readable.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.

'use strict';

const {
  ArrayIsArray,
  NumberIsInteger,
  NumberIsNaN,
  NumberParseInt,
  ObjectDefineProperties,
  ObjectSetPrototypeOf,
  Promise,
  Set,
  SymbolAsyncIterator,
  Symbol
} = primordials;

module.exports = Readable;
Readable.ReadableState = ReadableState;

const EE = require('events');
const Stream = require('internal/streams/legacy');
const { Buffer } = require('buffer');

let debug = require('internal/util/debuglog').debuglog('stream', (fn) => {
  debug = fn;
});
const BufferList = require('internal/streams/buffer_list');
const destroyImpl = require('internal/streams/destroy');
const {
  getHighWaterMark,
  getDefaultHighWaterMark
} = require('internal/streams/state');
const {
  ERR_INVALID_ARG_TYPE,
  ERR_STREAM_PUSH_AFTER_EOF,
  ERR_METHOD_NOT_IMPLEMENTED,
  ERR_STREAM_UNSHIFT_AFTER_END_EVENT
} = require('internal/errors').codes;

const kPaused = Symbol('kPaused');

// Lazy loaded to improve the startup performance.
let StringDecoder;
let from;

ObjectSetPrototypeOf(Readable.prototype, Stream.prototype);
ObjectSetPrototypeOf(Readable, Stream);
function nop() {}

const { errorOrDestroy } = destroyImpl;

function prependListener(emitter, event, fn) {
  // Sadly this is not cacheable as some libraries bundle their own
  // event emitter implementation with them.
  if (typeof emitter.prependListener === 'function')
    return emitter.prependListener(event, fn);

  // This is a hack to make sure that our error handler is attached before any
  // userland ones.  NEVER DO THIS. This is here only because this code needs
  // to continue to work with older versions of Node.js that do not include
  // the prependListener() method. The goal is to eventually remove this hack.
  if (!emitter._events || !emitter._events[event])
    emitter.on(event, fn);
  else if (ArrayIsArray(emitter._events[event]))
    emitter._events[event].unshift(fn);
  else
    emitter._events[event] = [fn, emitter._events[event]];
}

function ReadableState(options, stream, isDuplex) {
  // Duplex streams are both readable and writable, but share
  // the same options object.
  // However, some cases require setting options to different
  // values for the readable and the writable sides of the duplex stream.
  // These options can be provided separately as readableXXX and writableXXX.
  if (typeof isDuplex !== 'boolean')
    isDuplex = stream instanceof Stream.Duplex;

  // Object stream flag. Used to make read(n) ignore n and to
  // make all the buffer merging and length checks go away.
  this.objectMode = !!(options && options.objectMode);

  if (isDuplex)
    this.objectMode = this.objectMode ||
      !!(options && options.readableObjectMode);

  // The point at which it stops calling _read() to fill the buffer
  // Note: 0 is a valid value, means "don't call _read preemptively ever"
  this.highWaterMark = options ?
    getHighWaterMark(this, options, 'readableHighWaterMark', isDuplex) :
    getDefaultHighWaterMark(false);

  // A linked list is used to store data chunks instead of an array because the
  // linked list can remove elements from the beginning faster than
  // array.shift().
  this.buffer = new BufferList();
  this.length = 0;
  this.pipes = [];
  this.flowing = null;
  this.ended = false;
  this.endEmitted = false;
  this.reading = false;

  // A flag to be able to tell if the event 'readable'/'data' is emitted
  // immediately, or on a later tick.  We set this to true at first, because
  // any actions that shouldn't happen until "later" should generally also
  // not happen before the first read call.
  this.sync = true;

  // Whenever we return null, then we set a flag to say
  // that we're awaiting a 'readable' event emission.
  this.needReadable = false;
  this.emittedReadable = false;
  this.readableListening = false;
  this.resumeScheduled = false;
  this[kPaused] = null;

  // True if the error was already emitted and should not be thrown again.
  this.errorEmitted = false;

  // Should close be emitted on destroy. Defaults to true.
  this.emitClose = !options || options.emitClose !== false;

  // Should .destroy() be called after 'end' (and potentially 'finish').
  this.autoDestroy = !options || options.autoDestroy !== false;

  // Has it been destroyed.
  this.destroyed = false;

  // Indicates whether the stream has errored. When true no further
  // _read calls, 'data' or 'readable' events should occur. This is needed
  // since when autoDestroy is disabled we need a way to tell whether the
  // stream has failed.
  this.errored = null;

  // Indicates whether the stream has finished destroying.
  this.closed = false;

  // True if close has been emitted or would have been emitted
  // depending on emitClose.
  this.closeEmitted = false;

  // Crypto is kind of old and crusty.  Historically, its default string
  // encoding is 'binary' so we have to make this configurable.
  // Everything else in the universe uses 'utf8', though.
  this.defaultEncoding = (options && options.defaultEncoding) || 'utf8';

  // Ref the piped dest which we need a drain event on it
  // type: null | Writable | Set<Writable>.
  this.awaitDrainWriters = null;
  this.multiAwaitDrain = false;

  // If true, a maybeReadMore has been scheduled.
  this.readingMore = false;

  this.decoder = null;
  this.encoding = null;
  if (options && options.encoding) {
    if (!StringDecoder)
      StringDecoder = require('string_decoder').StringDecoder;
    this.decoder = new StringDecoder(options.encoding);
    this.encoding = options.encoding;
  }
}


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

  // Checking for a Stream.Duplex instance is faster here instead of inside
  // the ReadableState constructor, at least with V8 6.5.
  const isDuplex = this instanceof Stream.Duplex;

  this._readableState = new ReadableState(options, this, isDuplex);

  if (options) {
    if (typeof options.read === 'function')
      this._read = options.read;

    if (typeof options.destroy === 'function')
      this._destroy = options.destroy;
  }

  Stream.call(this, options);
}

Readable.prototype.destroy = destroyImpl.destroy;
Readable.prototype._undestroy = destroyImpl.undestroy;
Readable.prototype._destroy = function(err, cb) {
  cb(err);
};

Readable.prototype[EE.captureRejectionSymbol] = function(err) {
  this.destroy(err);
};

// Manually shove something into the read() buffer.
// This returns true if the highWaterMark has not been hit yet,
// similar to how Writable.write() returns true if you should
// write() some more.
Readable.prototype.push = function(chunk, encoding) {
  return readableAddChunk(this, chunk, encoding, false);
};

// Unshift should *always* be something directly out of read().
Readable.prototype.unshift = function(chunk, encoding) {
  return readableAddChunk(this, chunk, encoding, true);
};

function readableAddChunk(stream, chunk, encoding, addToFront) {
  debug('readableAddChunk', chunk);
  const state = stream._readableState;

  let err;
  if (!state.objectMode) {
    if (typeof chunk === 'string') {
      encoding = encoding || state.defaultEncoding;
      if (state.encoding !== encoding) {
        if (addToFront && state.encoding) {
          // When unshifting, if state.encoding is set, we have to save
          // the string in the BufferList with the state encoding.
          chunk = Buffer.from(chunk, encoding).toString(state.encoding);
        } else {
          chunk = Buffer.from(chunk, encoding);
          encoding = '';
        }
      }
    } else if (chunk instanceof Buffer) {
      encoding = '';
    } else if (Stream._isUint8Array(chunk)) {
      chunk = Stream._uint8ArrayToBuffer(chunk);
      encoding = '';
    } else if (chunk != null) {
      err = new ERR_INVALID_ARG_TYPE(
        'chunk', ['string', 'Buffer', 'Uint8Array'], chunk);
    }
  }

  if (err) {
    errorOrDestroy(stream, err);
  } else if (chunk === null) {
    state.reading = false;
    onEofChunk(stream, state);
  } else if (state.objectMode || (chunk && chunk.length > 0)) {
    if (addToFront) {
      if (state.endEmitted)
        errorOrDestroy(stream, new ERR_STREAM_UNSHIFT_AFTER_END_EVENT());
      else
        addChunk(stream, state, chunk, true);
    } else if (state.ended) {
      errorOrDestroy(stream, new ERR_STREAM_PUSH_AFTER_EOF());
    } else if (state.destroyed) {
      return false;
    } else {
      state.reading = false;
      if (state.decoder && !encoding) {
        chunk = state.decoder.write(chunk);
        if (state.objectMode || chunk.length !== 0)
          addChunk(stream, state, chunk, false);
        else
          maybeReadMore(stream, state);
      } else {
        addChunk(stream, state, chunk, false);
      }
    }
  } else if (!addToFront) {
    state.reading = false;
    maybeReadMore(stream, state);
  }

  // We can push more data if we are below the highWaterMark.
  // Also, if we have no data yet, we can stand some more bytes.
  // This is to work around cases where hwm=0, such as the repl.
  return !state.ended &&
    (state.length < state.highWaterMark || state.length === 0);
}

function addChunk(stream, state, chunk, addToFront) {
  if (state.flowing && state.length === 0 && !state.sync &&
      stream.listenerCount('data') > 0) {
    // Use the guard to avoid creating `Set()` repeatedly
    // when we have multiple pipes.
    if (state.multiAwaitDrain) {
      state.awaitDrainWriters.clear();
    } else {
      state.awaitDrainWriters = null;
    }
    stream.emit('data', chunk);
  } else {
    // Update the buffer info.
    state.length += state.objectMode ? 1 : chunk.length;
    if (addToFront)
      state.buffer.unshift(chunk);
    else
      state.buffer.push(chunk);

    if (state.needReadable)
      emitReadable(stream);
  }
  maybeReadMore(stream, state);
}

Readable.prototype.isPaused = function() {
  const state = this._readableState;
  return state[kPaused] === true || state.flowing === false;
};

// Backwards compatibility.
Readable.prototype.setEncoding = function(enc) {
  if (!StringDecoder)
    StringDecoder = require('string_decoder').StringDecoder;
  const decoder = new StringDecoder(enc);
  this._readableState.decoder = decoder;
  // If setEncoding(null), decoder.encoding equals utf8.
  this._readableState.encoding = this._readableState.decoder.encoding;

  const buffer = this._readableState.buffer;
  // Iterate over current buffer to convert already stored Buffers:
  let content = '';
  for (const data of buffer) {
    content += decoder.write(data);
  }
  buffer.clear();
  if (content !== '')
    buffer.push(content);
  this._readableState.length = content.length;
  return this;
};

// Don't raise the hwm > 1GB.
const MAX_HWM = 0x40000000;
function computeNewHighWaterMark(n) {
  if (n >= MAX_HWM) {
    // TODO(ronag): Throw ERR_VALUE_OUT_OF_RANGE.
    n = MAX_HWM;
  } else {
    // Get the next highest power of 2 to prevent increasing hwm excessively in
    // tiny amounts.
    n--;
    n |= n >>> 1;
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    n++;
  }
  return n;
}

// This function is designed to be inlinable, so please take care when making
// changes to the function body.
function howMuchToRead(n, state) {
  if (n <= 0 || (state.length === 0 && state.ended))
    return 0;
  if (state.objectMode)
    return 1;
  if (NumberIsNaN(n)) {
    // Only flow one buffer at a time.
    if (state.flowing && state.length)
      return state.buffer.first().length;
    return state.length;
  }
  if (n <= state.length)
    return n;
  return state.ended ? state.length : 0;
}

// You can override either this method, or the async _read(n) below.
Readable.prototype.read = function(n) {
  debug('read', n);
  // Same as NumberParseInt(undefined, 10), however V8 7.3 performance regressed
  // in this scenario, so we are doing it manually.
  if (n === undefined) {
    n = NaN;
  } else if (!NumberIsInteger(n)) {
    n = NumberParseInt(n, 10);
  }
  const state = this._readableState;
  const nOrig = n;

  // If we're asking for more than the current hwm, then raise the hwm.
  if (n > state.highWaterMark)
    state.highWaterMark = computeNewHighWaterMark(n);

  if (n !== 0)
    state.emittedReadable = false;

  // If we're doing read(0) to trigger a readable event, but we
  // already have a bunch of data in the buffer, then just trigger
  // the 'readable' event and move on.
  if (n === 0 &&
      state.needReadable &&
      ((state.highWaterMark !== 0 ?
        state.length >= state.highWaterMark :
        state.length > 0) ||
       state.ended)) {
    debug('read: emitReadable', state.length, state.ended);
    if (state.length === 0 && state.ended)
      endReadable(this);
    else
      emitReadable(this);
    return null;
  }

  n = howMuchToRead(n, state);

  // If we've ended, and we're now clear, then finish it up.
  if (n === 0 && state.ended) {
    if (state.length === 0)
      endReadable(this);
    return null;
  }

  // All the actual chunk generation logic needs to be
  // *below* the call to _read.  The reason is that in certain
  // synthetic stream cases, such as passthrough streams, _read
  // may be a completely synchronous operation which may change
  // the state of the read buffer, providing enough data when
  // before there was *not* enough.
  //
  // So, the steps are:
  // 1. Figure out what the state of things will be after we do
  // a read from the buffer.
  //
  // 2. If that resulting state will trigger a _read, then call _read.
  // Note that this may be asynchronous, or synchronous.  Yes, it is
  // deeply ugly to write APIs this way, but that still doesn't mean
  // that the Readable class should behave improperly, as streams are
  // designed to be sync/async agnostic.
  // Take note if the _read call is sync or async (ie, if the read call
  // has returned yet), so that we know whether or not it's safe to emit
  // 'readable' etc.
  //
  // 3. Actually pull the requested chunks out of the buffer and return.

  // if we need a readable event, then we need to do some reading.
  let doRead = state.needReadable;
  debug('need readable', doRead);

  // If we currently have less than the highWaterMark, then also read some.
  if (state.length === 0 || state.length - n < state.highWaterMark) {
    doRead = true;
    debug('length less than watermark', doRead);
  }

  // However, if we've ended, then there's no point, if we're already
  // reading, then it's unnecessary, and if we're destroyed, then it's
  // not allowed.
  if (state.ended || state.reading || state.destroyed) {
    doRead = false;
    debug('reading or ended', doRead);
  } else if (doRead) {
    debug('do read');
    state.reading = true;
    state.sync = true;
    // If the length is currently zero, then we *need* a readable event.
    if (state.length === 0)
      state.needReadable = true;
    // Call internal read method
    this._read(state.highWaterMark);
    state.sync = false;
    // If _read pushed data synchronously, then `reading` will be false,
    // and we need to re-evaluate how much data we can return to the user.
    if (!state.reading)
      n = howMuchToRead(nOrig, state);
  }

  let ret;
  if (n > 0)
    ret = fromList(n, state);
  else
    ret = null;

  if (ret === null) {
    state.needReadable = state.length <= state.highWaterMark;
    n = 0;
  } else {
    state.length -= n;
    if (state.multiAwaitDrain) {
      state.awaitDrainWriters.clear();
    } else {
      state.awaitDrainWriters = null;
    }
  }

  if (state.length === 0) {
    // If we have nothing in the buffer, then we want to know
    // as soon as we *do* get something into the buffer.
    if (!state.ended)
      state.needReadable = true;

    // If we tried to read() past the EOF, then emit end on the next tick.
    if (nOrig !== n && state.ended)
      endReadable(this);
  }

  if (ret !== null)
    this.emit('data', ret);

  return ret;
};

function onEofChunk(stream, state) {
  debug('onEofChunk');
  if (state.ended) return;
  if (state.decoder) {
    const chunk = state.decoder.end();
    if (chunk && chunk.length) {
      state.buffer.push(chunk);
      state.length += state.objectMode ? 1 : chunk.length;
    }
  }
  state.ended = true;

  if (state.sync) {
    // If we are sync, wait until next tick to emit the data.
    // Otherwise we risk emitting data in the flow()
    // the readable code triggers during a read() call.
    emitReadable(stream);
  } else {
    // Emit 'readable' now to make sure it gets picked up.
    state.needReadable = false;
    state.emittedReadable = true;
    // We have to emit readable now that we are EOF. Modules
    // in the ecosystem (e.g. dicer) rely on this event being sync.
    emitReadable_(stream);
  }
}

// Don't emit readable right away in sync mode, because this can trigger
// another read() call => stack overflow.  This way, it might trigger
// a nextTick recursion warning, but that's not so bad.
function emitReadable(stream) {
  const state = stream._readableState;
  debug('emitReadable', state.needReadable, state.emittedReadable);
  state.needReadable = false;
  if (!state.emittedReadable) {
    debug('emitReadable', state.flowing);
    state.emittedReadable = true;
    process.nextTick(emitReadable_, stream);
  }
}

function emitReadable_(stream) {
  const state = stream._readableState;
  debug('emitReadable_', state.destroyed, state.length, state.ended);
  if (!state.destroyed && (state.length || state.ended)) {
    stream.emit('readable');
    state.emittedReadable = false;
  }

  // The stream needs another readable event if:
  // 1. It is not flowing, as the flow mechanism will take
  //    care of it.
  // 2. It is not ended.
  // 3. It is below the highWaterMark, so we can schedule
  //    another readable later.
  state.needReadable =
    !state.flowing &&
    !state.ended &&
    state.length <= state.highWaterMark;
  flow(stream);
}


// At this point, the user has presumably seen the 'readable' event,
// and called read() to consume some data.  that may have triggered
// in turn another _read(n) call, in which case reading = true if
// it's in progress.
// However, if we're not ended, or reading, and the length < hwm,
// then go ahead and try to read some more preemptively.
function maybeReadMore(stream, state) {
  if (!state.readingMore) {
    state.readingMore = true;
    process.nextTick(maybeReadMore_, stream, state);
  }
}

function maybeReadMore_(stream, state) {
  // Attempt to read more data if we should.
  //
  // The conditions for reading more data are (one of):
  // - Not enough data buffered (state.length < state.highWaterMark). The loop
  //   is responsible for filling the buffer with enough data if such data
  //   is available. If highWaterMark is 0 and we are not in the flowing mode
  //   we should _not_ attempt to buffer any extra data. We'll get more data
  //   when the stream consumer calls read() instead.
  // - No data in the buffer, and the stream is in flowing mode. In this mode
  //   the loop below is responsible for ensuring read() is called. Failing to
  //   call read here would abort the flow and there's no other mechanism for
  //   continuing the flow if the stream consumer has just subscribed to the
  //   'data' event.
  //
  // In addition to the above conditions to keep reading data, the following
  // conditions prevent the data from being read:
  // - The stream has ended (state.ended).
  // - There is already a pending 'read' operation (state.reading). This is a
  //   case where the stream has called the implementation defined _read()
  //   method, but they are processing the call asynchronously and have _not_
  //   called push() with new data. In this case we skip performing more
  //   read()s. The execution ends in this method again after the _read() ends
  //   up calling push() with more data.
  while (!state.reading && !state.ended &&
         (state.length < state.highWaterMark ||
          (state.flowing && state.length === 0))) {
    const len = state.length;
    debug('maybeReadMore read 0');
    stream.read(0);
    if (len === state.length)
      // Didn't get any data, stop spinning.
      break;
  }
  state.readingMore = false;
}

// Abstract method.  to be overridden in specific implementation classes.
// call cb(er, data) where data is <= n in length.
// for virtual (non-string, non-buffer) streams, "length" is somewhat
// arbitrary, and perhaps not very meaningful.
Readable.prototype._read = function(n) {
  throw new ERR_METHOD_NOT_IMPLEMENTED('_read()');
};

Readable.prototype.pipe = function(dest, pipeOpts) {
  const src = this;
  const state = this._readableState;

  if (state.pipes.length === 1) {
    if (!state.multiAwaitDrain) {
      state.multiAwaitDrain = true;
      state.awaitDrainWriters = new Set(
        state.awaitDrainWriters ? [state.awaitDrainWriters] : []
      );
    }
  }

  state.pipes.push(dest);
  debug('pipe count=%d opts=%j', state.pipes.length, pipeOpts);

  const doEnd = (!pipeOpts || pipeOpts.end !== false) &&
              dest !== process.stdout &&
              dest !== process.stderr;

  const endFn = doEnd ? onend : unpipe;
  if (state.endEmitted)
    process.nextTick(endFn);
  else
    src.once('end', endFn);

  dest.on('unpipe', onunpipe);
  function onunpipe(readable, unpipeInfo) {
    debug('onunpipe');
    if (readable === src) {
      if (unpipeInfo && unpipeInfo.hasUnpiped === false) {
        unpipeInfo.hasUnpiped = true;
        cleanup();
      }
    }
  }

  function onend() {
    debug('onend');
    dest.end();
  }

  let ondrain;

  let cleanedUp = false;
  function cleanup() {
    debug('cleanup');
    // Cleanup event handlers once the pipe is broken.
    dest.removeListener('close', onclose);
    dest.removeListener('finish', onfinish);
    if (ondrain) {
      dest.removeListener('drain', ondrain);
    }
    dest.removeListener('error', onerror);
    dest.removeListener('unpipe', onunpipe);
    src.removeListener('end', onend);
    src.removeListener('end', unpipe);
    src.removeListener('data', ondata);

    cleanedUp = true;

    // If the reader is waiting for a drain event from this
    // specific writer, then it would cause it to never start
    // flowing again.
    // So, if this is awaiting a drain, then we just call it now.
    // If we don't know, then assume that we are waiting for one.
    if (ondrain && state.awaitDrainWriters &&
        (!dest._writableState || dest._writableState.needDrain))
      ondrain();
  }

  function pause() {
    // If the user unpiped during `dest.write()`, it is possible
    // to get stuck in a permanently paused state if that write
    // also returned false.
    // => Check whether `dest` is still a piping destination.
    if (!cleanedUp) {
      if (state.pipes.length === 1 && state.pipes[0] === dest) {
        debug('false write response, pause', 0);
        state.awaitDrainWriters = dest;
        state.multiAwaitDrain = false;
      } else if (state.pipes.length > 1 && state.pipes.includes(dest)) {
        debug('false write response, pause', state.awaitDrainWriters.size);
        state.awaitDrainWriters.add(dest);
      }
      src.pause();
    }
    if (!ondrain) {
      // When the dest drains, it reduces the awaitDrain counter
      // on the source.  This would be more elegant with a .once()
      // handler in flow(), but adding and removing repeatedly is
      // too slow.
      ondrain = pipeOnDrain(src, dest);
      dest.on('drain', ondrain);
    }
  }

  src.on('data', ondata);
  function ondata(chunk) {
    debug('ondata');
    const ret = dest.write(chunk);
    debug('dest.write', ret);
    if (ret === false) {
      pause();
    }
  }

  // If the dest has an error, then stop piping into it.
  // However, don't suppress the throwing behavior for this.
  function onerror(er) {
    debug('onerror', er);
    unpipe();
    dest.removeListener('error', onerror);
    if (EE.listenerCount(dest, 'error') === 0) {
      const s = dest._writableState || dest._readableState;
      if (s && !s.errorEmitted) {
        // User incorrectly emitted 'error' directly on the stream.
        errorOrDestroy(dest, er);
      } else {
        dest.emit('error', er);
      }
    }
  }

  // Make sure our error handler is attached before userland ones.
  prependListener(dest, 'error', onerror);

  // Both close and finish should trigger unpipe, but only once.
  function onclose() {
    dest.removeListener('finish', onfinish);
    unpipe();
  }
  dest.once('close', onclose);
  function onfinish() {
    debug('onfinish');
    dest.removeListener('close', onclose);
    unpipe();
  }
  dest.once('finish', onfinish);

  function unpipe() {
    debug('unpipe');
    src.unpipe(dest);
  }

  // Tell the dest that it's being piped to.
  dest.emit('pipe', src);

  // Start the flow if it hasn't been started already.

  if (dest.writableNeedDrain === true) {
    if (state.flowing) {
      pause();
    }
  } else if (!state.flowing) {
    debug('pipe resume');
    src.resume();
  }

  return dest;
};

function pipeOnDrain(src, dest) {
  return function pipeOnDrainFunctionResult() {
    const state = src._readableState;

    // `ondrain` will call directly,
    // `this` maybe not a reference to dest,
    // so we use the real dest here.
    if (state.awaitDrainWriters === dest) {
      debug('pipeOnDrain', 1);
      state.awaitDrainWriters = null;
    } else if (state.multiAwaitDrain) {
      debug('pipeOnDrain', state.awaitDrainWriters.size);
      state.awaitDrainWriters.delete(dest);
    }

    if ((!state.awaitDrainWriters || state.awaitDrainWriters.size === 0) &&
      EE.listenerCount(src, 'data')) {
      state.flowing = true;
      flow(src);
    }
  };
}


Readable.prototype.unpipe = function(dest) {
  const state = this._readableState;
  const unpipeInfo = { hasUnpiped: false };

  // If we're not piping anywhere, then do nothing.
  if (state.pipes.length === 0)
    return this;

  if (!dest) {
    // remove all.
    const dests = state.pipes;
    state.pipes = [];
    this.pause();

    for (const dest of dests)
      dest.emit('unpipe', this, { hasUnpiped: false });
    return this;
  }

  // Try to find the right one.
  const index = state.pipes.indexOf(dest);
  if (index === -1)
    return this;

  state.pipes.splice(index, 1);
  if (state.pipes.length === 0)
    this.pause();

  dest.emit('unpipe', this, unpipeInfo);

  return this;
};

// Set up data events if they are asked for
// Ensure readable listeners eventually get something.
Readable.prototype.on = function(ev, fn) {
  const res = Stream.prototype.on.call(this, ev, fn);
  const state = this._readableState;

  if (ev === 'data') {
    // Update readableListening so that resume() may be a no-op
    // a few lines down. This is needed to support once('readable').
    state.readableListening = this.listenerCount('readable') > 0;

    // Try start flowing on next tick if stream isn't explicitly paused.
    if (state.flowing !== false)
      this.resume();
  } else if (ev === 'readable') {
    if (!state.endEmitted && !state.readableListening) {
      state.readableListening = state.needReadable = true;
      state.flowing = false;
      state.emittedReadable = false;
      debug('on readable', state.length, state.reading);
      if (state.length) {
        emitReadable(this);
      } else if (!state.reading) {
        process.nextTick(nReadingNextTick, this);
      }
    }
  }

  return res;
};
Readable.prototype.addListener = Readable.prototype.on;

Readable.prototype.removeListener = function(ev, fn) {
  const res = Stream.prototype.removeListener.call(this, ev, fn);

  if (ev === 'readable') {
    // We need to check if there is someone still listening to
    // readable and reset the state. However this needs to happen
    // after readable has been emitted but before I/O (nextTick) to
    // support once('readable', fn) cycles. This means that calling
    // resume within the same tick will have no
    // effect.
    process.nextTick(updateReadableListening, this);
  }

  return res;
};
Readable.prototype.off = Readable.prototype.removeListener;

Readable.prototype.removeAllListeners = function(ev) {
  const res = Stream.prototype.removeAllListeners.apply(this, arguments);

  if (ev === 'readable' || ev === undefined) {
    // We need to check if there is someone still listening to
    // readable and reset the state. However this needs to happen
    // after readable has been emitted but before I/O (nextTick) to
    // support once('readable', fn) cycles. This means that calling
    // resume within the same tick will have no
    // effect.
    process.nextTick(updateReadableListening, this);
  }

  return res;
};

function updateReadableListening(self) {
  const state = self._readableState;
  state.readableListening = self.listenerCount('readable') > 0;

  if (state.resumeScheduled && state[kPaused] === false) {
    // Flowing needs to be set to true now, otherwise
    // the upcoming resume will not flow.
    state.flowing = true;

    // Crude way to check if we should resume.
  } else if (self.listenerCount('data') > 0) {
    self.resume();
  } else if (!state.readableListening) {
    state.flowing = null;
  }
}

function nReadingNextTick(self) {
  debug('readable nexttick read 0');
  self.read(0);
}

// pause() and resume() are remnants of the legacy readable stream API
// If the user uses them, then switch into old mode.
Readable.prototype.resume = function() {
  const state = this._readableState;
  if (!state.flowing) {
    debug('resume');
    // We flow only if there is no one listening
    // for readable, but we still have to call
    // resume().
    state.flowing = !state.readableListening;
    resume(this, state);
  }
  state[kPaused] = false;
  return this;
};

function resume(stream, state) {
  if (!state.resumeScheduled) {
    state.resumeScheduled = true;
    process.nextTick(resume_, stream, state);
  }
}

function resume_(stream, state) {
  debug('resume', state.reading);
  if (!state.reading) {
    stream.read(0);
  }

  state.resumeScheduled = false;
  stream.emit('resume');
  flow(stream);
  if (state.flowing && !state.reading)
    stream.read(0);
}

Readable.prototype.pause = function() {
  debug('call pause flowing=%j', this._readableState.flowing);
  if (this._readableState.flowing !== false) {
    debug('pause');
    this._readableState.flowing = false;
    this.emit('pause');
  }
  this._readableState[kPaused] = true;
  return this;
};

function flow(stream) {
  const state = stream._readableState;
  debug('flow', state.flowing);
  while (state.flowing && stream.read() !== null);
}

// Wrap an old-style stream as the async data source.
// This is *not* part of the readable stream interface.
// It is an ugly unfortunate mess of history.
Readable.prototype.wrap = function(stream) {
  const state = this._readableState;
  let paused = false;

  stream.on('end', () => {
    debug('wrapped end');
    if (state.decoder && !state.ended) {
      const chunk = state.decoder.end();
      if (chunk && chunk.length)
        this.push(chunk);
    }

    this.push(null);
  });

  stream.on('data', (chunk) => {
    debug('wrapped data');
    if (state.decoder)
      chunk = state.decoder.write(chunk);

    // Don't skip over falsy values in objectMode.
    if (state.objectMode && (chunk === null || chunk === undefined))
      return;
    else if (!state.objectMode && (!chunk || !chunk.length))
      return;

    const ret = this.push(chunk);
    if (!ret) {
      paused = true;
      stream.pause();
    }
  });

  // Proxy all the other methods. Important when wrapping filters and duplexes.
  for (const i in stream) {
    if (this[i] === undefined && typeof stream[i] === 'function') {
      this[i] = function methodWrap(method) {
        return function methodWrapReturnFunction() {
          return stream[method].apply(stream, arguments);
        };
      }(i);
    }
  }

  stream.on('error', (err) => {
    errorOrDestroy(this, err);
  });

  stream.on('close', () => {
    // TODO(ronag): Update readable state?
    this.emit('close');
  });

  stream.on('destroy', () => {
    // TODO(ronag): this.destroy()?
    this.emit('destroy');
  });

  stream.on('pause', () => {
    // TODO(ronag): this.pause()?
    this.emit('pause');
  });

  stream.on('resume', () => {
    // TODO(ronag): this.resume()?
    this.emit('resume');
  });

  // When we try to consume some more bytes, simply unpause the
  // underlying stream.
  this._read = (n) => {
    debug('wrapped _read', n);
    if (paused) {
      paused = false;
      stream.resume();
    }
  };

  return this;
};

Readable.prototype[SymbolAsyncIterator] = function() {
  let stream = this;

  if (typeof stream.read !== 'function') {
    // v1 stream
    const src = stream;
    stream = new Readable({
      objectMode: true,
      destroy(err, callback) {
        destroyImpl.destroyer(src, err);
        callback(err);
      }
    }).wrap(src);
  }

  const iter = createAsyncIterator(stream);
  iter.stream = stream;
  return iter;
};

async function* createAsyncIterator(stream) {
  let callback = nop;

  function next(resolve) {
    if (this === stream) {
      callback();
      callback = nop;
    } else {
      callback = resolve;
    }
  }

  const state = stream._readableState;

  let error = state.errored;
  let errorEmitted = state.errorEmitted;
  let endEmitted = state.endEmitted;
  let closeEmitted = state.closeEmitted;

  stream
    .on('readable', next)
    .on('error', function(err) {
      error = err;
      errorEmitted = true;
      next.call(this);
    })
    .on('end', function() {
      endEmitted = true;
      next.call(this);
    })
    .on('close', function() {
      closeEmitted = true;
      next.call(this);
    });

  try {
    while (true) {
      const chunk = stream.destroyed ? null : stream.read();
      if (chunk !== null) {
        yield chunk;
      } else if (errorEmitted) {
        throw error;
      } else if (endEmitted) {
        break;
      } else if (closeEmitted) {
        break;
      } else {
        await new Promise(next);
      }
    }
  } catch (err) {
    destroyImpl.destroyer(stream, err);
    throw err;
  } finally {
    if (state.autoDestroy || !endEmitted) {
      // TODO(ronag): ERR_PREMATURE_CLOSE?
      destroyImpl.destroyer(stream, null);
    }
  }
}

// Making it explicit these properties are not enumerable
// because otherwise some prototype manipulation in
// userland will fail.
ObjectDefineProperties(Readable.prototype, {
  readable: {
    get() {
      const r = this._readableState;
      // r.readable === false means that this is part of a Duplex stream
      // where the readable side was disabled upon construction.
      // Compat. The user might manually disable readable side through
      // deprecated setter.
      return !!r && r.readable !== false && !r.destroyed && !r.errorEmitted &&
        !r.endEmitted;
    },
    set(val) {
      // Backwards compat.
      if (this._readableState) {
        this._readableState.readable = !!val;
      }
    }
  },

  readableHighWaterMark: {
    enumerable: false,
    get: function() {
      return this._readableState.highWaterMark;
    }
  },

  readableBuffer: {
    enumerable: false,
    get: function() {
      return this._readableState && this._readableState.buffer;
    }
  },

  readableFlowing: {
    enumerable: false,
    get: function() {
      return this._readableState.flowing;
    },
    set: function(state) {
      if (this._readableState) {
        this._readableState.flowing = state;
      }
    }
  },

  readableLength: {
    enumerable: false,
    get() {
      return this._readableState.length;
    }
  },

  readableObjectMode: {
    enumerable: false,
    get() {
      return this._readableState ? this._readableState.objectMode : false;
    }
  },

  readableEncoding: {
    enumerable: false,
    get() {
      return this._readableState ? this._readableState.encoding : null;
    }
  },

  destroyed: {
    enumerable: false,
    get() {
      if (this._readableState === undefined) {
        return false;
      }
      return this._readableState.destroyed;
    },
    set(value) {
      // We ignore the value if the stream
      // has not been initialized yet.
      if (!this._readableState) {
        return;
      }

      // Backward compatibility, the user is explicitly
      // managing destroyed.
      this._readableState.destroyed = value;
    }
  },

  readableEnded: {
    enumerable: false,
    get() {
      return this._readableState ? this._readableState.endEmitted : false;
    }
  },

});

ObjectDefineProperties(ReadableState.prototype, {
  // Legacy getter for `pipesCount`.
  pipesCount: {
    get() {
      return this.pipes.length;
    }
  },

  // Legacy property for `paused`.
  paused: {
    get() {
      return this[kPaused] !== false;
    },
    set(value) {
      this[kPaused] = !!value;
    }
  }
});

// Exposed for testing purposes only.
Readable._fromList = fromList;

// Pluck off n bytes from an array of buffers.
// Length is the combined lengths of all the buffers in the list.
// This function is designed to be inlinable, so please take care when making
// changes to the function body.
function fromList(n, state) {
  // nothing buffered.
  if (state.length === 0)
    return null;

  let ret;
  if (state.objectMode)
    ret = state.buffer.shift();
  else if (!n || n >= state.length) {
    // Read it all, truncate the list.
    if (state.decoder)
      ret = state.buffer.join('');
    else if (state.buffer.length === 1)
      ret = state.buffer.first();
    else
      ret = state.buffer.concat(state.length);
    state.buffer.clear();
  } else {
    // read part of list.
    ret = state.buffer.consume(n, state.decoder);
  }

  return ret;
}

function endReadable(stream) {
  const state = stream._readableState;

  debug('endReadable', state.endEmitted);
  if (!state.endEmitted) {
    state.ended = true;
    process.nextTick(endReadableNT, state, stream);
  }
}

function endReadableNT(state, stream) {
  debug('endReadableNT', state.endEmitted, state.length);

  // Check that we didn't get one last unshift.
  if (!state.errorEmitted && !state.closeEmitted &&
      !state.endEmitted && state.length === 0) {
    state.endEmitted = true;
    stream.emit('end');

    if (stream.writable && stream.allowHalfOpen === false) {
      process.nextTick(endWritableNT, state, stream);
    } else if (state.autoDestroy) {
      // In case of duplex streams we need a way to detect
      // if the writable side is ready for autoDestroy as well.
      const wState = stream._writableState;
      const autoDestroy = !wState || (
        wState.autoDestroy &&
        // We don't expect the writable to ever 'finish'
        // if writable is explicitly set to false.
        (wState.finished || wState.writable === false)
      );

      if (autoDestroy) {
        stream.destroy();
      }
    }
  }
}

function endWritableNT(state, stream) {
  const writable = stream.writable && !stream.writableEnded &&
    !stream.destroyed;
  if (writable) {
    stream.end();
  }
}

Readable.from = function(iterable, opts) {
  if (from === undefined) {
    from = require('internal/streams/from');
  }
  return from(Readable, iterable, opts);
};