nodejs/lib/_stream_writable.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 bit simpler than readable streams.
// Implement an async ._write(chunk, cb), and it'll handle all
// the drain event emission and buffering.

module.exports = Writable;
Writable.WritableState = WritableState;

var util = require('util');
var Stream = require('stream');

util.inherits(Writable, Stream);

function WriteReq(chunk, encoding, cb) {
  this.chunk = chunk;
  this.encoding = encoding;
  this.callback = cb;
}

function WritableState(options, stream) {
  options = options || {};

  // the point at which write() starts returning false
  // Note: 0 is a valid value, means that we always return false if
  // the entire buffer is not flushed immediately on write()
  var hwm = options.highWaterMark;
  this.highWaterMark = (hwm || hwm === 0) ? hwm : 16 * 1024;

  // object stream flag to indicate whether or not this stream
  // contains buffers or objects.
  this.objectMode = !!options.objectMode;

  // cast to ints.
  this.highWaterMark = ~~this.highWaterMark;

  this.needDrain = false;
  // at the start of calling end()
  this.ending = false;
  // when end() has been called, and returned
  this.ended = false;
  // when 'finish' is emitted
  this.finished = false;

  // should we decode strings into buffers before passing to _write?
  // this is here so that some node-core streams can optimize string
  // handling at a lower level.
  var noDecode = options.decodeStrings === false;
  this.decodeStrings = !noDecode;

  // 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.defaultEncoding || 'utf8';

  // not an actual buffer we keep track of, but a measurement
  // of how much we're waiting to get pushed to some underlying
  // socket or file.
  this.length = 0;

  // a flag to see when we're in the middle of a write.
  this.writing = false;

  // a flag to be able to tell if the onwrite cb is called immediately,
  // or on a later tick.  We set this to true at first, becuase any
  // actions that shouldn't happen until "later" should generally also
  // not happen before the first write call.
  this.sync = true;

  // a flag to know if we're processing previously buffered items, which
  // may call the _write() callback in the same tick, so that we don't
  // end up in an overlapped onwrite situation.
  this.bufferProcessing = false;

  // the callback that's passed to _write(chunk,cb)
  this.onwrite = function(er) {
    onwrite(stream, er);
  };

  // the callback that the user supplies to write(chunk,encoding,cb)
  this.writecb = null;

  // the amount that is being written when _write is called.
  this.writelen = 0;

  this.buffer = [];

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

function Writable(options) {
  // Writable ctor is applied to Duplexes, though they're not
  // instanceof Writable, they're instanceof Readable.
  if (!(this instanceof Writable) && !(this instanceof Stream.Duplex))
    return new Writable(options);

  this._writableState = new WritableState(options, this);

  // legacy.
  this.writable = true;

  Stream.call(this);
}

// Otherwise people can pipe Writable streams, which is just wrong.
Writable.prototype.pipe = function() {
  this.emit('error', new Error('Cannot pipe. Not readable.'));
};


function writeAfterEnd(stream, state, cb) {
  var er = new Error('write after end');
  // TODO: defer error events consistently everywhere, not just the cb
  stream.emit('error', er);
  process.nextTick(function() {
    cb(er);
  });
}

// If we get something that is not a buffer, string, null, or undefined,
// and we're not in objectMode, then that's an error.
// Otherwise stream chunks are all considered to be of length=1, and the
// watermarks determine how many objects to keep in the buffer, rather than
// how many bytes or characters.
function validChunk(stream, state, chunk, cb) {
  var valid = true;
  if (!Buffer.isBuffer(chunk) &&
      'string' !== typeof chunk &&
      chunk !== null &&
      chunk !== undefined &&
      !state.objectMode) {
    var er = new TypeError('Invalid non-string/buffer chunk');
    stream.emit('error', er);
    process.nextTick(function() {
      cb(er);
    });
    valid = false;
  }
  return valid;
}

Writable.prototype.write = function(chunk, encoding, cb) {
  var state = this._writableState;
  var ret = false;

  if (typeof encoding === 'function') {
    cb = encoding;
    encoding = null;
  }

  if (Buffer.isBuffer(chunk))
    encoding = 'buffer';
  else if (!encoding)
    encoding = state.defaultEncoding;

  if (typeof cb !== 'function')
    cb = function() {};

  if (state.ended)
    writeAfterEnd(this, state, cb);
  else if (validChunk(this, state, chunk, cb))
    ret = writeOrBuffer(this, state, chunk, encoding, cb);

  return ret;
};

function decodeChunk(state, chunk, encoding) {
  if (!state.objectMode &&
      state.decodeStrings !== false &&
      typeof chunk === 'string') {
    chunk = new Buffer(chunk, encoding);
  }
  return chunk;
}

// if we're already writing something, then just put this
// in the queue, and wait our turn.  Otherwise, call _write
// If we return false, then we need a drain event, so set that flag.
function writeOrBuffer(stream, state, chunk, encoding, cb) {
  chunk = decodeChunk(state, chunk, encoding);
  if (Buffer.isBuffer(chunk))
    encoding = 'buffer';
  var len = state.objectMode ? 1 : chunk.length;

  state.length += len;

  var ret = state.length < state.highWaterMark;
  // we must ensure that previous needDrain will not be reset to false.
  if (!ret)
    state.needDrain = true;

  if (state.writing)
    state.buffer.push(new WriteReq(chunk, encoding, cb));
  else
    doWrite(stream, state, len, chunk, encoding, cb);

  return ret;
}

function doWrite(stream, state, len, chunk, encoding, cb) {
  state.writelen = len;
  state.writecb = cb;
  state.writing = true;
  state.sync = true;
  stream._write(chunk, encoding, state.onwrite);
  state.sync = false;
}

function onwriteError(stream, state, sync, er, cb) {
  if (sync)
    process.nextTick(function() {
      cb(er);
    });
  else
    cb(er);

  stream._writableState.errorEmitted = true;
  stream.emit('error', er);
}

function onwriteStateUpdate(state) {
  state.writing = false;
  state.writecb = null;
  state.length -= state.writelen;
  state.writelen = 0;
}

function onwrite(stream, er) {
  var state = stream._writableState;
  var sync = state.sync;
  var cb = state.writecb;

  onwriteStateUpdate(state);

  if (er)
    onwriteError(stream, state, sync, er, cb);
  else {
    // Check if we're actually ready to finish, but don't emit yet
    var finished = needFinish(stream, state);

    if (!finished && !state.bufferProcessing && state.buffer.length)
      clearBuffer(stream, state);

    if (sync) {
      process.nextTick(function() {
        afterWrite(stream, state, finished, cb);
      });
    } else {
      afterWrite(stream, state, finished, cb);
    }
  }
}

function afterWrite(stream, state, finished, cb) {
  if (!finished)
    onwriteDrain(stream, state);
  cb();
  if (finished)
    finishMaybe(stream, state);
}

// Must force callback to be called on nextTick, so that we don't
// emit 'drain' before the write() consumer gets the 'false' return
// value, and has a chance to attach a 'drain' listener.
function onwriteDrain(stream, state) {
  if (state.length === 0 && state.needDrain) {
    state.needDrain = false;
    stream.emit('drain');
  }
}


// if there's something in the buffer waiting, then process it
function clearBuffer(stream, state) {
  state.bufferProcessing = true;

  for (var c = 0; c < state.buffer.length; c++) {
    var entry = state.buffer[c];
    var chunk = entry.chunk;
    var encoding = entry.encoding;
    var cb = entry.callback;
    var len = state.objectMode ? 1 : chunk.length;

    doWrite(stream, state, len, chunk, encoding, cb);

    // if we didn't call the onwrite immediately, then
    // it means that we need to wait until it does.
    // also, that means that the chunk and cb are currently
    // being processed, so move the buffer counter past them.
    if (state.writing) {
      c++;
      break;
    }
  }

  state.bufferProcessing = false;
  if (c < state.buffer.length)
    state.buffer = state.buffer.slice(c);
  else
    state.buffer.length = 0;
}

Writable.prototype._write = function(chunk, encoding, cb) {
  cb(new Error('not implemented'));
};

Writable.prototype.end = function(chunk, encoding, cb) {
  var state = this._writableState;

  if (typeof chunk === 'function') {
    cb = chunk;
    chunk = null;
    encoding = null;
  } else if (typeof encoding === 'function') {
    cb = encoding;
    encoding = null;
  }

  if (typeof chunk !== 'undefined' && chunk !== null)
    this.write(chunk, encoding);

  // ignore unnecessary end() calls.
  if (!state.ending && !state.finished)
    endWritable(this, state, cb);
};


function needFinish(stream, state) {
  return (state.ending &&
          state.length === 0 &&
          !state.finished &&
          !state.writing);
}

function finishMaybe(stream, state) {
  var need = needFinish(stream, state);
  if (need) {
    state.finished = true;
    stream.emit('finish');
  }
  return need;
}

function endWritable(stream, state, cb) {
  state.ending = true;
  finishMaybe(stream, state);
  if (cb) {
    if (state.finished)
      process.nextTick(cb);
    else
      stream.once('finish', cb);
  }
  state.ended = true;
}