1 # Chrome Network Stack Common Coding Patterns
3 ## Combined error and byte count into a single value
5 At many places in the network stack, functions return a value that, if
6 positive, indicate a count of bytes that the the function read or
7 wrote, and if negative, indicates a network stack error code (see
9 Zero indicates either `net::OK` or zero bytes read (usually EOF)
10 depending on the context. This pattern is generally specified by
13 Many functions also have variables (often named `result` or `rv`) containing
14 such a value; this is especially common in the [DoLoop](#DoLoop) pattern
19 Many network stack routines may return synchronously or
20 asynchronously. These functions generally return an int as described
21 above. There are three cases:
23 * If the value is positive or zero, that indicates a synchronous
24 successful return, with a zero return value indicating either zero
25 bytes/EOF or indicating `net::OK`, depending on context.
26 * If the value is negative and != `net::ERR_IO_PENDING`, it is an error
27 code specifying a synchronous failure.
28 * If the return value is the special value `net::ERR_IO_PENDING`, it
29 indicates that the routine will complete asynchronously. A reference to
30 any provided IOBuffer will be retained by the called entity until
31 completion, to be written into or read from as required.
32 If there is a callback argument, that callback will be called upon
33 completion with the return value; if there is no callback argument, it
34 usually means that some known callback mechanism will be employed.
38 The DoLoop pattern is used in the network stack to construct simple
39 state machines. It is used for cases in which processing is basically
40 single-threaded and could be written in a single function, if that
41 function could block waiting for input. Generally, initiation of a
42 state machine is triggered by some method invocation by a class
43 consumer, and that state machine is driven (possibly across
44 asynchronous IO initiated by the class) until the operation requested
45 by the method invocation completes, at which point the state variable is
46 set to `STATE_NONE` and the consumer notified.
48 Cases which do not fit into this single-threaded, single consumer
49 operation model are generally adapted in some way to fit the model,
50 either by multiple state machines (e.g. independent state machines for
51 reading and writing, if each can be initiated while the other is
52 outstanding) or by storing information across consumer invocations and
53 returns that can be used to restart the state machine in the proper
56 Any class using this pattern will contain an enum listing all states
57 of that machine, and define a function, `DoLoop()`, to drive that state
58 machine. If a class has multiple state machines (as above) it will
59 have multiple methods (e.g. `DoReadLoop()` and `DoWriteLoop()`) to drive
60 those different machines.
62 The characteristics of the DoLoop pattern are:
64 * Each state has a corresponding function which is called by `DoLoop()`
65 for handling when the state machine is in that state. Generally the
66 states are named STATE`_<`STATE_NAME`>` (upper case separated by
67 underscores), and the routine is named Do`<`StateName`>` (CamelCase).
78 int DoFooComplete(int result);
80 * Each state handling function has two basic responsibilities in
81 addition to state specific handling: Setting the data member
82 (named `next_state_` or something similar)
83 to specify the next state, and returning a `net::Error` (or combined
84 error and byte count, as above).
86 * On each `DoLoop()` iteration, the function saves the next state to a local
87 variable and resets to a default state (`STATE_NONE`),
88 and then calls the appropriate state handling based on the
89 original value of the next state. This looks like:
92 State state = io_state_;
93 next_state_ = STATE_NONE;
100 This pattern is followed primarily to ensure that in the event of
101 a bug where the next state isn't set, the loop terminates rather
102 than loops infinitely. It's not a perfect mitigation, but works
103 well as a defensive measure.
105 * If a given state may complete asynchronously (for example,
106 writing to an underlying transport socket), then there will often
107 be split states, such as `STATE_WRITE` and
108 `STATE_WRITE_COMPLETE`. The first state is responsible for
109 starting/continuing the original operation, while the second state
110 is responsible for handling completion (e.g. success vs error,
111 complete vs. incomplete writes), and determining the next state to
114 * While the return value from each call is propagated through the loop
115 to the next state, it is expected that for most state transitions the
116 return value will be `net::OK`, and that an error return will also
117 set the state to `STATE_NONE` or fail to override the default
118 assignment to `STATE_DONE` to exit the loop and return that
119 error to the caller. This is often asserted with a DCHECK, e.g.
122 DCHECK_EQ(result, OK);
126 The exception to this pattern is split states, where an IO
127 operation has been dispatched, and the second state is handling
128 the result. In that case, the second state's function takes the
131 case STATE_FOO_COMPLETE:
132 result = DoFooComplete(result);
135 * If the return value from the state handling function is
136 `net::ERR_IO_PENDING`, that indicates that the function has arranged
137 for `DoLoop()` to be called at some point in the future, when further
138 progress can be made on the state transitions. The `next_state_` variable
139 will have been set to the proper value for handling that incoming
140 call. In this case, `DoLoop()` will exit. This often occurs between
141 split states, as described above.
143 * The DoLoop mechanism is generally invoked in response to a consumer
144 calling one of its methods. While the operation that method
145 requested is occuring, the state machine stays active, possibly
146 over multiple asynchronous operations and state transitions. When
147 that operation is complete, the state machine transitions to
148 `STATE_NONE` (by a `DoLoop()` callee not setting `next_state_`) or
149 explicitly to `STATE_DONE` (indicating that the operation is
150 complete *and* the state machine is not amenable to further
151 driving). At this point the consumer is notified of the completion
152 of the operation (by synchronous return or asynchronous callback).
154 Note that this implies that when `DoLoop()` returns, one of two
157 * The return value will be `net::ERR_IO_PENDING`, indicating that the
158 caller should take no action and instead wait for asynchronous
160 * The state of the machine will be either `STATE_DONE` or `STATE_NONE`,
161 indicating that the operation that first initiated the `DoLoop()` has
164 This invariant reflects and enforces the single-threaded (though
165 possibly asynchronous) nature of the driven state machine--the
166 machine is always executing one requested operation.
168 * `DoLoop()` is called from two places: a) methods exposed to the consumer
169 for specific operations (e.g. `ReadHeaders()`), and b) an IO completion
170 callbacks called asynchronously by spawned IO operations.
172 In the first case, the return value from `DoLoop()` is returned directly
173 to the caller; if the operation completed synchronously, that will
174 contain the operation result, and if it completed asynchronously, it
175 will be `net::ERR_IO_PENDING`. For example (from
176 `HttpStreamParser`, abridged for clarity):
178 int HttpStreamParser::ReadResponseHeaders(
179 CompletionOnceCallback callback) {
180 DCHECK(io_state_ == STATE_NONE || io_state_ == STATE_DONE);
181 DCHECK(callback_.is_null());
182 DCHECK(!callback.is_null());
185 io_state_ = STATE_READ_HEADERS;
187 result = DoLoop(result);
189 if (result == ERR_IO_PENDING)
190 callback_ = std::move(callback);
192 return result > 0 ? OK : result;
195 In the second case, the IO completion callback will examine the
196 return value from `DoLoop()`. If it is `net::ERR_IO_PENDING`, no
197 further action will be taken, and the IO completion callback will be
198 called again at some future point. If it is not
199 `net::ERR_IO_PENDING`, that is a signal that the operation has
200 completed, and the IO completion callback will call the appropriate
201 consumer callback to notify the consumer that the operation has
202 completed. Note that it is important that this callback be done
203 from the IO completion callback and not from `DoLoop()` or a
204 `DoLoop()` callee, both to support the sync/async error return
205 (DoLoop and its callees don't know the difference) and to avoid
206 consumer callbacks deleting the object out from under `DoLoop()`.
209 void HttpStreamParser::OnIOComplete(int result) {
210 result = DoLoop(result);
212 if (result != ERR_IO_PENDING && !callback_.is_null())
213 std::move(callback_).Run(result);
216 * The DoLoop pattern has no concept of different events arriving for
217 a single state; each state, if waiting, is waiting for one
218 particular event, and when `DoLoop()` is invoked when the machine is
219 in that state, it will handle that event. This reflects the
220 single-threaded model for operations spawned by the state machine.
222 Public class methods generally have very little processing, primarily wrapping
223 `DoLoop()`. For `DoLoop()` entry this involves setting the `next_state_`
224 variable, and possibly making copies of arguments into class members. For
225 `DoLoop()` exit, it involves inspecting the return and passing it back to
226 the caller, and in the asynchronous case, saving any passed completion callback
227 for executing by a future subsidiary IO completion (see above example).
229 This idiom allows synchronous and asynchronous logic to be written in
230 the same fashion; it's all just state transition handling. For mostly
231 linear state diagrams, the handling code can be very easy to
232 comprehend, as such code is usually written linearly (in different
233 handling functions) in the order it's executed.
235 For examples of this idiom, see
237 * [HttpStreamParser::DoLoop](https://source.chromium.org/chromium/chromium/src/+/HEAD:net/http/http_stream_parser.cc).
238 * [HttpNetworkTransaction::DoLoop](https://source.chromium.org/chromium/chromium/src/+/HEAD:net/http/http_network_transaction.cc)
240 [net_error_list.h]: https://chromium.googlesource.com/chromium/src/+/main/net/base/net_error_list.h#1