name : _stream_transform.js // 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; var _require$codes = require('../errors').codes, ERR_METHOD_NOT_IMPLEMENTED = _require$codes.ERR_METHOD_NOT_IMPLEMENTED, ERR_MULTIPLE_CALLBACK = _require$codes.ERR_MULTIPLE_CALLBACK, ERR_TRANSFORM_ALREADY_TRANSFORMING = _require$codes.ERR_TRANSFORM_ALREADY_TRANSFORMING, ERR_TRANSFORM_WITH_LENGTH_0 = _require$codes.ERR_TRANSFORM_WITH_LENGTH_0; var Duplex = require('./_stream_duplex'); require('inherits')(Transform, Duplex); function afterTransform(er, data) { var ts = this._transformState; ts.transforming = false; var cb = ts.writecb; if (cb === null) { return this.emit('error', new ERR_MULTIPLE_CALLBACK()); } ts.writechunk = null; ts.writecb = null; if (data != null) // single equals check for both `null` and `undefined` this.push(data); cb(er); var rs = this._readableState; rs.reading = false; if (rs.needReadable || rs.length < rs.highWaterMark) { this._read(rs.highWaterMark); } } function Transform(options) { if (!(this instanceof Transform)) return new Transform(options); Duplex.call(this, options); this._transformState = { afterTransform: afterTransform.bind(this), needTransform: false, transforming: false, writecb: null, writechunk: null, writeencoding: null }; // 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.on('prefinish', prefinish); } function prefinish() { var _this = this; if (typeof this._flush === 'function' && !this._readableState.destroyed) { this._flush(function (er, data) { done(_this, er, data); }); } else { done(this, null, null); } } 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) { cb(new ERR_METHOD_NOT_IMPLEMENTED('_transform()')); }; 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.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; } }; Transform.prototype._destroy = function (err, cb) { Duplex.prototype._destroy.call(this, err, function (err2) { cb(err2); }); }; function done(stream, er, data) { if (er) return stream.emit('error', er); if (data != null) // single equals check for both `null` and `undefined` stream.push(data); // TODO(BridgeAR): Write a test for these two error cases // if there's nothing in the write buffer, then that means // that nothing more will ever be provided if (stream._writableState.length) throw new ERR_TRANSFORM_WITH_LENGTH_0(); if (stream._transformState.transforming) throw new ERR_TRANSFORM_ALREADY_TRANSFORMING(); return stream.push(null); }