src/core/ext/filters/client_channel/resolving_lb_policy.cc

   1 /*
   2  *
   3  * Copyright 2015 gRPC authors.
   4  *
   5  * Licensed under the Apache License, Version 2.0 (the "License");
   6  * you may not use this file except in compliance with the License.
   7  * You may obtain a copy of the License at
   8  *
   9  *     http://www.apache.org/licenses/LICENSE-2.0
  10  *
  11  * Unless required by applicable law or agreed to in writing, software
  12  * distributed under the License is distributed on an "AS IS" BASIS,
  13  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14  * See the License for the specific language governing permissions and
  15  * limitations under the License.
  16  *
  17  */
  18
  19 #include <grpc/support/port_platform.h>
  20
  21 #include "src/core/ext/filters/client_channel/resolving_lb_policy.h"
  22
  23 #include <inttypes.h>
  24 #include <limits.h>
  25 #include <stdbool.h>
  26 #include <stdio.h>
  27 #include <string.h>
  28
  29 #include <grpc/support/alloc.h>
  30 #include <grpc/support/log.h>
  31 #include <grpc/support/string_util.h>
  32 #include <grpc/support/sync.h>
  33
  34 #include "src/core/ext/filters/client_channel/backup_poller.h"
  35 #include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
  36 #include "src/core/ext/filters/client_channel/lb_policy_registry.h"
  37 #include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
  38 #include "src/core/ext/filters/client_channel/resolver_registry.h"
  39 #include "src/core/ext/filters/client_channel/retry_throttle.h"
  40 #include "src/core/ext/filters/client_channel/server_address.h"
  41 #include "src/core/ext/filters/client_channel/service_config.h"
  42 #include "src/core/ext/filters/client_channel/subchannel.h"
  43 #include "src/core/ext/filters/deadline/deadline_filter.h"
  44 #include "src/core/lib/backoff/backoff.h"
  45 #include "src/core/lib/channel/channel_args.h"
  46 #include "src/core/lib/channel/connected_channel.h"
  47 #include "src/core/lib/channel/status_util.h"
  48 #include "src/core/lib/gpr/string.h"
  49 #include "src/core/lib/gprpp/inlined_vector.h"
  50 #include "src/core/lib/gprpp/manual_constructor.h"
  51 #include "src/core/lib/gprpp/mutex_lock.h"
  52 #include "src/core/lib/iomgr/combiner.h"
  53 #include "src/core/lib/iomgr/iomgr.h"
  54 #include "src/core/lib/iomgr/polling_entity.h"
  55 #include "src/core/lib/profiling/timers.h"
  56 #include "src/core/lib/slice/slice_internal.h"
  57 #include "src/core/lib/slice/slice_string_helpers.h"
  58 #include "src/core/lib/surface/channel.h"
  59 #include "src/core/lib/transport/connectivity_state.h"
  60 #include "src/core/lib/transport/error_utils.h"
  61 #include "src/core/lib/transport/metadata.h"
  62 #include "src/core/lib/transport/metadata_batch.h"
  63 #include "src/core/lib/transport/static_metadata.h"
  64 #include "src/core/lib/transport/status_metadata.h"
  65
  66 namespace grpc_core {
  67
  68 //
  69 // ResolvingLoadBalancingPolicy::ResolverResultHandler
  70 //
  71
  72 class ResolvingLoadBalancingPolicy::ResolverResultHandler
  73     : public Resolver::ResultHandler {
  74  public:
  75   explicit ResolverResultHandler(
  76       RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
  77       : parent_(std::move(parent)) {}
  78
  79   ~ResolverResultHandler() {
  80     if (parent_->tracer_->enabled()) {
  81       gpr_log(GPR_INFO, "resolving_lb=%p: resolver shutdown complete",
  82               parent_.get());
  83     }
  84   }
  85
  86   void ReturnResult(Resolver::Result result) override {
  87     parent_->OnResolverResultChangedLocked(std::move(result));
  88   }
  89
  90   void ReturnError(grpc_error* error) override {
  91     parent_->OnResolverError(error);
  92   }
  93
  94  private:
  95   RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
  96 };
  97
  98 //
  99 // ResolvingLoadBalancingPolicy::ResolvingControlHelper
 100 //
 101
 102 class ResolvingLoadBalancingPolicy::ResolvingControlHelper
 103     : public LoadBalancingPolicy::ChannelControlHelper {
 104  public:
 105   explicit ResolvingControlHelper(
 106       RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
 107       : parent_(std::move(parent)) {}
 108
 109   Subchannel* CreateSubchannel(const grpc_channel_args& args) override {
 110     if (parent_->resolver_ == nullptr) return nullptr;  // Shutting down.
 111     if (!CalledByCurrentChild() && !CalledByPendingChild()) return nullptr;
 112     return parent_->channel_control_helper()->CreateSubchannel(args);
 113   }
 114
 115   grpc_channel* CreateChannel(const char* target,
 116                               const grpc_channel_args& args) override {
 117     if (parent_->resolver_ == nullptr) return nullptr;  // Shutting down.
 118     if (!CalledByCurrentChild() && !CalledByPendingChild()) return nullptr;
 119     return parent_->channel_control_helper()->CreateChannel(target, args);
 120   }
 121
 122   void UpdateState(grpc_connectivity_state state, grpc_error* state_error,
 123                    UniquePtr<SubchannelPicker> picker) override {
 124     if (parent_->resolver_ == nullptr) {
 125       // shutting down.
 126       GRPC_ERROR_UNREF(state_error);
 127       return;
 128     }
 129     // If this request is from the pending child policy, ignore it until
 130     // it reports READY, at which point we swap it into place.
 131     if (CalledByPendingChild()) {
 132       if (parent_->tracer_->enabled()) {
 133         gpr_log(GPR_INFO,
 134                 "resolving_lb=%p helper=%p: pending child policy %p reports "
 135                 "state=%s",
 136                 parent_.get(), this, child_,
 137                 grpc_connectivity_state_name(state));
 138       }
 139       if (state != GRPC_CHANNEL_READY) {
 140         GRPC_ERROR_UNREF(state_error);
 141         return;
 142       }
 143       grpc_pollset_set_del_pollset_set(
 144           parent_->lb_policy_->interested_parties(),
 145           parent_->interested_parties());
 146       MutexLock lock(&parent_->lb_policy_mu_);
 147       parent_->lb_policy_ = std::move(parent_->pending_lb_policy_);
 148     } else if (!CalledByCurrentChild()) {
 149       // This request is from an outdated child, so ignore it.
 150       GRPC_ERROR_UNREF(state_error);
 151       return;
 152     }
 153     parent_->channel_control_helper()->UpdateState(state, state_error,
 154                                                    std::move(picker));
 155   }
 156
 157   void RequestReresolution() override {
 158     // If there is a pending child policy, ignore re-resolution requests
 159     // from the current child policy (or any outdated child).
 160     if (parent_->pending_lb_policy_ != nullptr && !CalledByPendingChild()) {
 161       return;
 162     }
 163     if (parent_->tracer_->enabled()) {
 164       gpr_log(GPR_INFO, "resolving_lb=%p: started name re-resolving",
 165               parent_.get());
 166     }
 167     if (parent_->resolver_ != nullptr) {
 168       parent_->resolver_->RequestReresolutionLocked();
 169     }
 170   }
 171
 172   void set_child(LoadBalancingPolicy* child) { child_ = child; }
 173
 174  private:
 175   bool CalledByPendingChild() const {
 176     GPR_ASSERT(child_ != nullptr);
 177     return child_ == parent_->pending_lb_policy_.get();
 178   }
 179
 180   bool CalledByCurrentChild() const {
 181     GPR_ASSERT(child_ != nullptr);
 182     return child_ == parent_->lb_policy_.get();
 183   };
 184
 185   RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
 186   LoadBalancingPolicy* child_ = nullptr;
 187 };
 188
 189 //
 190 // ResolvingLoadBalancingPolicy
 191 //
 192
 193 ResolvingLoadBalancingPolicy::ResolvingLoadBalancingPolicy(
 194     Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
 195     UniquePtr<char> child_policy_name, RefCountedPtr<Config> child_lb_config,
 196     grpc_error** error)
 197     : LoadBalancingPolicy(std::move(args)),
 198       tracer_(tracer),
 199       target_uri_(std::move(target_uri)),
 200       child_policy_name_(std::move(child_policy_name)),
 201       child_lb_config_(std::move(child_lb_config)) {
 202   GPR_ASSERT(child_policy_name_ != nullptr);
 203   // Don't fetch service config, since this ctor is for use in nested LB
 204   // policies, not at the top level, and we only fetch the service
 205   // config at the top level.
 206   grpc_arg arg = grpc_channel_arg_integer_create(
 207       const_cast<char*>(GRPC_ARG_SERVICE_CONFIG_DISABLE_RESOLUTION), 0);
 208   grpc_channel_args* new_args =
 209       grpc_channel_args_copy_and_add(args.args, &arg, 1);
 210   *error = Init(*new_args);
 211   grpc_channel_args_destroy(new_args);
 212 }
 213
 214 ResolvingLoadBalancingPolicy::ResolvingLoadBalancingPolicy(
 215     Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
 216     ProcessResolverResultCallback process_resolver_result,
 217     void* process_resolver_result_user_data, grpc_error** error)
 218     : LoadBalancingPolicy(std::move(args)),
 219       tracer_(tracer),
 220       target_uri_(std::move(target_uri)),
 221       process_resolver_result_(process_resolver_result),
 222       process_resolver_result_user_data_(process_resolver_result_user_data) {
 223   GPR_ASSERT(process_resolver_result != nullptr);
 224   gpr_mu_init(&lb_policy_mu_);
 225   *error = Init(*args.args);
 226 }
 227
 228 grpc_error* ResolvingLoadBalancingPolicy::Init(const grpc_channel_args& args) {
 229   resolver_ = ResolverRegistry::CreateResolver(
 230       target_uri_.get(), &args, interested_parties(), combiner(),
 231       UniquePtr<Resolver::ResultHandler>(New<ResolverResultHandler>(Ref())));
 232   if (resolver_ == nullptr) {
 233     return GRPC_ERROR_CREATE_FROM_STATIC_STRING("resolver creation failed");
 234   }
 235   // Return our picker to the channel.
 236   channel_control_helper()->UpdateState(
 237       GRPC_CHANNEL_IDLE, GRPC_ERROR_NONE,
 238       UniquePtr<SubchannelPicker>(New<QueuePicker>(Ref())));
 239   return GRPC_ERROR_NONE;
 240 }
 241
 242 ResolvingLoadBalancingPolicy::~ResolvingLoadBalancingPolicy() {
 243   GPR_ASSERT(resolver_ == nullptr);
 244   GPR_ASSERT(lb_policy_ == nullptr);
 245   gpr_mu_destroy(&lb_policy_mu_);
 246 }
 247
 248 void ResolvingLoadBalancingPolicy::ShutdownLocked() {
 249   if (resolver_ != nullptr) {
 250     resolver_.reset();
 251     MutexLock lock(&lb_policy_mu_);
 252     if (lb_policy_ != nullptr) {
 253       if (tracer_->enabled()) {
 254         gpr_log(GPR_INFO, "resolving_lb=%p: shutting down lb_policy=%p", this,
 255                 lb_policy_.get());
 256       }
 257       grpc_pollset_set_del_pollset_set(lb_policy_->interested_parties(),
 258                                        interested_parties());
 259       lb_policy_.reset();
 260     }
 261     if (pending_lb_policy_ != nullptr) {
 262       if (tracer_->enabled()) {
 263         gpr_log(GPR_INFO, "resolving_lb=%p: shutting down pending lb_policy=%p",
 264                 this, pending_lb_policy_.get());
 265       }
 266       grpc_pollset_set_del_pollset_set(pending_lb_policy_->interested_parties(),
 267                                        interested_parties());
 268       pending_lb_policy_.reset();
 269     }
 270   }
 271 }
 272
 273 void ResolvingLoadBalancingPolicy::ExitIdleLocked() {
 274   if (lb_policy_ != nullptr) {
 275     lb_policy_->ExitIdleLocked();
 276     if (pending_lb_policy_ != nullptr) pending_lb_policy_->ExitIdleLocked();
 277   } else {
 278     if (!started_resolving_ && resolver_ != nullptr) {
 279       StartResolvingLocked();
 280     }
 281   }
 282 }
 283
 284 void ResolvingLoadBalancingPolicy::ResetBackoffLocked() {
 285   if (resolver_ != nullptr) {
 286     resolver_->ResetBackoffLocked();
 287     resolver_->RequestReresolutionLocked();
 288   }
 289   if (lb_policy_ != nullptr) lb_policy_->ResetBackoffLocked();
 290   if (pending_lb_policy_ != nullptr) pending_lb_policy_->ResetBackoffLocked();
 291 }
 292
 293 void ResolvingLoadBalancingPolicy::FillChildRefsForChannelz(
 294     channelz::ChildRefsList* child_subchannels,
 295     channelz::ChildRefsList* child_channels) {
 296   // Delegate to the lb_policy_ to fill the children subchannels.
 297   // This must be done holding lb_policy_mu_, since this method does not
 298   // run in the combiner.
 299   MutexLock lock(&lb_policy_mu_);
 300   if (lb_policy_ != nullptr) {
 301     lb_policy_->FillChildRefsForChannelz(child_subchannels, child_channels);
 302   }
 303   if (pending_lb_policy_ != nullptr) {
 304     pending_lb_policy_->FillChildRefsForChannelz(child_subchannels,
 305                                                  child_channels);
 306   }
 307 }
 308
 309 void ResolvingLoadBalancingPolicy::StartResolvingLocked() {
 310   if (tracer_->enabled()) {
 311     gpr_log(GPR_INFO, "resolving_lb=%p: starting name resolution", this);
 312   }
 313   GPR_ASSERT(!started_resolving_);
 314   started_resolving_ = true;
 315   channel_control_helper()->UpdateState(
 316       GRPC_CHANNEL_CONNECTING, GRPC_ERROR_NONE,
 317       UniquePtr<SubchannelPicker>(New<QueuePicker>(Ref())));
 318   resolver_->StartLocked();
 319 }
 320
 321 void ResolvingLoadBalancingPolicy::OnResolverError(grpc_error* error) {
 322   if (resolver_ == nullptr) {
 323     GRPC_ERROR_UNREF(error);
 324     return;
 325   }
 326   if (tracer_->enabled()) {
 327     gpr_log(GPR_INFO, "resolving_lb=%p: resolver transient failure: %s", this,
 328             grpc_error_string(error));
 329   }
 330   // If we already have an LB policy from a previous resolution
 331   // result, then we continue to let it set the connectivity state.
 332   // Otherwise, we go into TRANSIENT_FAILURE.
 333   if (lb_policy_ == nullptr) {
 334     grpc_error* state_error = GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
 335         "Resolver transient failure", &error, 1);
 336     channel_control_helper()->UpdateState(
 337         GRPC_CHANNEL_TRANSIENT_FAILURE, GRPC_ERROR_REF(state_error),
 338         UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(state_error)));
 339   }
 340   GRPC_ERROR_UNREF(error);
 341 }
 342
 343 void ResolvingLoadBalancingPolicy::CreateOrUpdateLbPolicyLocked(
 344     const char* lb_policy_name, RefCountedPtr<Config> lb_policy_config,
 345     Resolver::Result result, TraceStringVector* trace_strings) {
 346   // If the child policy name changes, we need to create a new child
 347   // policy.  When this happens, we leave child_policy_ as-is and store
 348   // the new child policy in pending_child_policy_.  Once the new child
 349   // policy transitions into state READY, we swap it into child_policy_,
 350   // replacing the original child policy.  So pending_child_policy_ is
 351   // non-null only between when we apply an update that changes the child
 352   // policy name and when the new child reports state READY.
 353   //
 354   // Updates can arrive at any point during this transition.  We always
 355   // apply updates relative to the most recently created child policy,
 356   // even if the most recent one is still in pending_child_policy_.  This
 357   // is true both when applying the updates to an existing child policy
 358   // and when determining whether we need to create a new policy.
 359   //
 360   // As a result of this, there are several cases to consider here:
 361   //
 362   // 1. We have no existing child policy (i.e., we have started up but
 363   //    have not yet received a serverlist from the balancer or gone
 364   //    into fallback mode; in this case, both child_policy_ and
 365   //    pending_child_policy_ are null).  In this case, we create a
 366   //    new child policy and store it in child_policy_.
 367   //
 368   // 2. We have an existing child policy and have no pending child policy
 369   //    from a previous update (i.e., either there has not been a
 370   //    previous update that changed the policy name, or we have already
 371   //    finished swapping in the new policy; in this case, child_policy_
 372   //    is non-null but pending_child_policy_ is null).  In this case:
 373   //    a. If child_policy_->name() equals child_policy_name, then we
 374   //       update the existing child policy.
 375   //    b. If child_policy_->name() does not equal child_policy_name,
 376   //       we create a new policy.  The policy will be stored in
 377   //       pending_child_policy_ and will later be swapped into
 378   //       child_policy_ by the helper when the new child transitions
 379   //       into state READY.
 380   //
 381   // 3. We have an existing child policy and have a pending child policy
 382   //    from a previous update (i.e., a previous update set
 383   //    pending_child_policy_ as per case 2b above and that policy has
 384   //    not yet transitioned into state READY and been swapped into
 385   //    child_policy_; in this case, both child_policy_ and
 386   //    pending_child_policy_ are non-null).  In this case:
 387   //    a. If pending_child_policy_->name() equals child_policy_name,
 388   //       then we update the existing pending child policy.
 389   //    b. If pending_child_policy->name() does not equal
 390   //       child_policy_name, then we create a new policy.  The new
 391   //       policy is stored in pending_child_policy_ (replacing the one
 392   //       that was there before, which will be immediately shut down)
 393   //       and will later be swapped into child_policy_ by the helper
 394   //       when the new child transitions into state READY.
 395   const bool create_policy =
 396       // case 1
 397       lb_policy_ == nullptr ||
 398       // case 2b
 399       (pending_lb_policy_ == nullptr &&
 400        strcmp(lb_policy_->name(), lb_policy_name) != 0) ||
 401       // case 3b
 402       (pending_lb_policy_ != nullptr &&
 403        strcmp(pending_lb_policy_->name(), lb_policy_name) != 0);
 404   LoadBalancingPolicy* policy_to_update = nullptr;
 405   if (create_policy) {
 406     // Cases 1, 2b, and 3b: create a new child policy.
 407     // If lb_policy_ is null, we set it (case 1), else we set
 408     // pending_lb_policy_ (cases 2b and 3b).
 409     if (tracer_->enabled()) {
 410       gpr_log(GPR_INFO, "resolving_lb=%p: Creating new %schild policy %s", this,
 411               lb_policy_ == nullptr ? "" : "pending ", lb_policy_name);
 412     }
 413     auto new_policy =
 414         CreateLbPolicyLocked(lb_policy_name, *result.args, trace_strings);
 415     auto& lb_policy = lb_policy_ == nullptr ? lb_policy_ : pending_lb_policy_;
 416     {
 417       MutexLock lock(&lb_policy_mu_);
 418       lb_policy = std::move(new_policy);
 419     }
 420     policy_to_update = lb_policy.get();
 421   } else {
 422     // Cases 2a and 3a: update an existing policy.
 423     // If we have a pending child policy, send the update to the pending
 424     // policy (case 3a), else send it to the current policy (case 2a).
 425     policy_to_update = pending_lb_policy_ != nullptr ? pending_lb_policy_.get()
 426                                                      : lb_policy_.get();
 427   }
 428   GPR_ASSERT(policy_to_update != nullptr);
 429   // Update the policy.
 430   if (tracer_->enabled()) {
 431     gpr_log(GPR_INFO, "resolving_lb=%p: Updating %schild policy %p", this,
 432             policy_to_update == pending_lb_policy_.get() ? "pending " : "",
 433             policy_to_update);
 434   }
 435   UpdateArgs update_args;
 436   update_args.addresses = std::move(result.addresses);
 437   update_args.config = std::move(lb_policy_config);
 438   // TODO(roth): Once channel args is converted to C++, use std::move() here.
 439   update_args.args = result.args;
 440   result.args = nullptr;
 441   policy_to_update->UpdateLocked(std::move(update_args));
 442 }
 443
 444 // Creates a new LB policy.
 445 // Updates trace_strings to indicate what was done.
 446 OrphanablePtr<LoadBalancingPolicy>
 447 ResolvingLoadBalancingPolicy::CreateLbPolicyLocked(
 448     const char* lb_policy_name, const grpc_channel_args& args,
 449     TraceStringVector* trace_strings) {
 450   ResolvingControlHelper* helper = New<ResolvingControlHelper>(Ref());
 451   LoadBalancingPolicy::Args lb_policy_args;
 452   lb_policy_args.combiner = combiner();
 453   lb_policy_args.channel_control_helper =
 454       UniquePtr<ChannelControlHelper>(helper);
 455   lb_policy_args.args = &args;
 456   OrphanablePtr<LoadBalancingPolicy> lb_policy =
 457       LoadBalancingPolicyRegistry::CreateLoadBalancingPolicy(
 458           lb_policy_name, std::move(lb_policy_args));
 459   if (GPR_UNLIKELY(lb_policy == nullptr)) {
 460     gpr_log(GPR_ERROR, "could not create LB policy \"%s\"", lb_policy_name);
 461     if (channelz_node() != nullptr) {
 462       char* str;
 463       gpr_asprintf(&str, "Could not create LB policy \"%s\"", lb_policy_name);
 464       trace_strings->push_back(str);
 465     }
 466     return nullptr;
 467   }
 468   helper->set_child(lb_policy.get());
 469   if (tracer_->enabled()) {
 470     gpr_log(GPR_INFO, "resolving_lb=%p: created new LB policy \"%s\" (%p)",
 471             this, lb_policy_name, lb_policy.get());
 472   }
 473   if (channelz_node() != nullptr) {
 474     char* str;
 475     gpr_asprintf(&str, "Created new LB policy \"%s\"", lb_policy_name);
 476     trace_strings->push_back(str);
 477   }
 478   // Propagate channelz node.
 479   auto* channelz = channelz_node();
 480   if (channelz != nullptr) {
 481     lb_policy->set_channelz_node(channelz->Ref());
 482   }
 483   grpc_pollset_set_add_pollset_set(lb_policy->interested_parties(),
 484                                    interested_parties());
 485   return lb_policy;
 486 }
 487
 488 void ResolvingLoadBalancingPolicy::MaybeAddTraceMessagesForAddressChangesLocked(
 489     bool resolution_contains_addresses, TraceStringVector* trace_strings) {
 490   if (!resolution_contains_addresses &&
 491       previous_resolution_contained_addresses_) {
 492     trace_strings->push_back(gpr_strdup("Address list became empty"));
 493   } else if (resolution_contains_addresses &&
 494              !previous_resolution_contained_addresses_) {
 495     trace_strings->push_back(gpr_strdup("Address list became non-empty"));
 496   }
 497   previous_resolution_contained_addresses_ = resolution_contains_addresses;
 498 }
 499
 500 void ResolvingLoadBalancingPolicy::ConcatenateAndAddChannelTraceLocked(
 501     TraceStringVector* trace_strings) const {
 502   if (!trace_strings->empty()) {
 503     gpr_strvec v;
 504     gpr_strvec_init(&v);
 505     gpr_strvec_add(&v, gpr_strdup("Resolution event: "));
 506     bool is_first = 1;
 507     for (size_t i = 0; i < trace_strings->size(); ++i) {
 508       if (!is_first) gpr_strvec_add(&v, gpr_strdup(", "));
 509       is_first = false;
 510       gpr_strvec_add(&v, (*trace_strings)[i]);
 511     }
 512     char* flat;
 513     size_t flat_len = 0;
 514     flat = gpr_strvec_flatten(&v, &flat_len);
 515     channelz_node()->AddTraceEvent(channelz::ChannelTrace::Severity::Info,
 516                                    grpc_slice_new(flat, flat_len, gpr_free));
 517     gpr_strvec_destroy(&v);
 518   }
 519 }
 520
 521 void ResolvingLoadBalancingPolicy::OnResolverResultChangedLocked(
 522     Resolver::Result result) {
 523   // Handle race conditions.
 524   if (resolver_ == nullptr) return;
 525   if (tracer_->enabled()) {
 526     gpr_log(GPR_INFO, "resolving_lb=%p: got resolver result", this);
 527   }
 528   // We only want to trace the address resolution in the follow cases:
 529   // (a) Address resolution resulted in service config change.
 530   // (b) Address resolution that causes number of backends to go from
 531   //     zero to non-zero.
 532   // (c) Address resolution that causes number of backends to go from
 533   //     non-zero to zero.
 534   // (d) Address resolution that causes a new LB policy to be created.
 535   //
 536   // We track a list of strings to eventually be concatenated and traced.
 537   TraceStringVector trace_strings;
 538   const bool resolution_contains_addresses = result.addresses.size() > 0;
 539   // Process the resolver result.
 540   const char* lb_policy_name = nullptr;
 541   RefCountedPtr<Config> lb_policy_config;
 542   bool service_config_changed = false;
 543   if (process_resolver_result_ != nullptr) {
 544     service_config_changed =
 545         process_resolver_result_(process_resolver_result_user_data_, &result,
 546                                  &lb_policy_name, &lb_policy_config);
 547   } else {
 548     lb_policy_name = child_policy_name_.get();
 549     lb_policy_config = child_lb_config_;
 550   }
 551   GPR_ASSERT(lb_policy_name != nullptr);
 552   // Create or update LB policy, as needed.
 553   CreateOrUpdateLbPolicyLocked(lb_policy_name, std::move(lb_policy_config),
 554                                std::move(result), &trace_strings);
 555   // Add channel trace event.
 556   if (channelz_node() != nullptr) {
 557     if (service_config_changed) {
 558       // TODO(ncteisen): might be worth somehow including a snippet of the
 559       // config in the trace, at the risk of bloating the trace logs.
 560       trace_strings.push_back(gpr_strdup("Service config changed"));
 561     }
 562     MaybeAddTraceMessagesForAddressChangesLocked(resolution_contains_addresses,
 563                                                  &trace_strings);
 564     ConcatenateAndAddChannelTraceLocked(&trace_strings);
 565   }
 566 }
 567
 568 }  // namespace grpc_core