node/lib/_stream_transform.js at master · anchnk/node · GitHub

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// Copyright Joyent, Inc. and other Node contributors.

// Permission is hereby granted, free of charge, to any person obtaining a

// copy of this software and associated documentation files (the

// "Software"), to deal in the Software without restriction, including

// without limitation the rights to use, copy, modify, merge, publish,

// distribute, sublicense, and/or sell copies of the Software, and to permit

// persons to whom the Software is furnished to do so, subject to the

// following conditions:

// The above copyright notice and this permission notice shall be included

// in all copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS

// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN

// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,

// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR

// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE

// USE OR OTHER DEALINGS IN THE SOFTWARE.

// a transform stream is a readable/writable stream where you do

// something with the data. Sometimes it's called a "filter",

// but that's not a great name for it, since that implies a thing where

// some bits pass through, and others are simply ignored. (That would

// be a valid example of a transform, of course.)

// While the output is causally related to the input, it's not a

// necessarily symmetric or synchronous transformation. For example,

// a zlib stream might take multiple plain-text writes(), and then

// emit a single compressed chunk some time in the future.

// Here's how this works:

// The Transform stream has all the aspects of the readable and writable

// stream classes. When you write(chunk), that calls _write(chunk,cb)

// internally, and returns false if there's a lot of pending writes

// buffered up. When you call read(), that calls _read(n) until

// there's enough pending readable data buffered up.

// In a transform stream, the written data is placed in a buffer. When

// _read(n) is called, it transforms the queued up data, calling the

// buffered _write cb's as it consumes chunks. If consuming a single

// written chunk would result in multiple output chunks, then the first

// outputted bit calls the readcb, and subsequent chunks just go into

// the read buffer, and will cause it to emit 'readable' if necessary.

// This way, back-pressure is actually determined by the reading side,

// since _read has to be called to start processing a new chunk. However,

// a pathological inflate type of transform can cause excessive buffering

// here. For example, imagine a stream where every byte of input is

// interpreted as an integer from 0-255, and then results in that many

// bytes of output. Writing the 4 bytes {ff,ff,ff,ff} would result in

// 1kb of data being output. In this case, you could write a very small

// amount of input, and end up with a very large amount of output. In

// such a pathological inflating mechanism, there'd be no way to tell

// the system to stop doing the transform. A single 4MB write could

// cause the system to run out of memory.

// However, even in such a pathological case, only a single written chunk

// would be consumed, and then the rest would wait (un-transformed) until

// the results of the previous transformed chunk were consumed.

'use strict';

module.exports = Transform;

const Duplex = require('_stream_duplex');

const util = require('util');

util.inherits(Transform, Duplex);

function TransformState(stream) {

this.afterTransform = function(er, data) {

return afterTransform(stream, er, data);

};

this.needTransform = false;

this.transforming = false;

this.writecb = null;

this.writechunk = null;

this.writeencoding = null;

}

function afterTransform(stream, er, data) {

var ts = stream._transformState;

ts.transforming = false;

var cb = ts.writecb;

if (!cb)

return stream.emit('error', new Error('no writecb in Transform class'));

ts.writechunk = null;

ts.writecb = null;

if (data !== null && data !== undefined)

stream.push(data);

cb(er);

var rs = stream._readableState;

rs.reading = false;

if (rs.needReadable || rs.length < rs.highWaterMark) {

stream._read(rs.highWaterMark);

}

function Transform(options) {

if (!(this instanceof Transform))

return new Transform(options);

Duplex.call(this, options);

this._transformState = new TransformState(this);

var stream = this;

// start out asking for a readable event once data is transformed.

this._readableState.needReadable = true;

// we have implemented the _read method, and done the other things

// that Readable wants before the first _read call, so unset the

// sync guard flag.

this._readableState.sync = false;

if (options) {

if (typeof options.transform === 'function')

this._transform = options.transform;

if (typeof options.flush === 'function')

this._flush = options.flush;

}

// When the writable side finishes, then flush out anything remaining.

this.once('prefinish', function() {

if (typeof this._flush === 'function')

this._flush(function(er, data) {

done(stream, er, data);

});

else

done(stream);

});

}

Transform.prototype.push = function(chunk, encoding) {

this._transformState.needTransform = false;

return Duplex.prototype.push.call(this, chunk, encoding);

};

// This is the part where you do stuff!

// override this function in implementation classes.

// 'chunk' is an input chunk.

// Call `push(newChunk)` to pass along transformed output

// to the readable side. You may call 'push' zero or more times.

// Call `cb(err)` when you are done with this chunk. If you pass

// an error, then that'll put the hurt on the whole operation. If you

// never call cb(), then you'll never get another chunk.

Transform.prototype._transform = function(chunk, encoding, cb) {

throw new Error('_transform() is not implemented');

};

Transform.prototype._write = function(chunk, encoding, cb) {

var ts = this._transformState;

ts.writecb = cb;

ts.writechunk = chunk;

ts.writeencoding = encoding;

if (!ts.transforming) {

var rs = this._readableState;

if (ts.needTransform ||

rs.needReadable ||

rs.length < rs.highWaterMark)

this._read(rs.highWaterMark);

}

};

// Doesn't matter what the args are here.

// _transform does all the work.

// That we got here means that the readable side wants more data.

Transform.prototype._read = function(n) {

var ts = this._transformState;

if (ts.writechunk !== null && ts.writecb && !ts.transforming) {

ts.transforming = true;

this._transform(ts.writechunk, ts.writeencoding, ts.afterTransform);

} else {

// mark that we need a transform, so that any data that comes in

// will get processed, now that we've asked for it.

ts.needTransform = true;

}

};

function done(stream, er, data) {

if (er)

return stream.emit('error', er);

if (data !== null && data !== undefined)

stream.push(data);

// if there's nothing in the write buffer, then that means

// that nothing more will ever be provided

var ws = stream._writableState;

var ts = stream._transformState;

if (ws.length)

throw new Error('Calling transform done when ws.length != 0');

if (ts.transforming)

throw new Error('Calling transform done when still transforming');

return stream.push(null);

}

node/lib/_stream_transform.js at master · anchnk/node · GitHub

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