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var assert = require('assert');
// 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).
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
//
// 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
//
// * **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;
}
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.
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 = 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.
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._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.listenerCount('window_update')) {
this.once('window_update', this._read);
}
}
};
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 if (limit === -1) {
limit = 0;
} else if ((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
if (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;
};