stream.js

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- # stream.js

- ¶

      var assert = require('assert');

- ¶

  # The Stream class

- ¶

  Stream is a Duplex stream subclass that implements the HTTP/2 Stream concept. It has two 'sides': one that is used by the user to send/receive data (the `stream` object itself) and one that is used by a Connection to read/write frames to/from the other peer (`stream.upstream`).

      var Duplex = require('stream').Duplex;

      exports.Stream = Stream;

- ¶

  ## Public API

- ¶

  - new Stream(log): create a new Stream

  - Event: 'headers' (headers): signals incoming headers

  - Event: 'promise' (stream, headers): signals an incoming push promise

  - Event: 'priority' (priority): signals a priority change. `priority` is a number between 0 (highest priority) and 2^31-1 (lowest priority). Default value is 2^30.

  - Event: 'error' (type): signals an error

  - headers(headers): send headers

  - promise(headers): Stream: promise a stream

  - priority(priority): set the priority of the stream. Priority can be changed by the peer too, but once it is set locally, it can not be changed remotely.

  - reset(error): reset the stream with an error code

  - upstream: a Flow that is used by the parent connection to write/read frames that are to be sent/arrived to/from the peer and are related to this stream.

  Headers are always in the regular node.js header format.

  ## Constructor

- ¶

  The main aspects of managing the stream are:

      function Stream(log) {
        Duplex.call(this);

- ¶
  - logging

        this._log = log.child({ component: 'stream', s: this });

- ¶
  - receiving and sending stream management commands

        this._initializeManagement();

- ¶
  - sending and receiving frames to/from the upstream connection

        this._initializeDataFlow();

- ¶
  - maintaining the state of the stream (idle, open, closed, etc.) and error detection

        this._initializeState();
      }

      Stream.prototype = Object.create(Duplex.prototype, { constructor: { value: Stream } });

- ¶

  ## Managing the stream

- ¶

  the default stream priority is 2^30

      var DEFAULT_PRIORITY = Math.pow(2, 30);
      var MAX_PRIORITY = Math.pow(2, 31) - 1;

- ¶

  PUSH_PROMISE and HEADERS are forwarded to the user through events.

      Stream.prototype._initializeManagement = function _initializeManagement() {
        this._resetSent = false;
        this._priority = DEFAULT_PRIORITY;
        this._letPeerPrioritize = true;
      };

      Stream.prototype.promise = function promise(headers) {
        var stream = new Stream(this._log);
        stream._priority = Math.min(this._priority + 1, MAX_PRIORITY);
        this._pushUpstream({
          type: 'PUSH_PROMISE',
          flags: {},
          stream: this.id,
          promised_stream: stream,
          headers: headers
        });
        return stream;
      };

      Stream.prototype._onPromise = function _onPromise(frame) {
        this.emit('promise', frame.promised_stream, frame.headers);
      };

      Stream.prototype.headers = function headers(headers) {
        this._pushUpstream({
          type: 'HEADERS',
          flags: {},
          stream: this.id,
          headers: headers
        });
      };

      Stream.prototype._onHeaders = function _onHeaders(frame) {
        if (frame.priority !== undefined) {
          this.priority(frame.priority, true);
        }
        this.emit('headers', frame.headers);
      };

      Stream.prototype.priority = function priority(priority, peer) {
        if ((peer && this._letPeerPrioritize) || !peer) {
          if (!peer) {
            this._letPeerPrioritize = false;

            var lastFrame = this.upstream.getLastQueuedFrame();
            if (lastFrame && ((lastFrame.type === 'HEADERS') || (lastFrame.type === 'PRIORITY'))) {
              lastFrame.priority = priority;
            } else {
              this._pushUpstream({
                type: 'PRIORITY',
                flags: {},
                stream: this.id,
                priority: priority
              });
            }
          }

          this._log.debug({ priority: priority }, 'Changing priority');
          this.emit('priority', priority);
          this._priority = priority;
        }
      };

      Stream.prototype._onPriority = function _onPriority(frame) {
        this.priority(frame.priority, true);
      };

- ¶

  Resetting the stream. Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream.

      Stream.prototype.reset = function reset(error) {
        if (!this._resetSent) {
          this._resetSent = true;
          this._pushUpstream({
            type: 'RST_STREAM',
            flags: {},
            stream: this.id,
            error: error
          });
        }
      };

- ¶

  ## Data flow

- ¶

  The incoming and the generated outgoing frames are received/transmitted on the `this.upstream` Flow. The Connection object instantiating the stream will read and write frames to/from it. The stream itself is a regular Duplex stream, and is used by the user to write or read the body of the request.

      upstream side                  stream                  user side

                     +------------------------------------+
                     |                                    |
                     +------------------+                 |
                     |     upstream     |                 |
                     |                  |                 |
                     +--+               |              +--|
             read()  |  |  _send()      |    _write()  |  |  write(buf)
      <--------------|B |<--------------|--------------| B|<------------
                     |  |               |              |  |
             frames  +--+               |              +--|  buffers
                     |  |               |              |  |
      -------------->|B |---------------|------------->| B|------------>
       write(frame)  |  |  _receive()   |     _read()  |  |  read()
                     +--+               |              +--|
                     |                  |                 |
                     |                  |                 |
                     +------------------+                 |
                     |                                    |
                     +------------------------------------+

      B: input or output buffer

      var Flow = require('./flow').Flow;

      Stream.prototype._initializeDataFlow = function _initializeDataFlow() {
        this.id = undefined;

        this._ended = false;

        this.upstream = new Flow();
        this.upstream._log = this._log;
        this.upstream._send = this._send.bind(this);
        this.upstream._receive = this._receive.bind(this);
        this.upstream.write = this._writeUpstream.bind(this);
        this.upstream.on('error', this.emit.bind(this, 'error'));

        this.on('finish', this._finishing);
      };

      Stream.prototype._pushUpstream = function _pushUpstream(frame) {
        this.upstream.push(frame);
        this._transition(true, frame);
      };

- ¶

  Overriding the upstream's `write` allows us to act immediately instead of waiting for the input queue to empty. This is important in case of control frames.

      Stream.prototype._writeUpstream = function _writeUpstream(frame) {
        this._log.debug({ frame: frame }, 'Receiving frame');

        var moreNeeded = Flow.prototype.write.call(this.upstream, frame);

- ¶
  - Transition to a new state if that's the effect of receiving the frame

        this._transition(false, frame);

- ¶
  - If it's a control frame. Call the appropriate handler method.

        if (frame.type === 'HEADERS') {
          this._onHeaders(frame);
        } else if (frame.type === 'PUSH_PROMISE') {
          this._onPromise(frame);
        } else if (frame.type === 'PRIORITY') {
          this._onPriority(frame);
        }

- ¶
  - If it's an invalid stream level frame, emit error

        else if ((frame.type !== 'DATA') &&
                 (frame.type !== 'WINDOW_UPDATE') &&
                 (frame.type !== 'RST_STREAM')) {
          this._log.error({ frame: frame }, 'Invalid stream level frame');
          this.emit('error', 'PROTOCOL_ERROR');
        }

        return moreNeeded;
      };

- ¶

  The `_receive` method (= `upstream._receive`) gets called when there's an incoming frame.

      Stream.prototype._receive = function _receive(frame, ready) {

- ¶
  - If it's a DATA frame, then push the payload into the output buffer on the other side. Call ready when the other side is ready to receive more.

        if (!this._ended && (frame.type === 'DATA')) {
          var moreNeeded = this.push(frame.data);
          if (!moreNeeded) {
            this._receiveMore = ready;
          }
        }

- ¶
  - Any frame may signal the end of the stream with the END_STREAM flag

        if (!this._ended && (frame.flags.END_STREAM || (frame.type === 'RST_STREAM'))) {
          this.push(null);
          this._ended = true;
        }

- ¶
  - Postpone calling `ready` if `push()` returned a falsy value

        if (this._receiveMore !== ready) {
          ready();
        }
      };

- ¶

  The `_read` method is called when the user side is ready to receive more data. If there's a pending write on the upstream, then call its pending ready callback to receive more frames.

      Stream.prototype._read = function _read() {
        if (this._receiveMore) {
          var receiveMore = this._receiveMore;
          delete this._receiveMore;
          receiveMore();
        }
      };

- ¶

  The `write` method gets called when there's a write request from the user.

      Stream.prototype._write = function _write(buffer, encoding, ready) {

- ¶
  - Chunking is done by the upstream Flow.

        var moreNeeded = this._pushUpstream({
          type: 'DATA',
          flags: {},
          stream: this.id,
          data: buffer
        });

- ¶
  - Call ready when upstream is ready to receive more frames.

        if (moreNeeded) {
          ready();
        } else {
          this._sendMore = ready;
        }
      };

- ¶

  The `_send` (= `upstream._send`) method is called when upstream is ready to receive more frames. If there's a pending write on the user side, then call its pending ready callback to receive more writes.

      Stream.prototype._send = function _send() {
        if (this._sendMore) {
          var sendMore = this._sendMore;
          delete this._sendMore;
          sendMore();
        }
      };

- ¶

  When the stream is finishing (the user calls `end()` on it), then we have to set the `END_STREAM` flag on the last frame. If there's no frame in the queue, or if it doesn't support this flag, then we create a 0 length DATA frame. We could do this all the time, but putting the flag on an existing frame is a nice optimization.

      var emptyBuffer = new Buffer(0);
      Stream.prototype._finishing = function _finishing() {
        var endFrame = {
          type: 'DATA',
          flags: { END_STREAM: true },
          stream: this.id,
          data: emptyBuffer
        };
        var lastFrame = this.upstream.getLastQueuedFrame();
        if (lastFrame && ((lastFrame.type === 'DATA') || (lastFrame.type === 'HEADERS'))) {
          this._log.debug({ frame: lastFrame }, 'Marking last frame with END_STREAM flag.');
          lastFrame.flags.END_STREAM = true;
          this._transition(true, endFrame);
        } else {
          this._pushUpstream(endFrame);
        }
      };

- ¶

  ## Stream States

- ¶

                            +--------+
                      PP    |        |    PP
                   ,--------|  idle  |--------.
                  /         |        |         \
                 v          +--------+          v
          +----------+          |           +----------+
          |          |          | H         |          |
      ,---| reserved |          |           | reserved |---.
      |   | (local)  |          v           | (remote) |   |
      |   +----------+      +--------+      +----------+   |
      |      |          ES  |        |  ES          |      |
      |      | H    ,-------|  open  |-------.      | H    |
      |      |     /        |        |        \     |      |
      |      v    v         +--------+         v    v      |
      |   +----------+          |           +----------+   |
      |   |   half   |          |           |   half   |   |
      |   |  closed  |          | R         |  closed  |   |
      |   | (remote) |          |           | (local)  |   |
      |   +----------+          |           +----------+   |
      |        |                v                 |        |
      |        |  ES / R    +--------+  ES / R    |        |
      |        `----------->|        |<-----------'        |
      |  R                  | closed |                  R  |
      `-------------------->|        |<--------------------'
                            +--------+

  Streams begin in the IDLE state and transitions happen when there's an incoming or outgoing frame

      Stream.prototype._initializeState = function _initializeState() {
        this.state = 'IDLE';
        this._initiated = undefined;
        this._closedByUs = undefined;
        this._closedWithRst = undefined;
      };

- ¶

  Only `_setState` should change `this.state` directly. It also logs the state change and notifies interested parties using the 'state' event.

      Stream.prototype._setState = function transition(state) {
        assert(this.state !== state);
        this._log.debug({ from: this.state, to: state }, 'State transition');
        this.state = state;
        this.emit('state', state);
      };

- ¶

  A state is 'active' if the stream in that state counts towards the concurrency limit. Streams that are in the "open" state, or either of the "half closed" states count toward this limit.

      function activeState(state) {
        return ((state === 'HALF_CLOSED_LOCAL') || (state === 'HALF_CLOSED_REMOTE') || (state === 'OPEN'));
      }

- ¶

  `_transition` is called every time there's an incoming or outgoing frame. It manages state transitions, and detects stream errors. A stream error is always caused by a frame that is not allowed in the current state.

      Stream.prototype._transition = function transition(sending, frame) {
        var receiving = !sending;
        var error = undefined;

        var DATA = false, HEADERS = false, PRIORITY = false;
        var RST_STREAM = false, PUSH_PROMISE = false, WINDOW_UPDATE = false;
        switch(frame.type) {
          case 'DATA'         : DATA          = true; break;
          case 'HEADERS'      : HEADERS       = true; break;
          case 'PRIORITY'     : PRIORITY      = true; break;
          case 'RST_STREAM'   : RST_STREAM    = true; break;
          case 'PUSH_PROMISE' : PUSH_PROMISE  = true; break;
          case 'WINDOW_UPDATE': WINDOW_UPDATE = true; break;
        }

        var previousState = this.state;

        switch (this.state) {

- ¶

  All streams start in the idle state. In this state, no frames have been exchanged.

  - Sending or receiving a HEADERS frame causes the stream to become "open".

  When the HEADERS frame contains the END_STREAM flags, then two state transitions happen.

          case 'IDLE':
            if (HEADERS) {
              this._setState('OPEN');
              if (frame.flags.END_STREAM) {
                this._setState(sending ? 'HALF_CLOSED_LOCAL' : 'HALF_CLOSED_REMOTE');
              }
              this._initiated = sending;
            } else if (sending && RST_STREAM) {
              this._setState('CLOSED');
            } else {
              error = 'PROTOCOL_ERROR';
            }
            break;

- ¶

  A stream in the reserved (local) state is one that has been promised by sending a PUSH_PROMISE frame.

  - The endpoint can send a HEADERS frame. This causes the stream to open in a "half closed (remote)" state.
  - Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.
  - An endpoint may receive PRIORITY frame in this state.
  - An endpoint MUST NOT send any other type of frame in this state.

          case 'RESERVED_LOCAL':
            if (sending && HEADERS) {
              this._setState('HALF_CLOSED_REMOTE');
            } else if (RST_STREAM) {
              this._setState('CLOSED');
            } else if (receiving && PRIORITY) {
              /* No state change */
            } else {
              error = 'PROTOCOL_ERROR';
            }
            break;

- ¶

  A stream in the reserved (remote) state has been reserved by a remote peer.

  - Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.
  - Receiving a HEADERS frame causes the stream to transition to "half closed (local)".
  - An endpoint MAY send PRIORITY frames in this state to reprioritize the stream.
  - Receiving any other type of frame MUST be treated as a stream error of type PROTOCOL_ERROR.

          case 'RESERVED_REMOTE':
            if (RST_STREAM) {
              this._setState('CLOSED');
            } else if (receiving && HEADERS) {
              this._setState('HALF_CLOSED_LOCAL');
            } else if (sending && PRIORITY) {
              /* No state change */
            } else {
              error = 'PROTOCOL_ERROR';
            }
            break;

- ¶

  The open state is where both peers can send frames. In this state, sending peers observe advertised stream level flow control limits.

  - From this state either endpoint can send a frame with a END_STREAM flag set, which causes the stream to transition into one of the "half closed" states: an endpoint sending a END_STREAM flag causes the stream state to become "half closed (local)"; an endpoint receiving a END_STREAM flag causes the stream state to become "half closed (remote)".
  - Either endpoint can send a RST_STREAM frame from this state, causing it to transition immediately to "closed".

          case 'OPEN':
            if (frame.flags.END_STREAM) {
              this._setState(sending ? 'HALF_CLOSED_LOCAL' : 'HALF_CLOSED_REMOTE');
            } else if (RST_STREAM) {
              this._setState('CLOSED');
            } else {
              /* No state change */
            }
            break;

- ¶

  A stream that is half closed (local) cannot be used for sending frames.

  - A stream transitions from this state to "closed" when a frame that contains a END_STREAM flag is received, or when either peer sends a RST_STREAM frame.
  - An endpoint MAY send or receive PRIORITY frames in this state to reprioritize the stream.
  - WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag.

          case 'HALF_CLOSED_LOCAL':
            if (RST_STREAM || (receiving && frame.flags.END_STREAM)) {
              this._setState('CLOSED');
            } else if (receiving || (sending && (PRIORITY || WINDOW_UPDATE))) {
              /* No state change */
            } else {
              error = 'PROTOCOL_ERROR';
            }
            break;

- ¶

  A stream that is half closed (remote) is no longer being used by the peer to send frames. In this state, an endpoint is no longer obligated to maintain a receiver flow control window if it performs flow control.

  - If an endpoint receives additional frames for a stream that is in this state it MUST respond with a stream error of type STREAM_CLOSED.
  - A stream can transition from this state to "closed" by sending a frame that contains a END_STREAM flag, or when either peer sends a RST_STREAM frame.
  - An endpoint MAY send or receive PRIORITY frames in this state to reprioritize the stream.
  - A receiver MAY receive a WINDOW_UPDATE frame on a "half closed (remote)" stream.

          case 'HALF_CLOSED_REMOTE':
            if (RST_STREAM || (sending && frame.flags.END_STREAM)) {
              this._setState('CLOSED');
            } else if (sending || (receiving && (WINDOW_UPDATE || PRIORITY))) {
              /* No state change */
            } else {
              error = 'PROTOCOL_ERROR';
            }
            break;

- ¶

  The closed state is the terminal state.

  - An endpoint MUST NOT send frames on a closed stream. An endpoint that receives a frame after receiving a RST_STREAM or a frame containing a END_STREAM flag on that stream MUST treat that as a stream error of type STREAM_CLOSED.
  - WINDOW_UPDATE, PRIORITY or RST_STREAM frames can be received in this state for a short period after a frame containing an END_STREAM flag is sent. Until the remote peer receives and processes the frame bearing the END_STREAM flag, it might send either frame type. Endpoints MUST ignore WINDOW_UPDATE frames received in this state, though endpoints MAY choose to treat WINDOW_UPDATE frames that arrive a significant time after sending END_STREAM as a connection error of type PROTOCOL_ERROR.
  - If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAM might have already sent - or enqueued for sending - frames on the stream that cannot be withdrawn. An endpoint that sends a RST_STREAM frame MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. An endpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after this time as being in error.
  - An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causes a stream to become "reserved". If promised streams are not desired, a RST_STREAM can be used to close any of those streams.

          case 'CLOSED':
            if ((sending && RST_STREAM) ||
                (receiving && this._closedByUs &&
                 (this._closedWithRst || WINDOW_UPDATE || PRIORITY || RST_STREAM))) {
              /* No state change */
            } else {
              error = 'STREAM_CLOSED';
            }
            break;
        }

- ¶

  Noting that the connection was closed by the other endpoint. It may be important in edge cases. For example, when the peer tries to cancel a promised stream, but we already sent every data on it, then the stream is in CLOSED state, yet we want to ignore the incoming RST_STREAM.

        if ((this.state === 'CLOSED') && (previousState !== 'CLOSED')) {
          this._closedByUs = sending;
          this._closedWithRst = RST_STREAM;
        }

- ¶

  Sending/receiving a PUSH_PROMISE

  - Sending a PUSH_PROMISE frame marks the associated stream for later use. The stream state for the reserved stream transitions to "reserved (local)".
  - Receiving a PUSH_PROMISE frame marks the associated stream as reserved by the remote peer. The state of the stream becomes "reserved (remote)".

        if (PUSH_PROMISE && !error) {
          /* This assertion must hold, because _transition is called immediately when a frame is written
             to the stream. If it would be called when a frame gets out of the input queue, the state
             of the reserved could have been changed by then. */
          assert(frame.promised_stream.state === 'IDLE', frame.promised_stream.state);
          frame.promised_stream._setState(sending ? 'RESERVED_LOCAL' : 'RESERVED_REMOTE');
          frame.promised_stream._initiated = sending;
        }

- ¶

  Signaling how sending/receiving this frame changes the active stream count (-1, 0 or +1)

        if (this._initiated) {
          var change = (activeState(this.state) - activeState(previousState));
          if (sending) {
            frame.count_change = change;
          } else {
            frame.count_change(change);
          }
        } else if (sending) {
          frame.count_change = 0;
        }

- ¶

  Common error handling.

        if (error) {
          var info = {
            error: error,
            frame: frame,
            state: this.state,
            closedByUs: this._closedByUs,
            closedWithRst: this._closedWithRst
          };

- ¶
  - When sending something invalid, throwing an exception, since it is probably a bug.

          if (sending) {
            this._log.error(info, 'Sending illegal frame.');
            throw new Error('Sending illegal frame (' + frame.type + ') in ' + this.state + ' state.');
          }

- ¶
  - When receiving something invalid, sending an RST_STREAM using the `reset` method. This will automatically cause a transition to the CLOSED state.

          else {
            this._log.error(info, 'Received illegal frame.');
            this.emit('error', error);
          }
        }
      };

- ¶

  ## Bunyan serializers

- ¶

      exports.serializers = {};

      var nextId = 0;
      exports.serializers.s = function(stream) {
        if (!('_id' in stream)) {
          stream._id = nextId;
          nextId += 1;
        }
        return stream._id;
      };