_stream_transform.js

// Copyright Joyent, Inc. and other Node contributors.
//
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// following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
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// 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
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// 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 Duplex = require('./_stream_duplex');

/*<replacement>*/
var util = require('core-util-is');
util.inherits = require('inherits');
/*</replacement>*/

util.inherits(Transform, Duplex);

function afterTransform(er, data) {
  var ts = this._transformState;
  ts.transforming = false;

  var cb = ts.writecb;

  if (!cb) {
    return this.emit('error', new Error('write callback called multiple times'));
  }

  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._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) {
  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;
  }
};

Transform.prototype._destroy = function (err, cb) {
  var _this2 = this;

  Duplex.prototype._destroy.call(this, err, function (err2) {
    cb(err2);
    _this2.emit('close');
  });
};

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);

  // if there's nothing in the write buffer, then that means
  // that nothing more will ever be provided
  if (stream._writableState.length) throw new Error('Calling transform done when ws.length != 0');

  if (stream._transformState.transforming) throw new Error('Calling transform done when still transforming');

  return stream.push(null);
}