transform.js

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//
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// 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';

const {
  ObjectSetPrototypeOf,
  Symbol,
} = primordials;

module.exports = Transform;
const {
  ERR_METHOD_NOT_IMPLEMENTED,
} = require('internal/errors').codes;
const Duplex = require('internal/streams/duplex');
const { getHighWaterMark } = require('internal/streams/state');
ObjectSetPrototypeOf(Transform.prototype, Duplex.prototype);
ObjectSetPrototypeOf(Transform, Duplex);

const kCallback = Symbol('kCallback');

function Transform(options) {
  if (!(this instanceof Transform))
    return new Transform(options);

  // TODO (ronag): This should preferably always be
  // applied but would be semver-major. Or even better;
  // make Transform a Readable with the Writable interface.
  const readableHighWaterMark = options ? getHighWaterMark(this, options, 'readableHighWaterMark', true) : null;
  if (readableHighWaterMark === 0) {
    // A Duplex will buffer both on the writable and readable side while
    // a Transform just wants to buffer hwm number of elements. To avoid
    // buffering twice we disable buffering on the writable side.
    options = {
      ...options,
      highWaterMark: null,
      readableHighWaterMark,
      writableHighWaterMark: options.writableHighWaterMark || 0,
    };
  }

  Duplex.call(this, options);

  // 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;

  this[kCallback] = null;

  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.
  // Backwards compat. Some Transform streams incorrectly implement _final
  // instead of or in addition to _flush. By using 'prefinish' instead of
  // implementing _final we continue supporting this unfortunate use case.
  this.on('prefinish', prefinish);
}

function final(cb) {
  if (typeof this._flush === 'function' && !this.destroyed) {
    this._flush((er, data) => {
      if (er) {
        if (cb) {
          cb(er);
        } else {
          this.destroy(er);
        }
        return;
      }

      if (data != null) {
        this.push(data);
      }
      this.push(null);
      if (cb) {
        cb();
      }
    });
  } else {
    this.push(null);
    if (cb) {
      cb();
    }
  }
}

function prefinish() {
  if (this._final !== final) {
    final.call(this);
  }
}

Transform.prototype._final = final;

Transform.prototype._transform = function(chunk, encoding, callback) {
  throw new ERR_METHOD_NOT_IMPLEMENTED('_transform()');
};

Transform.prototype._write = function(chunk, encoding, callback) {
  const rState = this._readableState;
  const wState = this._writableState;
  const length = rState.length;

  this._transform(chunk, encoding, (err, val) => {
    if (err) {
      callback(err);
      return;
    }

    if (val != null) {
      this.push(val);
    }

    if (rState.ended) {
      // If user has called this.push(null) we have to
      // delay the callback to properly propagate the new
      // state.
      process.nextTick(callback);
    } else if (
      wState.ended || // Backwards compat.
      length === rState.length || // Backwards compat.
      rState.length < rState.highWaterMark
    ) {
      callback();
    } else {
      this[kCallback] = callback;
    }
  });
};

Transform.prototype._read = function() {
  if (this[kCallback]) {
    const callback = this[kCallback];
    this[kCallback] = null;
    callback();
  }
};