# all files / lib/protocol/ flow.js

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     var assert = require('assert');
    var timers = require('timers');

    // The Flow class
    // ==============

    // Flow is a [Duplex stream][1] subclass which implements HTTP/2 flow control. It is designed to be
    // subclassed by [Connection](connection.html) and the `upstream` component of [Stream](stream.html).
    // [1]: https://nodejs.org/api/stream.html#stream_class_stream_duplex

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

    exports.Flow = Flow;

    // Public API
    // ----------

    // * **Event: 'error' (type)**: signals an error
    //
    // * **setInitialWindow(size)**: the initial flow control window size can be changed *any time*
    //   ([as described in the standard][1]) using this method
    //
    // [1]: https://tools.ietf.org/html/rfc7540#section-6.9.2

    // API for child classes
    // ---------------------

    // * **new Flow([flowControlId])**: creating a new flow that will listen for WINDOW_UPDATES frames
    //   with the given `flowControlId` (or every update frame if not given)
    //
    // * **_send()**: called when more frames should be pushed. The child class is expected to override
    //   this (instead of the `_read` method of the Duplex class).
    //
    // * **_receive(frame, readyCallback)**: called when there's an incoming frame. The child class is
    //   expected to override this (instead of the `_write` method of the Duplex class).
    //
    // * **push(frame): bool**: schedules `frame` for sending.
    //
    //   Returns `true` if it needs more frames in the output queue, `false` if the output queue is
    //   full, and `null` if did not push the frame into the output queue (instead, it pushed it into
    //   the flow control queue).
    //
    // * **read(limit): frame**: like the regular `read`, but the 'flow control size' (0 for non-DATA
    //   frames, length of the payload for DATA frames) of the returned frame will be under `limit`.
    //   Small exception: pass -1 as `limit` if the max. flow control size is 0. `read(0)` means the
    //   same thing as [in the original API](https://nodejs.org/api/stream.html#stream_stream_read_0).
    //
    // * **getLastQueuedFrame(): frame**: returns the last frame in output buffers
    //
    // * **_log**: the Flow class uses the `_log` object of the parent

    // Constructor
    // -----------

    // When a HTTP/2.0 connection is first established, new streams are created with an initial flow
    // control window size of 65535 bytes.
    var INITIAL_WINDOW_SIZE = 65535;

    // `flowControlId` is needed if only specific WINDOW_UPDATEs should be watched.
    function Flow(flowControlId) {
      Duplex.call(this, { objectMode: true });

      this._window = this._initialWindow = INITIAL_WINDOW_SIZE;
      this._flowControlId = flowControlId;
      this._queue = [];
      this._ended = false;
      this._received = 0;
      this._blocked = false;
    }
    Flow.prototype = Object.create(Duplex.prototype, { constructor: { value: Flow } });

    // Incoming frames
    // ---------------

    // `_receive` is called when there's an incoming frame.
    Flow.prototype._receive = function _receive(frame, callback) {
      throw new Error('The _receive(frame, callback) method has to be overridden by the child class!');
    };

    // `_receive` is called by `_write` which in turn is [called by Duplex][1] when someone `write()`s
    // to the flow. It emits the 'receiving' event and notifies the window size tracking code if the
    // incoming frame is a WINDOW_UPDATE.
    // [1]: https://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback_1
    Flow.prototype._write = function _write(frame, encoding, callback) {
      var sentToUs = (this._flowControlId === undefined) || (frame.stream === this._flowControlId);

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

      if ((frame.type === 'DATA') && (frame.data.length > 0)) {
        this._receive(frame, function() {
          this._received += frame.data.length;
          if (!this._restoreWindowTimer) {
            this._restoreWindowTimer = timers.setImmediate(this._restoreWindow.bind(this));
          }
          callback();
        }.bind(this));
      }

      else {
        this._receive(frame, callback);
      }

      if (sentToUs && (frame.type === 'WINDOW_UPDATE')) {
        this._updateWindow(frame);
      }
    };

    // `_restoreWindow` basically acknowledges the DATA frames received since it's last call. It sends
    // a WINDOW_UPDATE that restores the flow control window of the remote end.
    // TODO: push this directly into the output queue. No need to wait for DATA frames in the queue.
    Flow.prototype._restoreWindow = function _restoreWindow() {
      delete this._restoreWindowTimer;
      if (!this._ended && (this._received > 0)) {
        this.push({
          type: 'WINDOW_UPDATE',
          flags: {},
          stream: this._flowControlId,
          window_size: this._received
        });
        this._received = 0;
      }
    };

    // Outgoing frames - sending procedure
    // -----------------------------------

    //                                         flow
    //                +-------------------------------------------------+
    //                |                                                 |
    //                +--------+           +---------+                  |
    //        read()  | output |  _read()  | flow    |  _send()         |
    //     <----------|        |<----------| control |<-------------    |
    //                | buffer |           | buffer  |                  |
    //                +--------+           +---------+                  |
    //                | input  |                                        |
    //     ---------->|        |----------------------------------->    |
    //       write()  | buffer |  _write()              _receive()      |
    //                +--------+                                        |
    //                |                                                 |
    //                +-------------------------------------------------+

    // `_send` is called when more frames should be pushed to the output buffer.
    Flow.prototype._send = function _send() {
      throw new Error('The _send() method has to be overridden by the child class!');
    };

    // `_send` is called by `_read` which is in turn [called by Duplex][1] when it wants to have more
    // items in the output queue.
    // [1]: https://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback_1
    Flow.prototype._read = function _read() {
      // * if the flow control queue is empty, then let the user push more frames
      if (this._queue.length === 0) {
        this._send();
      }

      // * if there are items in the flow control queue, then let's put them into the output queue (to
      //   the extent it is possible with respect to the window size and output queue feedback)
      else if (this._window > 0) {
        this._blocked = false;
        this._readableState.sync = true; // to avoid reentrant calls
        do {
          var moreNeeded = this._push(this._queue[0]);
          if (moreNeeded !== null) {
            this._queue.shift();
          }
        } while (moreNeeded && (this._queue.length > 0));
        this._readableState.sync = false;

        assert((!moreNeeded) ||                              // * output queue is full
               (this._queue.length === 0) ||                         // * flow control queue is empty
               (!this._window && (this._queue[0].type === 'DATA'))); // * waiting for window update
      }

      // * otherwise, come back when the flow control window is positive
      else if (!this._blocked) {
        this._parentPush({
          type: 'BLOCKED',
          flags: {},
          stream: this._flowControlId
        });
        this.once('window_update', this._read);
        this._blocked = true;
      }
    };

    var MAX_PAYLOAD_SIZE = 4096; // Must not be greater than MAX_HTTP_PAYLOAD_SIZE which is 16383

    // `read(limit)` is like the `read` of the Readable class, but it guarantess that the 'flow control
    // size' (0 for non-DATA frames, length of the payload for DATA frames) of the returned frame will
    // be under `limit`.
    Flow.prototype.read = function read(limit) {
      if (limit === 0) {
        return Duplex.prototype.read.call(this, 0);
      } else Iif (limit === -1) {
        limit = 0;
      } else Eif ((limit === undefined) || (limit > MAX_PAYLOAD_SIZE)) {
        limit = MAX_PAYLOAD_SIZE;
      }

      // * Looking at the first frame in the queue without pulling it out if possible.
      var frame = this._readableState.buffer[0];
      if (!frame && !this._readableState.ended) {
        this._read();
        frame = this._readableState.buffer[0];
      }

      if (frame && (frame.type === 'DATA')) {
        // * If the frame is DATA, then there's two special cases:
        //   * if the limit is 0, we shouldn't return anything
        //   * if the size of the frame is larger than limit, then the frame should be split
        Iif (limit === 0) {
          return Duplex.prototype.read.call(this, 0);
        }

        else if (frame.data.length > limit) {
          this._log.trace({ frame: frame, size: frame.data.length, forwardable: limit },
            'Splitting out forwardable part of a DATA frame.');
          this.unshift({
            type: 'DATA',
            flags: {},
            stream: frame.stream,
            data: frame.data.slice(0, limit)
          });
          frame.data = frame.data.slice(limit);
        }
      }

      return Duplex.prototype.read.call(this);
    };

    // `_parentPush` pushes the given `frame` into the output queue
    Flow.prototype._parentPush = function _parentPush(frame) {
      this._log.trace({ frame: frame }, 'Pushing frame into the output queue');

      if (frame && (frame.type === 'DATA') && (this._window !== Infinity)) {
        this._log.trace({ window: this._window, by: frame.data.length },
                        'Decreasing flow control window size.');
        this._window -= frame.data.length;
        assert(this._window >= 0);
      }

      return Duplex.prototype.push.call(this, frame);
    };

    // `_push(frame)` pushes `frame` into the output queue and decreases the flow control window size.
    // It is capable of splitting DATA frames into smaller parts, if the window size is not enough to
    // push the whole frame. The return value is similar to `push` except that it returns `null` if it
    // did not push the whole frame to the output queue (but maybe it did push part of the frame).
    Flow.prototype._push = function _push(frame) {
      var data = frame && (frame.type === 'DATA') && frame.data;
      var maxFrameLength = (this._window < 16384) ? this._window : 16384;

      if (!data || (data.length <= maxFrameLength)) {
        return this._parentPush(frame);
      }

      else if (this._window <= 0) {
        return null;
      }

      else {
        this._log.trace({ frame: frame, size: frame.data.length, forwardable: this._window },
                        'Splitting out forwardable part of a DATA frame.');
        frame.data = data.slice(maxFrameLength);
        this._parentPush({
          type: 'DATA',
          flags: {},
          stream: frame.stream,
          data: data.slice(0, maxFrameLength)
        });
        return null;
      }
    };

    // Push `frame` into the flow control queue, or if it's empty, then directly into the output queue
    Flow.prototype.push = function push(frame) {
      if (frame === null) {
        this._log.debug('Enqueueing outgoing End Of Stream');
      } else {
        this._log.debug({ frame: frame }, 'Enqueueing outgoing frame');
      }

      var moreNeeded = null;
      if (this._queue.length === 0) {
        moreNeeded = this._push(frame);
      }

      if (moreNeeded === null) {
        this._queue.push(frame);
      }

      return moreNeeded;
    };

    // `getLastQueuedFrame` returns the last frame in output buffers. This is primarily used by the
    // [Stream](stream.html) class to mark the last frame with END_STREAM flag.
    Flow.prototype.getLastQueuedFrame = function getLastQueuedFrame() {
      var readableQueue = this._readableState.buffer;
      return this._queue[this._queue.length - 1] || readableQueue[readableQueue.length - 1];
    };

    // Outgoing frames - managing the window size
    // ------------------------------------------

    // Flow control window size is manipulated using the `_increaseWindow` method.
    //
    // * Invoking it with `Infinite` means turning off flow control. Flow control cannot be enabled
    //   again once disabled. Any attempt to re-enable flow control MUST be rejected with a
    //   FLOW_CONTROL_ERROR error code.
    // * A sender MUST NOT allow a flow control window to exceed 2^31 - 1 bytes. The action taken
    //   depends on it being a stream or the connection itself.

    var WINDOW_SIZE_LIMIT = Math.pow(2, 31) - 1;

    Flow.prototype._increaseWindow = function _increaseWindow(size) {
      if ((this._window === Infinity) && (size !== Infinity)) {
        this._log.error('Trying to increase flow control window after flow control was turned off.');
        this.emit('error', 'FLOW_CONTROL_ERROR');
      } else {
        this._log.trace({ window: this._window, by: size }, 'Increasing flow control window size.');
        this._window += size;
        if ((this._window !== Infinity) && (this._window > WINDOW_SIZE_LIMIT)) {
          this._log.error('Flow control window grew too large.');
          this.emit('error', 'FLOW_CONTROL_ERROR');
        } else {
          if (size != 0) {
            this.emit('window_update');
          }
        }
      }
    };

    // The `_updateWindow` method gets called every time there's an incoming WINDOW_UPDATE frame. It
    // modifies the flow control window:
    //
    // * Flow control can be disabled for an individual stream by sending a WINDOW_UPDATE with the
    //   END_FLOW_CONTROL flag set. The payload of a WINDOW_UPDATE frame that has the END_FLOW_CONTROL
    //   flag set is ignored.
    // * A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount
    //   specified in the frame.
    Flow.prototype._updateWindow = function _updateWindow(frame) {
      this._increaseWindow(frame.flags.END_FLOW_CONTROL ? Infinity : frame.window_size);
    };

    // A SETTINGS frame can alter the initial flow control window size for all current streams. When the
    // value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream by
    // calling the `setInitialWindow` method. The window size has to be modified by the difference
    // between the new value and the old value.
    Flow.prototype.setInitialWindow = function setInitialWindow(initialWindow) {
      this._increaseWindow(initialWindow - this._initialWindow);
      this._initialWindow = initialWindow;
    };