand=%p calld=%p: retry already dispatched", chand,
                this);
      }
      return true;
    }
  }
  // Check status.
  if (GPR_LIKELY(status == GRPC_STATUS_OK)) {
    if (retry_throttle_data_ != nullptr) {
      retry_throttle_data_->RecordSuccess();
    }
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: call succeeded", chand, this);
    }
    return false;
  }
  // Status is not OK.  Check whether the status is retryable.
  if (!retry_policy->retryable_status_codes.Contains(status)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: status %s not configured as retryable", chand,
              this, grpc_status_code_to_string(status));
    }
    return false;
  }
  // Record the failure and check whether retries are throttled.
  // Note that it's important for this check to come after the status
  // code check above, since we should only record failures whose statuses
  // match the configured retryable status codes, so that we don't count
  // things like failures due to malformed requests (INVALID_ARGUMENT).
  // Conversely, it's important for this to come before the remaining
  // checks, so that we don't fail to record failures due to other factors.
  if (retry_throttle_data_ != nullptr &&
      !retry_throttle_data_->RecordFailure()) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: retries throttled", chand, this);
    }
    return false;
  }
  // Check whether the call is committed.
  if (retry_committed_) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: retries already committed", chand,
              this);
    }
    return false;
  }
  // Check whether we have retries remaining.
  ++num_attempts_completed_;
  if (num_attempts_completed_ >= retry_policy->max_attempts) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO, "chand=%p calld=%p: exceeded %d retry attempts", chand,
              this, retry_policy->max_attempts);
    }
    return false;
  }
  // If the call was cancelled from the surface, don't retry.
  if (cancel_error_ != GRPC_ERROR_NONE) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: call cancelled from surface, not retrying",
              chand, this);
    }
    return false;
  }
  // Check server push-back.
  grpc_millis server_pushback_ms = -1;
  if (server_pushback_md != nullptr) {
    // If the value is "-1" or any other unparseable string, we do not retry.
    uint32_t ms;
    if (!grpc_parse_slice_to_uint32(GRPC_MDVALUE(*server_pushback_md), &ms)) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p calld=%p: not retrying due to server push-back",
                chand, this);
      }
      return false;
    } else {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
        gpr_log(GPR_INFO, "chand=%p calld=%p: server push-back: retry in %u ms",
                chand, this, ms);
      }
      server_pushback_ms = (grpc_millis)ms;
    }
  }
  DoRetry(elem, retry_state, server_pushback_ms);
  return true;
}

//
// CallData::SubchannelCallBatchData
//

CallData::SubchannelCallBatchData* CallData::SubchannelCallBatchData::Create(
    grpc_call_element* elem, int refcount, bool set_on_complete) {
  CallData* calld = static_cast<CallData*>(elem->call_data);
  return calld->arena_->New<SubchannelCallBatchData>(elem, calld, refcount,
                                                     set_on_complete);
}

CallData::SubchannelCallBatchData::SubchannelCallBatchData(
    grpc_call_element* elem, CallData* calld, int refcount,
    bool set_on_complete)
    : elem(elem), subchannel_call(calld->subchannel_call_) {
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          calld->subchannel_call_->GetParentData());
  batch.payload = &retry_state->batch_payload;
  gpr_ref_init(&refs, refcount);
  if (set_on_complete) {
    GRPC_CLOSURE_INIT(&on_complete, CallData::OnComplete, this,
                      grpc_schedule_on_exec_ctx);
    batch.on_complete = &on_complete;
  }
  GRPC_CALL_STACK_REF(calld->owning_call_, "batch_data");
}

void CallData::SubchannelCallBatchData::Destroy() {
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(subchannel_call->GetParentData());
  if (batch.send_initial_metadata) {
    grpc_metadata_batch_destroy(&retry_state->send_initial_metadata);
  }
  if (batch.send_trailing_metadata) {
    grpc_metadata_batch_destroy(&retry_state->send_trailing_metadata);
  }
  if (batch.recv_initial_metadata) {
    grpc_metadata_batch_destroy(&retry_state->recv_initial_metadata);
  }
  if (batch.recv_trailing_metadata) {
    grpc_metadata_batch_destroy(&retry_state->recv_trailing_metadata);
  }
  subchannel_call.reset();
  CallData* calld = static_cast<CallData*>(elem->call_data);
  GRPC_CALL_STACK_UNREF(calld->owning_call_, "batch_data");
}

//
// recv_initial_metadata callback handling
//

void CallData::InvokeRecvInitialMetadataCallback(void* arg, grpc_error* error) {
  SubchannelCallBatchData* batch_data =
      static_cast<SubchannelCallBatchData*>(arg);
  CallData* calld = static_cast<CallData*>(batch_data->elem->call_data);
  // Find pending batch.
  PendingBatch* pending = calld->PendingBatchFind(
      batch_data->elem, "invoking recv_initial_metadata_ready for",
      [](grpc_transport_stream_op_batch* batch) {
        return batch->recv_initial_metadata &&
               batch->payload->recv_initial_metadata
                       .recv_initial_metadata_ready != nullptr;
      });
  GPR_ASSERT(pending != nullptr);
  // Return metadata.
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  grpc_metadata_batch_move(
      &retry_state->recv_initial_metadata,
      pending->batch->payload->recv_initial_metadata.recv_initial_metadata);
  // Update bookkeeping.
  // Note: Need to do this before invoking the callback, since invoking
  // the callback will result in yielding the call combiner.
  grpc_closure* recv_initial_metadata_ready =
      pending->batch->payload->recv_initial_metadata
          .recv_initial_metadata_ready;
  pending->batch->payload->recv_initial_metadata.recv_initial_metadata_ready =
      nullptr;
  calld->MaybeClearPendingBatch(batch_data->elem, pending);
  batch_data->Unref();
  // Invoke callback.
  GRPC_CLOSURE_RUN(recv_initial_metadata_ready, GRPC_ERROR_REF(error));
}

void CallData::RecvInitialMetadataReady(void* arg, grpc_error* error) {
  SubchannelCallBatchData* batch_data =
      static_cast<SubchannelCallBatchData*>(arg);
  grpc_call_element* elem = batch_data->elem;
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: got recv_initial_metadata_ready, error=%s",
            chand, calld, grpc_error_string(error));
  }
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  retry_state->completed_recv_initial_metadata = true;
  // If a retry was already dispatched, then we're not going to use the
  // result of this recv_initial_metadata op, so do nothing.
  if (retry_state->retry_dispatched) {
    GRPC_CALL_COMBINER_STOP(
        calld->call_combiner_,
        "recv_initial_metadata_ready after retry dispatched");
    return;
  }
  // If we got an error or a Trailers-Only response and have not yet gotten
  // the recv_trailing_metadata_ready callback, then defer propagating this
  // callback back to the surface.  We can evaluate whether to retry when
  // recv_trailing_metadata comes back.
  if (GPR_UNLIKELY((retry_state->trailing_metadata_available ||
                    error != GRPC_ERROR_NONE) &&
                   !retry_state->completed_recv_trailing_metadata)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: deferring recv_initial_metadata_ready "
              "(Trailers-Only)",
              chand, calld);
    }
    retry_state->recv_initial_metadata_ready_deferred_batch = batch_data;
    retry_state->recv_initial_metadata_error = GRPC_ERROR_REF(error);
    if (!retry_state->started_recv_trailing_metadata) {
      // recv_trailing_metadata not yet started by application; start it
      // ourselves to get status.
      calld->StartInternalRecvTrailingMetadata(elem);
    } else {
      GRPC_CALL_COMBINER_STOP(
          calld->call_combiner_,
          "recv_initial_metadata_ready trailers-only or error");
    }
    return;
  }
  // Received valid initial metadata, so commit the call.
  calld->RetryCommit(elem, retry_state);
  // Invoke the callback to return the result to the surface.
  // Manually invoking a callback function; it does not take ownership of error.
  calld->InvokeRecvInitialMetadataCallback(batch_data, error);
}

//
// recv_message callback handling
//

void CallData::InvokeRecvMessageCallback(void* arg, grpc_error* error) {
  SubchannelCallBatchData* batch_data =
      static_cast<SubchannelCallBatchData*>(arg);
  CallData* calld = static_cast<CallData*>(batch_data->elem->call_data);
  // Find pending op.
  PendingBatch* pending = calld->PendingBatchFind(
      batch_data->elem, "invoking recv_message_ready for",
      [](grpc_transport_stream_op_batch* batch) {
        return batch->recv_message &&
               batch->payload->recv_message.recv_message_ready != nullptr;
      });
  GPR_ASSERT(pending != nullptr);
  // Return payload.
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  *pending->batch->payload->recv_message.recv_message =
      std::move(retry_state->recv_message);
  // Update bookkeeping.
  // Note: Need to do this before invoking the callback, since invoking
  // the callback will result in yielding the call combiner.
  grpc_closure* recv_message_ready =
      pending->batch->payload->recv_message.recv_message_ready;
  pending->batch->payload->recv_message.recv_message_ready = nullptr;
  calld->MaybeClearPendingBatch(batch_data->elem, pending);
  batch_data->Unref();
  // Invoke callback.
  GRPC_CLOSURE_RUN(recv_message_ready, GRPC_ERROR_REF(error));
}

void CallData::RecvMessageReady(void* arg, grpc_error* error) {
  SubchannelCallBatchData* batch_data =
      static_cast<SubchannelCallBatchData*>(arg);
  grpc_call_element* elem = batch_data->elem;
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: got recv_message_ready, error=%s",
            chand, calld, grpc_error_string(error));
  }
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  ++retry_state->completed_recv_message_count;
  // If a retry was already dispatched, then we're not going to use the
  // result of this recv_message op, so do nothing.
  if (retry_state->retry_dispatched) {
    GRPC_CALL_COMBINER_STOP(calld->call_combiner_,
                            "recv_message_ready after retry dispatched");
    return;
  }
  // If we got an error or the payload was nullptr and we have not yet gotten
  // the recv_trailing_metadata_ready callback, then defer propagating this
  // callback back to the surface.  We can evaluate whether to retry when
  // recv_trailing_metadata comes back.
  if (GPR_UNLIKELY(
          (retry_state->recv_message == nullptr || error != GRPC_ERROR_NONE) &&
          !retry_state->completed_recv_trailing_metadata)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: deferring recv_message_ready (nullptr "
              "message and recv_trailing_metadata pending)",
              chand, calld);
    }
    retry_state->recv_message_ready_deferred_batch = batch_data;
    retry_state->recv_message_error = GRPC_ERROR_REF(error);
    if (!retry_state->started_recv_trailing_metadata) {
      // recv_trailing_metadata not yet started by application; start it
      // ourselves to get status.
      calld->StartInternalRecvTrailingMetadata(elem);
    } else {
      GRPC_CALL_COMBINER_STOP(calld->call_combiner_, "recv_message_ready null");
    }
    return;
  }
  // Received a valid message, so commit the call.
  calld->RetryCommit(elem, retry_state);
  // Invoke the callback to return the result to the surface.
  // Manually invoking a callback function; it does not take ownership of error.
  calld->InvokeRecvMessageCallback(batch_data, error);
}

//
// recv_trailing_metadata handling
//

void CallData::GetCallStatus(grpc_call_element* elem,
                             grpc_metadata_batch* md_batch, grpc_error* error,
                             grpc_status_code* status,
                             grpc_mdelem** server_pushback_md) {
  if (error != GRPC_ERROR_NONE) {
    grpc_error_get_status(error, deadline_, status, nullptr, nullptr, nullptr);
  } else {
    GPR_ASSERT(md_batch->idx.named.grpc_status != nullptr);
    *status =
        grpc_get_status_code_from_metadata(md_batch->idx.named.grpc_status->md);
    if (server_pushback_md != nullptr &&
        md_batch->idx.named.grpc_retry_pushback_ms != nullptr) {
      *server_pushback_md = &md_batch->idx.named.grpc_retry_pushback_ms->md;
    }
  }
  GRPC_ERROR_UNREF(error);
}

void CallData::AddClosureForRecvTrailingMetadataReady(
    grpc_call_element* elem, SubchannelCallBatchData* batch_data,
    grpc_error* error, CallCombinerClosureList* closures) {
  // Find pending batch.
  PendingBatch* pending = PendingBatchFind(
      elem, "invoking recv_trailing_metadata for",
      [](grpc_transport_stream_op_batch* batch) {
        return batch->recv_trailing_metadata &&
               batch->payload->recv_trailing_metadata
                       .recv_trailing_metadata_ready != nullptr;
      });
  // If we generated the recv_trailing_metadata op internally via
  // StartInternalRecvTrailingMetadata(), then there will be no pending batch.
  if (pending == nullptr) {
    GRPC_ERROR_UNREF(error);
    return;
  }
  // Return metadata.
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  grpc_metadata_batch_move(
      &retry_state->recv_trailing_metadata,
      pending->batch->payload->recv_trailing_metadata.recv_trailing_metadata);
  // Add closure.
  closures->Add(pending->batch->payload->recv_trailing_metadata
                    .recv_trailing_metadata_ready,
                error, "recv_trailing_metadata_ready for pending batch");
  // Update bookkeeping.
  pending->batch->payload->recv_trailing_metadata.recv_trailing_metadata_ready =
      nullptr;
  MaybeClearPendingBatch(elem, pending);
}

void CallData::AddClosuresForDeferredRecvCallbacks(
    SubchannelCallBatchData* batch_data, SubchannelCallRetryState* retry_state,
    CallCombinerClosureList* closures) {
  if (batch_data->batch.recv_trailing_metadata) {
    // Add closure for deferred recv_initial_metadata_ready.
    if (GPR_UNLIKELY(retry_state->recv_initial_metadata_ready_deferred_batch !=
                     nullptr)) {
      GRPC_CLOSURE_INIT(&retry_state->recv_initial_metadata_ready,
                        InvokeRecvInitialMetadataCallback,
                        retry_state->recv_initial_metadata_ready_deferred_batch,
                        grpc_schedule_on_exec_ctx);
      closures->Add(&retry_state->recv_initial_metadata_ready,
                    retry_state->recv_initial_metadata_error,
                    "resuming recv_initial_metadata_ready");
      retry_state->recv_initial_metadata_ready_deferred_batch = nullptr;
    }
    // Add closure for deferred recv_message_ready.
    if (GPR_UNLIKELY(retry_state->recv_message_ready_deferred_batch !=
                     nullptr)) {
      GRPC_CLOSURE_INIT(&retry_state->recv_message_ready,
                        InvokeRecvMessageCallback,
                        retry_state->recv_message_ready_deferred_batch,
                        grpc_schedule_on_exec_ctx);
      closures->Add(&retry_state->recv_message_ready,
                    retry_state->recv_message_error,
                    "resuming recv_message_ready");
      retry_state->recv_message_ready_deferred_batch = nullptr;
    }
  }
}

bool CallData::PendingBatchIsUnstarted(PendingBatch* pending,
                                       SubchannelCallRetryState* retry_state) {
  if (pending->batch == nullptr || pending->batch->on_complete == nullptr) {
    return false;
  }
  if (pending->batch->send_initial_metadata &&
      !retry_state->started_send_initial_metadata) {
    return true;
  }
  if (pending->batch->send_message &&
      retry_state->started_send_message_count < send_messages_.size()) {
    return true;
  }
  if (pending->batch->send_trailing_metadata &&
      !retry_state->started_send_trailing_metadata) {
    return true;
  }
  return false;
}

void CallData::AddClosuresToFailUnstartedPendingBatches(
    grpc_call_element* elem, SubchannelCallRetryState* retry_state,
    grpc_error* error, CallCombinerClosureList* closures) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
    PendingBatch* pending = &pending_batches_[i];
    if (PendingBatchIsUnstarted(pending, retry_state)) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
        gpr_log(GPR_INFO,
                "chand=%p calld=%p: failing unstarted pending batch at index "
                "%" PRIuPTR,
                chand, this, i);
      }
      closures->Add(pending->batch->on_complete, GRPC_ERROR_REF(error),
                    "failing on_complete for pending batch");
      pending->batch->on_complete = nullptr;
      MaybeClearPendingBatch(elem, pending);
    }
  }
  GRPC_ERROR_UNREF(error);
}

void CallData::RunClosuresForCompletedCall(SubchannelCallBatchData* batch_data,
                                           grpc_error* error) {
  grpc_call_element* elem = batch_data->elem;
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  // Construct list of closures to execute.
  CallCombinerClosureList closures;
  // First, add closure for recv_trailing_metadata_ready.
  AddClosureForRecvTrailingMetadataReady(elem, batch_data,
                                         GRPC_ERROR_REF(error), &closures);
  // If there are deferred recv_initial_metadata_ready or recv_message_ready
  // callbacks, add them to closures.
  AddClosuresForDeferredRecvCallbacks(batch_data, retry_state, &closures);
  // Add closures to fail any pending batches that have not yet been started.
  AddClosuresToFailUnstartedPendingBatches(elem, retry_state,
                                           GRPC_ERROR_REF(error), &closures);
  // Don't need batch_data anymore.
  batch_data->Unref();
  // Schedule all of the closures identified above.
  // Note: This will release the call combiner.
  closures.RunClosures(call_combiner_);
  GRPC_ERROR_UNREF(error);
}

void CallData::RecvTrailingMetadataReady(void* arg, grpc_error* error) {
  SubchannelCallBatchData* batch_data =
      static_cast<SubchannelCallBatchData*>(arg);
  grpc_call_element* elem = batch_data->elem;
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: got recv_trailing_metadata_ready, error=%s",
            chand, calld, grpc_error_string(error));
  }
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  retry_state->completed_recv_trailing_metadata = true;
  // Get the call's status and check for server pushback metadata.
  grpc_status_code status = GRPC_STATUS_OK;
  grpc_mdelem* server_pushback_md = nullptr;
  grpc_metadata_batch* md_batch =
      batch_data->batch.payload->recv_trailing_metadata.recv_trailing_metadata;
  calld->GetCallStatus(elem, md_batch, GRPC_ERROR_REF(error), &status,
                       &server_pushback_md);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: call finished, status=%s", chand,
            calld, grpc_status_code_to_string(status));
  }
  // Check if we should retry.
  if (calld->MaybeRetry(elem, batch_data, status, server_pushback_md)) {
    // Unref batch_data for deferred recv_initial_metadata_ready or
    // recv_message_ready callbacks, if any.
    if (retry_state->recv_initial_metadata_ready_deferred_batch != nullptr) {
      batch_data->Unref();
      GRPC_ERROR_UNREF(retry_state->recv_initial_metadata_error);
    }
    if (retry_state->recv_message_ready_deferred_batch != nullptr) {
      batch_data->Unref();
      GRPC_ERROR_UNREF(retry_state->recv_message_error);
    }
    batch_data->Unref();
    return;
  }
  // Not retrying, so commit the call.
  calld->RetryCommit(elem, retry_state);
  // Run any necessary closures.
  calld->RunClosuresForCompletedCall(batch_data, GRPC_ERROR_REF(error));
}

//
// on_complete callback handling
//

void CallData::AddClosuresForCompletedPendingBatch(
    grpc_call_element* elem, SubchannelCallBatchData* batch_data,
    SubchannelCallRetryState* retry_state, grpc_error* error,
    CallCombinerClosureList* closures) {
  PendingBatch* pending = PendingBatchFind(
      elem, "completed", [batch_data](grpc_transport_stream_op_batch* batch) {
        // Match the pending batch with the same set of send ops as the
        // subchannel batch we've just completed.
        return batch->on_complete != nullptr &&
               batch_data->batch.send_initial_metadata ==
                   batch->send_initial_metadata &&
               batch_data->batch.send_message == batch->send_message &&
               batch_data->batch.send_trailing_metadata ==
                   batch->send_trailing_metadata;
      });
  // If batch_data is a replay batch, then there will be no pending
  // batch to complete.
  if (pending == nullptr) {
    GRPC_ERROR_UNREF(error);
    return;
  }
  // Add closure.
  closures->Add(pending->batch->on_complete, error,
                "on_complete for pending batch");
  pending->batch->on_complete = nullptr;
  MaybeClearPendingBatch(elem, pending);
}

void CallData::AddClosuresForReplayOrPendingSendOps(
    grpc_call_element* elem, SubchannelCallBatchData* batch_data,
    SubchannelCallRetryState* retry_state, CallCombinerClosureList* closures) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  bool have_pending_send_message_ops =
      retry_state->started_send_message_count < send_messages_.size();
  bool have_pending_send_trailing_metadata_op =
      seen_send_trailing_metadata_ &&
      !retry_state->started_send_trailing_metadata;
  if (!have_pending_send_message_ops &&
      !have_pending_send_trailing_metadata_op) {
    for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
      PendingBatch* pending = &pending_batches_[i];
      grpc_transport_stream_op_batch* batch = pending->batch;
      if (batch == nullptr || pending->send_ops_cached) continue;
      if (batch->send_message) have_pending_send_message_ops = true;
      if (batch->send_trailing_metadata) {
        have_pending_send_trailing_metadata_op = true;
      }
    }
  }
  if (have_pending_send_message_ops || have_pending_send_trailing_metadata_op) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: starting next batch for pending send op(s)",
              chand, this);
    }
    GRPC_CLOSURE_INIT(&batch_data->batch.handler_private.closure,
                      StartRetriableSubchannelBatches, elem,
                      grpc_schedule_on_exec_ctx);
    closures->Add(&batch_data->batch.handler_private.closure, GRPC_ERROR_NONE,
                  "starting next batch for send_* op(s)");
  }
}

void CallData::OnComplete(void* arg, grpc_error* error) {
  SubchannelCallBatchData* batch_data =
      static_cast<SubchannelCallBatchData*>(arg);
  grpc_call_element* elem = batch_data->elem;
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    char* batch_str = grpc_transport_stream_op_batch_string(&batch_data->batch);
    gpr_log(GPR_INFO, "chand=%p calld=%p: got on_complete, error=%s, batch=%s",
            chand, calld, grpc_error_string(error), batch_str);
    gpr_free(batch_str);
  }
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          batch_data->subchannel_call->GetParentData());
  // Update bookkeeping in retry_state.
  if (batch_data->batch.send_initial_metadata) {
    retry_state->completed_send_initial_metadata = true;
  }
  if (batch_data->batch.send_message) {
    ++retry_state->completed_send_message_count;
  }
  if (batch_data->batch.send_trailing_metadata) {
    retry_state->completed_send_trailing_metadata = true;
  }
  // If the call is committed, free cached data for send ops that we've just
  // completed.
  if (calld->retry_committed_) {
    calld->FreeCachedSendOpDataForCompletedBatch(elem, batch_data, retry_state);
  }
  // Construct list of closures to execute.
  CallCombinerClosureList closures;
  // If a retry was already dispatched, that means we saw
  // recv_trailing_metadata before this, so we do nothing here.
  // Otherwise, invoke the callback to return the result to the surface.
  if (!retry_state->retry_dispatched) {
    // Add closure for the completed pending batch, if any.
    calld->AddClosuresForCompletedPendingBatch(
        elem, batch_data, retry_state, GRPC_ERROR_REF(error), &closures);
    // If needed, add a callback to start any replay or pending send ops on
    // the subchannel call.
    if (!retry_state->completed_recv_trailing_metadata) {
      calld->AddClosuresForReplayOrPendingSendOps(elem, batch_data, retry_state,
                                                  &closures);
    }
  }
  // Track number of pending subchannel send batches and determine if this
  // was the last one.
  --calld->num_pending_retriable_subchannel_send_batches_;
  const bool last_send_batch_complete =
      calld->num_pending_retriable_subchannel_send_batches_ == 0;
  // Don't need batch_data anymore.
  batch_data->Unref();
  // Schedule all of the closures identified above.
  // Note: This yeilds the call combiner.
  closures.RunClosures(calld->call_combiner_);
  // If this was the last subchannel send batch, unref the call stack.
  if (last_send_batch_complete) {
    GRPC_CALL_STACK_UNREF(calld->owning_call_, "subchannel_send_batches");
  }
}

//
// subchannel batch construction
//

void CallData::StartBatchInCallCombiner(void* arg, grpc_error* ignored) {
  grpc_transport_stream_op_batch* batch =
      static_cast<grpc_transport_stream_op_batch*>(arg);
  SubchannelCall* subchannel_call =
      static_cast<SubchannelCall*>(batch->handler_private.extra_arg);
  // Note: This will release the call combiner.
  subchannel_call->StartTransportStreamOpBatch(batch);
}

void CallData::AddClosureForSubchannelBatch(
    grpc_call_element* elem, grpc_transport_stream_op_batch* batch,
    CallCombinerClosureList* closures) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  batch->handler_private.extra_arg = subchannel_call_.get();
  GRPC_CLOSURE_INIT(&batch->handler_private.closure, StartBatchInCallCombiner,
                    batch, grpc_schedule_on_exec_ctx);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    char* batch_str = grpc_transport_stream_op_batch_string(batch);
    gpr_log(GPR_INFO, "chand=%p calld=%p: starting subchannel batch: %s", chand,
            this, batch_str);
    gpr_free(batch_str);
  }
  closures->Add(&batch->handler_private.closure, GRPC_ERROR_NONE,
                "start_subchannel_batch");
}

void CallData::AddRetriableSendInitialMetadataOp(
    SubchannelCallRetryState* retry_state,
    SubchannelCallBatchData* batch_data) {
  // Maps the number of retries to the corresponding metadata value slice.
  static const grpc_slice* retry_count_strings[] = {
      &GRPC_MDSTR_1, &GRPC_MDSTR_2, &GRPC_MDSTR_3, &GRPC_MDSTR_4};
  // We need to make a copy of the metadata batch for each attempt, since
  // the filters in the subchannel stack may modify this batch, and we don't
  // want those modifications to be passed forward to subsequent attempts.
  //
  // If we've already completed one or more attempts, add the
  // grpc-retry-attempts header.
  retry_state->send_initial_metadata_storage =
      static_cast<grpc_linked_mdelem*>(arena_->Alloc(
          sizeof(grpc_linked_mdelem) *
          (send_initial_metadata_.list.count + (num_attempts_completed_ > 0))));
  grpc_metadata_batch_copy(&send_initial_metadata_,
                           &retry_state->send_initial_metadata,
                           retry_state->send_initial_metadata_storage);
  if (GPR_UNLIKELY(retry_state->send_initial_metadata.idx.named
                       .grpc_previous_rpc_attempts != nullptr)) {
    grpc_metadata_batch_remove(&retry_state->send_initial_metadata,
                               retry_state->send_initial_metadata.idx.named
                                   .grpc_previous_rpc_attempts);
  }
  if (GPR_UNLIKELY(num_attempts_completed_ > 0)) {
    grpc_mdelem retry_md = grpc_mdelem_create(
        GRPC_MDSTR_GRPC_PREVIOUS_RPC_ATTEMPTS,
        *retry_count_strings[num_attempts_completed_ - 1], nullptr);
    grpc_error* error = grpc_metadata_batch_add_tail(
        &retry_state->send_initial_metadata,
        &retry_state
             ->send_initial_metadata_storage[send_initial_metadata_.list.count],
        retry_md);
    if (GPR_UNLIKELY(error != GRPC_ERROR_NONE)) {
      gpr_log(GPR_ERROR, "error adding retry metadata: %s",
              grpc_error_string(error));
      GPR_ASSERT(false);
    }
  }
  retry_state->started_send_initial_metadata = true;
  batch_data->batch.send_initial_metadata = true;
  batch_data->batch.payload->send_initial_metadata.send_initial_metadata =
      &retry_state->send_initial_metadata;
  batch_data->batch.payload->send_initial_metadata.send_initial_metadata_flags =
      send_initial_metadata_flags_;
  batch_data->batch.payload->send_initial_metadata.peer_string = peer_string_;
}

void CallData::AddRetriableSendMessageOp(grpc_call_element* elem,
                                         SubchannelCallRetryState* retry_state,
                                         SubchannelCallBatchData* batch_data) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: starting calld->send_messages[%" PRIuPTR "]",
            chand, this, retry_state->started_send_message_count);
  }
  ByteStreamCache* cache =
      send_messages_[retry_state->started_send_message_count];
  ++retry_state->started_send_message_count;
  retry_state->send_message.Init(cache);
  batch_data->batch.send_message = true;
  batch_data->batch.payload->send_message.send_message.reset(
      retry_state->send_message.get());
}

void CallData::AddRetriableSendTrailingMetadataOp(
    SubchannelCallRetryState* retry_state,
    SubchannelCallBatchData* batch_data) {
  // We need to make a copy of the metadata batch for each attempt, since
  // the filters in the subchannel stack may modify this batch, and we don't
  // want those modifications to be passed forward to subsequent attempts.
  retry_state->send_trailing_metadata_storage =
      static_cast<grpc_linked_mdelem*>(arena_->Alloc(
          sizeof(grpc_linked_mdelem) * send_trailing_metadata_.list.count));
  grpc_metadata_batch_copy(&send_trailing_metadata_,
                           &retry_state->send_trailing_metadata,
                           retry_state->send_trailing_metadata_storage);
  retry_state->started_send_trailing_metadata = true;
  batch_data->batch.send_trailing_metadata = true;
  batch_data->batch.payload->send_trailing_metadata.send_trailing_metadata =
      &retry_state->send_trailing_metadata;
}

void CallData::AddRetriableRecvInitialMetadataOp(
    SubchannelCallRetryState* retry_state,
    SubchannelCallBatchData* batch_data) {
  retry_state->started_recv_initial_metadata = true;
  batch_data->batch.recv_initial_metadata = true;
  grpc_metadata_batch_init(&retry_state->recv_initial_metadata);
  batch_data->batch.payload->recv_initial_metadata.recv_initial_metadata =
      &retry_state->recv_initial_metadata;
  batch_data->batch.payload->recv_initial_metadata.trailing_metadata_available =
      &retry_state->trailing_metadata_available;
  GRPC_CLOSURE_INIT(&retry_state->recv_initial_metadata_ready,
                    RecvInitialMetadataReady, batch_data,
                    grpc_schedule_on_exec_ctx);
  batch_data->batch.payload->recv_initial_metadata.recv_initial_metadata_ready =
      &retry_state->recv_initial_metadata_ready;
}

void CallData::AddRetriableRecvMessageOp(SubchannelCallRetryState* retry_state,
                                         SubchannelCallBatchData* batch_data) {
  ++retry_state->started_recv_message_count;
  batch_data->batch.recv_message = true;
  batch_data->batch.payload->recv_message.recv_message =
      &retry_state->recv_message;
  GRPC_CLOSURE_INIT(&retry_state->recv_message_ready, RecvMessageReady,
                    batch_data, grpc_schedule_on_exec_ctx);
  batch_data->batch.payload->recv_message.recv_message_ready =
      &retry_state->recv_message_ready;
}

void CallData::AddRetriableRecvTrailingMetadataOp(
    SubchannelCallRetryState* retry_state,
    SubchannelCallBatchData* batch_data) {
  retry_state->started_recv_trailing_metadata = true;
  batch_data->batch.recv_trailing_metadata = true;
  grpc_metadata_batch_init(&retry_state->recv_trailing_metadata);
  batch_data->batch.payload->recv_trailing_metadata.recv_trailing_metadata =
      &retry_state->recv_trailing_metadata;
  batch_data->batch.payload->recv_trailing_metadata.collect_stats =
      &retry_state->collect_stats;
  GRPC_CLOSURE_INIT(&retry_state->recv_trailing_metadata_ready,
                    RecvTrailingMetadataReady, batch_data,
                    grpc_schedule_on_exec_ctx);
  batch_data->batch.payload->recv_trailing_metadata
      .recv_trailing_metadata_ready =
      &retry_state->recv_trailing_metadata_ready;
  MaybeInjectRecvTrailingMetadataReadyForLoadBalancingPolicy(
      pick_.pick, &batch_data->batch);
}

void CallData::StartInternalRecvTrailingMetadata(grpc_call_element* elem) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: call failed but recv_trailing_metadata not "
            "started; starting it internally",
            chand, this);
  }
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(subchannel_call_->GetParentData());
  // Create batch_data with 2 refs, since this batch will be unreffed twice:
  // once for the recv_trailing_metadata_ready callback when the subchannel
  // batch returns, and again when we actually get a recv_trailing_metadata
  // op from the surface.
  SubchannelCallBatchData* batch_data =
      SubchannelCallBatchData::Create(elem, 2, false /* set_on_complete */);
  AddRetriableRecvTrailingMetadataOp(retry_state, batch_data);
  retry_state->recv_trailing_metadata_internal_batch = batch_data;
  // Note: This will release the call combiner.
  subchannel_call_->StartTransportStreamOpBatch(&batch_data->batch);
}

// If there are any cached send ops that need to be replayed on the
// current subchannel call, creates and returns a new subchannel batch
// to replay those ops.  Otherwise, returns nullptr.
CallData::SubchannelCallBatchData*
CallData::MaybeCreateSubchannelBatchForReplay(
    grpc_call_element* elem, SubchannelCallRetryState* retry_state) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  SubchannelCallBatchData* replay_batch_data = nullptr;
  // send_initial_metadata.
  if (seen_send_initial_metadata_ &&
      !retry_state->started_send_initial_metadata &&
      !pending_send_initial_metadata_) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: replaying previously completed "
              "send_initial_metadata op",
              chand, this);
    }
    replay_batch_data =
        SubchannelCallBatchData::Create(elem, 1, true /* set_on_complete */);
    AddRetriableSendInitialMetadataOp(retry_state, replay_batch_data);
  }
  // send_message.
  // Note that we can only have one send_message op in flight at a time.
  if (retry_state->started_send_message_count < send_messages_.size() &&
      retry_state->started_send_message_count ==
          retry_state->completed_send_message_count &&
      !pending_send_message_) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: replaying previously completed "
              "send_message op",
              chand, this);
    }
    if (replay_batch_data == nullptr) {
      replay_batch_data =
          SubchannelCallBatchData::Create(elem, 1, true /* set_on_complete */);
    }
    AddRetriableSendMessageOp(elem, retry_state, replay_batch_data);
  }
  // send_trailing_metadata.
  // Note that we only add this op if we have no more send_message ops
  // to start, since we can't send down any more send_message ops after
  // send_trailing_metadata.
  if (seen_send_trailing_metadata_ &&
      retry_state->started_send_message_count == send_messages_.size() &&
      !retry_state->started_send_trailing_metadata &&
      !pending_send_trailing_metadata_) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: replaying previously completed "
              "send_trailing_metadata op",
              chand, this);
    }
    if (replay_batch_data == nullptr) {
      replay_batch_data =
          SubchannelCallBatchData::Create(elem, 1, true /* set_on_complete */);
    }
    AddRetriableSendTrailingMetadataOp(retry_state, replay_batch_data);
  }
  return replay_batch_data;
}

void CallData::AddSubchannelBatchesForPendingBatches(
    grpc_call_element* elem, SubchannelCallRetryState* retry_state,
    CallCombinerClosureList* closures) {
  for (size_t i = 0; i < GPR_ARRAY_SIZE(pending_batches_); ++i) {
    PendingBatch* pending = &pending_batches_[i];
    grpc_transport_stream_op_batch* batch = pending->batch;
    if (batch == nullptr) continue;
    // Skip any batch that either (a) has already been started on this
    // subchannel call or (b) we can't start yet because we're still
    // replaying send ops that need to be completed first.
    // TODO(roth): Note that if any one op in the batch can't be sent
    // yet due to ops that we're replaying, we don't start any of the ops
    // in the batch.  This is probably okay, but it could conceivably
    // lead to increased latency in some cases -- e.g., we could delay
    // starting a recv op due to it being in the same batch with a send
    // op.  If/when we revamp the callback protocol in
    // transport_stream_op_batch, we may be able to fix this.
    if (batch->send_initial_metadata &&
        retry_state->started_send_initial_metadata) {
      continue;
    }
    if (batch->send_message && retry_state->completed_send_message_count <
                                   retry_state->started_send_message_count) {
      continue;
    }
    // Note that we only start send_trailing_metadata if we have no more
    // send_message ops to start, since we can't send down any more
    // send_message ops after send_trailing_metadata.
    if (batch->send_trailing_metadata &&
        (retry_state->started_send_message_count + batch->send_message <
             send_messages_.size() ||
         retry_state->started_send_trailing_metadata)) {
      continue;
    }
    if (batch->recv_initial_metadata &&
        retry_state->started_recv_initial_metadata) {
      continue;
    }
    if (batch->recv_message && retry_state->completed_recv_message_count <
                                   retry_state->started_recv_message_count) {
      continue;
    }
    if (batch->recv_trailing_metadata &&
        retry_state->started_recv_trailing_metadata) {
      // If we previously completed a recv_trailing_metadata op
      // initiated by StartInternalRecvTrailingMetadata(), use the
      // result of that instead of trying to re-start this op.
      if (GPR_UNLIKELY((retry_state->recv_trailing_metadata_internal_batch !=
                        nullptr))) {
        // If the batch completed, then trigger the completion callback
        // directly, so that we return the previously returned results to
        // the application.  Otherwise, just unref the internally
        // started subchannel batch, since we'll propagate the
        // completion when it completes.
        if (retry_state->completed_recv_trailing_metadata) {
          // Batches containing recv_trailing_metadata always succeed.
          closures->Add(
              &retry_state->recv_trailing_metadata_ready, GRPC_ERROR_NONE,
              "re-executing recv_trailing_metadata_ready to propagate "
              "internally triggered result");
        } else {
          retry_state->recv_trailing_metadata_internal_batch->Unref();
        }
        retry_state->recv_trailing_metadata_internal_batch = nullptr;
      }
      continue;
    }
    // If we're not retrying, just send the batch as-is.
    if (method_params_ == nullptr ||
        method_params_->retry_policy() == nullptr || retry_committed_) {
      // TODO(roth) : We should probably call
      // MaybeInjectRecvTrailingMetadataReadyForLoadBalancingPolicy here.
      AddClosureForSubchannelBatch(elem, batch, closures);
      PendingBatchClear(pending);
      continue;
    }
    // Create batch with the right number of callbacks.
    const bool has_send_ops = batch->send_initial_metadata ||
                              batch->send_message ||
                              batch->send_trailing_metadata;
    const int num_callbacks = has_send_ops + batch->recv_initial_metadata +
                              batch->recv_message +
                              batch->recv_trailing_metadata;
    SubchannelCallBatchData* batch_data = SubchannelCallBatchData::Create(
        elem, num_callbacks, has_send_ops /* set_on_complete */);
    // Cache send ops if needed.
    MaybeCacheSendOpsForBatch(pending);
    // send_initial_metadata.
    if (batch->send_initial_metadata) {
      AddRetriableSendInitialMetadataOp(retry_state, batch_data);
    }
    // send_message.
    if (batch->send_message) {
      AddRetriableSendMessageOp(elem, retry_state, batch_data);
    }
    // send_trailing_metadata.
    if (batch->send_trailing_metadata) {
      AddRetriableSendTrailingMetadataOp(retry_state, batch_data);
    }
    // recv_initial_metadata.
    if (batch->recv_initial_metadata) {
      // recv_flags is only used on the server side.
      GPR_ASSERT(batch->payload->recv_initial_metadata.recv_flags == nullptr);
      AddRetriableRecvInitialMetadataOp(retry_state, batch_data);
    }
    // recv_message.
    if (batch->recv_message) {
      AddRetriableRecvMessageOp(retry_state, batch_data);
    }
    // recv_trailing_metadata.
    if (batch->recv_trailing_metadata) {
      AddRetriableRecvTrailingMetadataOp(retry_state, batch_data);
    }
    AddClosureForSubchannelBatch(elem, &batch_data->batch, closures);
    // Track number of pending subchannel send batches.
    // If this is the first one, take a ref to the call stack.
    if (batch->send_initial_metadata || batch->send_message ||
        batch->send_trailing_metadata) {
      if (num_pending_retriable_subchannel_send_batches_ == 0) {
        GRPC_CALL_STACK_REF(owning_call_, "subchannel_send_batches");
      }
      ++num_pending_retriable_subchannel_send_batches_;
    }
  }
}

void CallData::StartRetriableSubchannelBatches(void* arg, grpc_error* ignored) {
  grpc_call_element* elem = static_cast<grpc_call_element*>(arg);
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: constructing retriable batches",
            chand, calld);
  }
  SubchannelCallRetryState* retry_state =
      static_cast<SubchannelCallRetryState*>(
          calld->subchannel_call_->GetParentData());
  // Construct list of closures to execute, one for each pending batch.
  CallCombinerClosureList closures;
  // Replay previously-returned send_* ops if needed.
  SubchannelCallBatchData* replay_batch_data =
      calld->MaybeCreateSubchannelBatchForReplay(elem, retry_state);
  if (replay_batch_data != nullptr) {
    calld->AddClosureForSubchannelBatch(elem, &replay_batch_data->batch,
                                        &closures);
    // Track number of pending subchannel send batches.
    // If this is the first one, take a ref to the call stack.
    if (calld->num_pending_retriable_subchannel_send_batches_ == 0) {
      GRPC_CALL_STACK_REF(calld->owning_call_, "subchannel_send_batches");
    }
    ++calld->num_pending_retriable_subchannel_send_batches_;
  }
  // Now add pending batches.
  calld->AddSubchannelBatchesForPendingBatches(elem, retry_state, &closures);
  // Start batches on subchannel call.
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: starting %" PRIuPTR
            " retriable batches on subchannel_call=%p",
            chand, calld, closures.size(), calld->subchannel_call_.get());
  }
  // Note: This will yield the call combiner.
  closures.RunClosures(calld->call_combiner_);
}

//
// LB pick
//

void CallData::CreateSubchannelCall(grpc_call_element* elem) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  const size_t parent_data_size =
      enable_retries_ ? sizeof(SubchannelCallRetryState) : 0;
  const ConnectedSubchannel::CallArgs call_args = {
      pollent_, path_, call_start_time_, deadline_, arena_,
      // TODO(roth): When we implement hedging support, we will probably
      // need to use a separate call context for each subchannel call.
      call_context_, call_combiner_, parent_data_size};
  grpc_error* error = GRPC_ERROR_NONE;
  subchannel_call_ =
      pick_.pick.connected_subchannel->CreateCall(call_args, &error);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: create subchannel_call=%p: error=%s",
            chand, this, subchannel_call_.get(), grpc_error_string(error));
  }
  if (GPR_UNLIKELY(error != GRPC_ERROR_NONE)) {
    PendingBatchesFail(elem, error, YieldCallCombiner);
  } else {
    if (parent_data_size > 0) {
      new (subchannel_call_->GetParentData())
          SubchannelCallRetryState(call_context_);
    }
    PendingBatchesResume(elem);
  }
}

void CallData::PickDone(void* arg, grpc_error* error) {
  grpc_call_element* elem = static_cast<grpc_call_element*>(arg);
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  if (error != GRPC_ERROR_NONE) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: failed to pick subchannel: error=%s", chand,
              calld, grpc_error_string(error));
    }
    calld->PendingBatchesFail(elem, GRPC_ERROR_REF(error), YieldCallCombiner);
    return;
  }
  calld->CreateSubchannelCall(elem);
}

// A class to handle the call combiner cancellation callback for a
// queued pick.
class CallData::QueuedPickCanceller {
 public:
  explicit QueuedPickCanceller(grpc_call_element* elem) : elem_(elem) {
    auto* calld = static_cast<CallData*>(elem->call_data);
    auto* chand = static_cast<ChannelData*>(elem->channel_data);
    GRPC_CALL_STACK_REF(calld->owning_call_, "QueuedPickCanceller");
    GRPC_CLOSURE_INIT(&closure_, &CancelLocked, this,
                      grpc_combiner_scheduler(chand->data_plane_combiner()));
    calld->call_combiner_->SetNotifyOnCancel(&closure_);
  }

 private:
  static void CancelLocked(void* arg, grpc_error* error) {
    auto* self = static_cast<QueuedPickCanceller*>(arg);
    auto* chand = static_cast<ChannelData*>(self->elem_->channel_data);
    auto* calld = static_cast<CallData*>(self->elem_->call_data);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
      gpr_log(GPR_INFO,
              "chand=%p calld=%p: cancelling queued pick: "
              "error=%s self=%p calld->pick_canceller=%p",
              chand, calld, grpc_error_string(error), self,
              calld->pick_canceller_);
    }
    if (calld->pick_canceller_ == self && error != GRPC_ERROR_NONE) {
      // Remove pick from list of queued picks.
      calld->RemoveCallFromQueuedPicksLocked(self->elem_);
      // Fail pending batches on the call.
      calld->PendingBatchesFail(self->elem_, GRPC_ERROR_REF(error),
                                YieldCallCombinerIfPendingBatchesFound);
    }
    GRPC_CALL_STACK_UNREF(calld->owning_call_, "QueuedPickCanceller");
    Delete(self);
  }

  grpc_call_element* elem_;
  grpc_closure closure_;
};

void CallData::RemoveCallFromQueuedPicksLocked(grpc_call_element* elem) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: removing from queued picks list",
            chand, this);
  }
  chand->RemoveQueuedPick(&pick_, pollent_);
  pick_queued_ = false;
  // Lame the call combiner canceller.
  pick_canceller_ = nullptr;
}

void CallData::AddCallToQueuedPicksLocked(grpc_call_element* elem) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: adding to queued picks list", chand,
            this);
  }
  pick_queued_ = true;
  pick_.elem = elem;
  chand->AddQueuedPick(&pick_, pollent_);
  // Register call combiner cancellation callback.
  pick_canceller_ = New<QueuedPickCanceller>(elem);
}

void CallData::ApplyServiceConfigToCallLocked(grpc_call_element* elem) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
    gpr_log(GPR_INFO, "chand=%p calld=%p: applying service config to call",
            chand, this);
  }
  // Store a ref to the service_config in service_config_call_data_. Also, save
  // a pointer to this in the call_context so that all future filters can access
  // it.
  service_config_call_data_ =
      ServiceConfig::CallData(chand->service_config(), path_);
  if (service_config_call_data_.service_config() != nullptr) {
    call_context_[GRPC_SERVICE_CONFIG_CALL_DATA].value =
        &service_config_call_data_;
    method_params_ = static_cast<ClientChannelMethodParsedObject*>(
        service_config_call_data_.GetMethodParsedObject(
            internal::ClientChannelServiceConfigParser::ParserIndex()));
  }
  retry_throttle_data_ = chand->retry_throttle_data();
  if (method_params_ != nullptr) {
    // If the deadline from the service config is shorter than the one
    // from the client API, reset the deadline timer.
    if (chand->deadline_checking_enabled() && method_params_->timeout() != 0) {
      const grpc_millis per_method_deadline =
          grpc_timespec_to_millis_round_up(call_start_time_) +
          method_params_->timeout();
      if (per_method_deadline < deadline_) {
        deadline_ = per_method_deadline;
        grpc_deadline_state_reset(elem, deadline_);
      }
    }
    // If the service config set wait_for_ready and the application
    // did not explicitly set it, use the value from the service config.
    uint32_t* send_initial_metadata_flags =
        &pending_batches_[0]
             .batch->payload->send_initial_metadata.send_initial_metadata_flags;
    if (method_params_->wait_for_ready().has_value() &&
        !(*send_initial_metadata_flags &
          GRPC_INITIAL_METADATA_WAIT_FOR_READY_EXPLICITLY_SET)) {
      if (method_params_->wait_for_ready().value()) {
        *send_initial_metadata_flags |= GRPC_INITIAL_METADATA_WAIT_FOR_READY;
      } else {
        *send_initial_metadata_flags &= ~GRPC_INITIAL_METADATA_WAIT_FOR_READY;
      }
    }
  }
  // If no retry policy, disable retries.
  // TODO(roth): Remove this when adding support for transparent retries.
  if (method_params_ == nullptr || method_params_->retry_policy() == nullptr) {
    enable_retries_ = false;
  }
}

void CallData::MaybeApplyServiceConfigToCallLocked(grpc_call_element* elem) {
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  // Apply service config data to the call only once, and only if the
  // channel has the data available.
  if (GPR_LIKELY(chand->received_service_config_data() &&
                 !service_config_applied_)) {
    service_config_applied_ = true;
    ApplyServiceConfigToCallLocked(elem);
  }
}

const char* PickResultName(LoadBalancingPolicy::PickResult result) {
  switch (result) {
    case LoadBalancingPolicy::PICK_COMPLETE:
      return "COMPLETE";
    case LoadBalancingPolicy::PICK_QUEUE:
      return "QUEUE";
    case LoadBalancingPolicy::PICK_TRANSIENT_FAILURE:
      return "TRANSIENT_FAILURE";
  }
  GPR_UNREACHABLE_CODE(return "UNKNOWN");
}

void CallData::StartPickLocked(void* arg, grpc_error* error) {
  grpc_call_element* elem = static_cast<grpc_call_element*>(arg);
  CallData* calld = static_cast<CallData*>(elem->call_data);
  ChannelData* chand = static_cast<ChannelData*>(elem->channel_data);
  GPR_ASSERT(calld->pick_.pick.connected_subchannel == nullptr);
  GPR_ASSERT(calld->subchannel_call_ == nullptr);
  // If this is a retry, use the send_initial_metadata payload that
  // we've cached; otherwise, use the pending batch.  The
  // send_initial_metadata batch will be the first pending batch in the
  // list, as set by GetBatchIndex() above.
  // TODO(roth): What if the LB policy needs to add something to the
  // call's initial metadata, and then there's a retry?  We don't want
  // the new metadata to be added twice.  We might need to somehow
  // allocate the subchannel batch earlier so that we can give the
  // subchannel's copy of the metadata batch (which is copied for each
  // attempt) to the LB policy instead the one from the parent channel.
  calld->pick_.pick.initial_metadata =
      calld->seen_send_initial_metadata_
          ? &calld->send_initial_metadata_
          : calld->pending_batches_[0]
                .batch->payload->send_initial_metadata.send_initial_metadata;
  uint32_t* send_initial_metadata_flags =
      calld->seen_send_initial_metadata_
          ? &calld->send_initial_metadata_flags_
          : &calld->pending_batches_[0]
                 .batch->payload->send_initial_metadata
                 .send_initial_metadata_flags;
  // Apply service config to call if needed.
  calld->MaybeApplyServiceConfigToCallLocked(elem);
  // When done, we schedule this closure to leave the data plane combiner.
  GRPC_CLOSURE_INIT(&calld->pick_closure_, PickDone, elem,
                    grpc_schedule_on_exec_ctx);
  // Attempt pick.
  error = GRPC_ERROR_NONE;
  auto pick_result = chand->picker()->Pick(&calld->pick_.pick, &error);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
    gpr_log(GPR_INFO,
            "chand=%p calld=%p: LB pick returned %s (connected_subchannel=%p, "
            "error=%s)",
            chand, calld, PickResultName(pick_result),
            calld->pick_.pick.connected_subchannel.get(),
            grpc_error_string(error));
  }
  switch (pick_result) {
    case LoadBalancingPolicy::PICK_TRANSIENT_FAILURE: {
      // If we're shutting down, fail all RPCs.
      grpc_error* disconnect_error = chand->disconnect_error();
      if (disconnect_error != GRPC_ERROR_NONE) {
        GRPC_ERROR_UNREF(error);
        GRPC_CLOSURE_SCHED(&calld->pick_closure_,
                           GRPC_ERROR_REF(disconnect_error));
        break;
      }
      // If wait_for_ready is false, then the error indicates the RPC
      // attempt's final status.
      if ((*send_initial_metadata_flags &
           GRPC_INITIAL_METADATA_WAIT_FOR_READY) == 0) {
        // Retry if appropriate; otherwise, fail.
        grpc_status_code status = GRPC_STATUS_OK;
        grpc_error_get_status(error, calld->deadline_, &status, nullptr,
                              nullptr, nullptr);
        if (!calld->enable_retries_ ||
            !calld->MaybeRetry(elem, nullptr /* batch_data */, status,
                               nullptr /* server_pushback_md */)) {
          grpc_error* new_error =
              GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
                  "Failed to pick subchannel", &error, 1);
          GRPC_ERROR_UNREF(error);
          GRPC_CLOSURE_SCHED(&calld->pick_closure_, new_error);
        }
        if (calld->pick_queued_) calld->RemoveCallFromQueuedPicksLocked(elem);
        break;
      }
      // If wait_for_ready is true, then queue to retry when we get a new
      // picker.
      GRPC_ERROR_UNREF(error);
    }
    // Fallthrough
    case LoadBalancingPolicy::PICK_QUEUE:
      if (!calld->pick_queued_) calld->AddCallToQueuedPicksLocked(elem);
      break;
    default:  // PICK_COMPLETE
      // Handle drops.
      if (GPR_UNLIKELY(calld->pick_.pick.connected_subchannel == nullptr)) {
        error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "Call dropped by load balancing policy");
      }
      GRPC_CLOSURE_SCHED(&calld->pick_closure_, error);
      if (calld->pick_queued_) calld->RemoveCallFromQueuedPicksLocked(elem);
  }
}

}  // namespace
}  // namespace grpc_core

/*************************************************************************
 * EXPORTED SYMBOLS
 */

using grpc_core::CallData;
using grpc_core::ChannelData;

const grpc_channel_filter grpc_client_channel_filter = {
    CallData::StartTransportStreamOpBatch,
    ChannelData::StartTransportOp,
    sizeof(CallData),
    CallData::Init,
    CallData::SetPollent,
    CallData::Destroy,
    sizeof(ChannelData),
    ChannelData::Init,
    ChannelData::Destroy,
    ChannelData::GetChannelInfo,
    "client-channel",
};

void grpc_client_channel_set_channelz_node(
    grpc_channel_element* elem, grpc_core::channelz::ClientChannelNode* node) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  chand->set_channelz_node(node);
}

void grpc_client_channel_populate_child_refs(
    grpc_channel_element* elem,
    grpc_core::channelz::ChildRefsList* child_subchannels,
    grpc_core::channelz::ChildRefsList* child_channels) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  chand->FillChildRefsForChannelz(child_subchannels, child_channels);
}

grpc_connectivity_state grpc_client_channel_check_connectivity_state(
    grpc_channel_element* elem, int try_to_connect) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  return chand->CheckConnectivityState(try_to_connect);
}

int grpc_client_channel_num_external_connectivity_watchers(
    grpc_channel_element* elem) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  return chand->NumExternalConnectivityWatchers();
}

void grpc_client_channel_watch_connectivity_state(
    grpc_channel_element* elem, grpc_polling_entity pollent,
    grpc_connectivity_state* state, grpc_closure* closure,
    grpc_closure* watcher_timer_init) {
  auto* chand = static_cast<ChannelData*>(elem->channel_data);
  return chand->AddExternalConnectivityWatcher(pollent, state, closure,
                                               watcher_timer_init);
}

grpc_core::RefCountedPtr<grpc_core::SubchannelCall>
grpc_client_channel_get_subchannel_call(grpc_call_element* elem) {
  auto* calld = static_cast<CallData*>(elem->call_data);
  return calld->subchannel_call();
}