node/lib/_http_outgoing.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 assert = require('assert').ok;
const Stream = require('stream');
const timers = require('timers');
const util = require('util');
const internalUtil = require('internal/util');
const Buffer = require('buffer').Buffer;
const common = require('_http_common');
const checkIsHttpToken = common._checkIsHttpToken;
const checkInvalidHeaderChar = common._checkInvalidHeaderChar;
const outHeadersKey = require('internal/http').outHeadersKey;
const StorageObject = require('internal/querystring').StorageObject;

const CRLF = common.CRLF;
const debug = common.debug;

var RE_FIELDS = new RegExp('^(?:Connection|Transfer-Encoding|Content-Length|' +
                           'Date|Expect|Trailer|Upgrade)$', 'i');
var RE_CONN_VALUES = /(?:^|\W)close|upgrade(?:$|\W)/ig;
var RE_TE_CHUNKED = common.chunkExpression;

// isCookieField performs a case-insensitive comparison of a provided string
// against the word "cookie." This method (at least as of V8 5.4) is faster than
// the equivalent case-insensitive regexp, even if isCookieField does not get
// inlined.
function isCookieField(s) {
  if (s.length !== 6) return false;
  var ch = s.charCodeAt(0);
  if (ch !== 99 && ch !== 67) return false;
  ch = s.charCodeAt(1);
  if (ch !== 111 && ch !== 79) return false;
  ch = s.charCodeAt(2);
  if (ch !== 111 && ch !== 79) return false;
  ch = s.charCodeAt(3);
  if (ch !== 107 && ch !== 75) return false;
  ch = s.charCodeAt(4);
  if (ch !== 105 && ch !== 73) return false;
  ch = s.charCodeAt(5);
  if (ch !== 101 && ch !== 69) return false;
  return true;
}

var dateCache;
function utcDate() {
  if (!dateCache) {
    var d = new Date();
    dateCache = d.toUTCString();
    timers.enroll(utcDate, 1000 - d.getMilliseconds());
    timers._unrefActive(utcDate);
  }
  return dateCache;
}
utcDate._onTimeout = function _onTimeout() {
  dateCache = undefined;
};


function OutgoingMessage() {
  Stream.call(this);

  // Queue that holds all currently pending data, until the response will be
  // assigned to the socket (until it will its turn in the HTTP pipeline).
  this.output = [];
  this.outputEncodings = [];
  this.outputCallbacks = [];

  // `outputSize` is an approximate measure of how much data is queued on this
  // response. `_onPendingData` will be invoked to update similar global
  // per-connection counter. That counter will be used to pause/unpause the
  // TCP socket and HTTP Parser and thus handle the backpressure.
  this.outputSize = 0;

  this.writable = true;

  this._last = false;
  this.upgrading = false;
  this.chunkedEncoding = false;
  this.shouldKeepAlive = true;
  this.useChunkedEncodingByDefault = true;
  this.sendDate = false;
  this._removedConnection = false;
  this._removedContLen = false;
  this._removedTE = false;

  this._contentLength = null;
  this._hasBody = true;
  this._trailer = '';

  this.finished = false;
  this._headerSent = false;

  this.socket = null;
  this.connection = null;
  this._header = null;
  this[outHeadersKey] = null;

  this._onPendingData = null;
}
util.inherits(OutgoingMessage, Stream);


Object.defineProperty(OutgoingMessage.prototype, '_headers', {
  get: function() {
    return this.getHeaders();
  },
  set: function(val) {
    if (val == null) {
      this[outHeadersKey] = null;
    } else if (typeof val === 'object') {
      const headers = this[outHeadersKey] = {};
      const keys = Object.keys(val);
      for (var i = 0; i < keys.length; ++i) {
        const name = keys[i];
        headers[name.toLowerCase()] = [name, val[name]];
      }
    }
  }
});

Object.defineProperty(OutgoingMessage.prototype, '_headerNames', {
  get: function() {
    const headers = this[outHeadersKey];
    if (headers) {
      const out = new StorageObject();
      const keys = Object.keys(headers);
      for (var i = 0; i < keys.length; ++i) {
        const key = keys[i];
        const val = headers[key][0];
        out[key] = val;
      }
      return out;
    } else {
      return headers;
    }
  },
  set: function(val) {
    if (typeof val === 'object' && val !== null) {
      const headers = this[outHeadersKey];
      if (!headers)
        return;
      const keys = Object.keys(val);
      for (var i = 0; i < keys.length; ++i) {
        const header = headers[keys[i]];
        if (header)
          header[0] = val[keys[i]];
      }
    }
  }
});


OutgoingMessage.prototype._renderHeaders = function _renderHeaders() {
  if (this._header) {
    throw new Error('Can\'t render headers after they are sent to the client');
  }

  var headersMap = this[outHeadersKey];
  if (!headersMap) return {};

  var headers = {};
  var keys = Object.keys(headersMap);

  for (var i = 0, l = keys.length; i < l; i++) {
    var key = keys[i];
    headers[headersMap[key][0]] = headersMap[key][1];
  }
  return headers;
};


exports.OutgoingMessage = OutgoingMessage;


OutgoingMessage.prototype.setTimeout = function setTimeout(msecs, callback) {

  if (callback) {
    this.on('timeout', callback);
  }

  if (!this.socket) {
    this.once('socket', function(socket) {
      socket.setTimeout(msecs);
    });
  } else {
    this.socket.setTimeout(msecs);
  }
  return this;
};


// It's possible that the socket will be destroyed, and removed from
// any messages, before ever calling this.  In that case, just skip
// it, since something else is destroying this connection anyway.
OutgoingMessage.prototype.destroy = function destroy(error) {
  if (this.socket)
    this.socket.destroy(error);
  else
    this.once('socket', function(socket) {
      socket.destroy(error);
    });
};


// This abstract either writing directly to the socket or buffering it.
OutgoingMessage.prototype._send = function _send(data, encoding, callback) {
  // This is a shameful hack to get the headers and first body chunk onto
  // the same packet. Future versions of Node are going to take care of
  // this at a lower level and in a more general way.
  if (!this._headerSent) {
    if (typeof data === 'string' &&
        encoding !== 'hex' &&
        encoding !== 'base64') {
      data = this._header + data;
    } else {
      this.output.unshift(this._header);
      this.outputEncodings.unshift('latin1');
      this.outputCallbacks.unshift(null);
      this.outputSize += this._header.length;
      if (typeof this._onPendingData === 'function')
        this._onPendingData(this._header.length);
    }
    this._headerSent = true;
  }
  return this._writeRaw(data, encoding, callback);
};


OutgoingMessage.prototype._writeRaw = _writeRaw;
function _writeRaw(data, encoding, callback) {
  const conn = this.connection;
  if (conn && conn.destroyed) {
    // The socket was destroyed. If we're still trying to write to it,
    // then we haven't gotten the 'close' event yet.
    return false;
  }

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

  if (conn && conn._httpMessage === this && conn.writable && !conn.destroyed) {
    // There might be pending data in the this.output buffer.
    if (this.output.length) {
      this._flushOutput(conn);
    } else if (!data.length) {
      if (typeof callback === 'function')
        process.nextTick(callback);
      return true;
    }
    // Directly write to socket.
    return conn.write(data, encoding, callback);
  }
  // Buffer, as long as we're not destroyed.
  return this._buffer(data, encoding, callback);
}


OutgoingMessage.prototype._buffer = function _buffer(data, encoding, callback) {
  this.output.push(data);
  this.outputEncodings.push(encoding);
  this.outputCallbacks.push(callback);
  this.outputSize += data.length;
  if (typeof this._onPendingData === 'function')
    this._onPendingData(data.length);
  return false;
};


OutgoingMessage.prototype._storeHeader = _storeHeader;
function _storeHeader(firstLine, headers) {
  // firstLine in the case of request is: 'GET /index.html HTTP/1.1\r\n'
  // in the case of response it is: 'HTTP/1.1 200 OK\r\n'
  var state = {
    connection: false,
    connUpgrade: false,
    contLen: false,
    te: false,
    date: false,
    expect: false,
    trailer: false,
    upgrade: false,
    header: firstLine
  };

  var field;
  var key;
  var value;
  var i;
  var j;
  if (headers === this[outHeadersKey]) {
    for (key in headers) {
      var entry = headers[key];
      field = entry[0];
      value = entry[1];

      if (value instanceof Array) {
        if (value.length < 2 || !isCookieField(field)) {
          for (j = 0; j < value.length; j++)
            storeHeader(this, state, field, value[j], false);
          continue;
        }
        value = value.join('; ');
      }
      storeHeader(this, state, field, value, false);
    }
  } else if (headers instanceof Array) {
    for (i = 0; i < headers.length; i++) {
      field = headers[i][0];
      value = headers[i][1];

      if (value instanceof Array) {
        for (j = 0; j < value.length; j++) {
          storeHeader(this, state, field, value[j], true);
        }
      } else {
        storeHeader(this, state, field, value, true);
      }
    }
  } else if (headers) {
    var keys = Object.keys(headers);
    for (i = 0; i < keys.length; i++) {
      field = keys[i];
      value = headers[field];

      if (value instanceof Array) {
        if (value.length < 2 || !isCookieField(field)) {
          for (j = 0; j < value.length; j++)
            storeHeader(this, state, field, value[j], true);
          continue;
        }
        value = value.join('; ');
      }
      storeHeader(this, state, field, value, true);
    }
  }

  // Are we upgrading the connection?
  if (state.connUpgrade && state.upgrade)
    this.upgrading = true;

  // Date header
  if (this.sendDate && !state.date) {
    state.header += 'Date: ' + utcDate() + CRLF;
  }

  // Force the connection to close when the response is a 204 No Content or
  // a 304 Not Modified and the user has set a "Transfer-Encoding: chunked"
  // header.
  //
  // RFC 2616 mandates that 204 and 304 responses MUST NOT have a body but
  // node.js used to send out a zero chunk anyway to accommodate clients
  // that don't have special handling for those responses.
  //
  // It was pointed out that this might confuse reverse proxies to the point
  // of creating security liabilities, so suppress the zero chunk and force
  // the connection to close.
  var statusCode = this.statusCode;
  if ((statusCode === 204 || statusCode === 304) && this.chunkedEncoding) {
    debug(statusCode + ' response should not use chunked encoding,' +
          ' closing connection.');
    this.chunkedEncoding = false;
    this.shouldKeepAlive = false;
  }

  // keep-alive logic
  if (this._removedConnection) {
    this._last = true;
    this.shouldKeepAlive = false;
  } else if (!state.connection) {
    var shouldSendKeepAlive = this.shouldKeepAlive &&
        (state.contLen || this.useChunkedEncodingByDefault || this.agent);
    if (shouldSendKeepAlive) {
      state.header += 'Connection: keep-alive\r\n';
    } else {
      this._last = true;
      state.header += 'Connection: close\r\n';
    }
  }

  if (!state.contLen && !state.te) {
    if (!this._hasBody) {
      // Make sure we don't end the 0\r\n\r\n at the end of the message.
      this.chunkedEncoding = false;
    } else if (!this.useChunkedEncodingByDefault) {
      this._last = true;
    } else {
      if (!state.trailer &&
          !this._removedContLen &&
          typeof this._contentLength === 'number') {
        state.header += 'Content-Length: ' + this._contentLength + CRLF;
      } else if (!this._removedTE) {
        state.header += 'Transfer-Encoding: chunked\r\n';
        this.chunkedEncoding = true;
      } else {
        // We should only be able to get here if both Content-Length and
        // Transfer-Encoding are removed by the user.
        // See: test/parallel/test-http-remove-header-stays-removed.js
        debug('Both Content-Length and Transfer-Encoding are removed');
      }
    }
  }

  this._header = state.header + CRLF;
  this._headerSent = false;

  // wait until the first body chunk, or close(), is sent to flush,
  // UNLESS we're sending Expect: 100-continue.
  if (state.expect) this._send('');
}

function storeHeader(self, state, key, value, validate) {
  if (validate) {
    if (typeof key !== 'string' || !key || !checkIsHttpToken(key)) {
      throw new TypeError(
        'Header name must be a valid HTTP Token ["' + key + '"]');
    }
    if (value === undefined) {
      throw new Error('Header "%s" value must not be undefined', key);
    } else if (checkInvalidHeaderChar(value)) {
      debug('Header "%s" contains invalid characters', key);
      throw new TypeError('The header content contains invalid characters');
    }
  }
  state.header += key + ': ' + escapeHeaderValue(value) + CRLF;
  matchHeader(self, state, key, value);
}

function matchConnValue(self, state, value) {
  var sawClose = false;
  var m = RE_CONN_VALUES.exec(value);
  while (m) {
    if (m[0].length === 5)
      sawClose = true;
    else
      state.connUpgrade = true;
    m = RE_CONN_VALUES.exec(value);
  }
  if (sawClose)
    self._last = true;
  else
    self.shouldKeepAlive = true;
}

function matchHeader(self, state, field, value) {
  var m = RE_FIELDS.exec(field);
  if (!m)
    return;
  var len = m[0].length;
  if (len === 10) {
    state.connection = true;
    matchConnValue(self, state, value);
  } else if (len === 17) {
    state.te = true;
    if (RE_TE_CHUNKED.test(value)) self.chunkedEncoding = true;
  } else if (len === 14) {
    state.contLen = true;
  } else if (len === 4) {
    state.date = true;
  } else if (len === 6) {
    state.expect = true;
  } else if (len === 7) {
    var ch = m[0].charCodeAt(0);
    if (ch === 85 || ch === 117)
      state.upgrade = true;
    else
      state.trailer = true;
  }
}

function validateHeader(msg, name, value) {
  if (typeof name !== 'string' || !name || !checkIsHttpToken(name))
    throw new TypeError(
      'Header name must be a valid HTTP Token ["' + name + '"]');
  if (value === undefined)
    throw new Error('"value" required in setHeader("' + name + '", value)');
  if (msg._header)
    throw new Error('Can\'t set headers after they are sent.');
  if (checkInvalidHeaderChar(value)) {
    debug('Header "%s" contains invalid characters', name);
    throw new TypeError('The header content contains invalid characters');
  }
}
OutgoingMessage.prototype.setHeader = function setHeader(name, value) {
  validateHeader(this, name, value);

  if (!this[outHeadersKey])
    this[outHeadersKey] = {};

  const key = name.toLowerCase();
  this[outHeadersKey][key] = [name, value];

  switch (key.length) {
    case 10:
      if (key === 'connection')
        this._removedConnection = false;
      break;
    case 14:
      if (key === 'content-length')
        this._removedContLen = false;
      break;
    case 17:
      if (key === 'transfer-encoding')
        this._removedTE = false;
      break;
  }
};


OutgoingMessage.prototype.getHeader = function getHeader(name) {
  if (typeof name !== 'string') {
    throw new TypeError('"name" argument must be a string');
  }

  if (!this[outHeadersKey]) return;

  var entry = this[outHeadersKey][name.toLowerCase()];
  if (!entry)
    return;
  return entry[1];
};


// Returns an array of the names of the current outgoing headers.
OutgoingMessage.prototype.getHeaderNames = function getHeaderNames() {
  return (this[outHeadersKey] ? Object.keys(this[outHeadersKey]) : []);
};


// Returns a shallow copy of the current outgoing headers.
OutgoingMessage.prototype.getHeaders = function getHeaders() {
  const headers = this[outHeadersKey];
  const ret = new StorageObject();
  if (headers) {
    const keys = Object.keys(headers);
    for (var i = 0; i < keys.length; ++i) {
      const key = keys[i];
      const val = headers[key][1];
      ret[key] = val;
    }
  }
  return ret;
};


OutgoingMessage.prototype.hasHeader = function hasHeader(name) {
  if (typeof name !== 'string') {
    throw new TypeError('"name" argument must be a string');
  }

  return !!(this[outHeadersKey] && this[outHeadersKey][name.toLowerCase()]);
};


OutgoingMessage.prototype.removeHeader = function removeHeader(name) {
  if (typeof name !== 'string') {
    throw new TypeError('"name" argument must be a string');
  }

  if (this._header) {
    throw new Error('Can\'t remove headers after they are sent');
  }

  var key = name.toLowerCase();

  switch (key.length) {
    case 10:
      if (key === 'connection')
        this._removedConnection = true;
      break;
    case 14:
      if (key === 'content-length')
        this._removedContLen = true;
      break;
    case 17:
      if (key === 'transfer-encoding')
        this._removedTE = true;
      break;
    case 4:
      if (key === 'date')
        this.sendDate = false;
      break;
  }

  if (this[outHeadersKey]) {
    delete this[outHeadersKey][key];
  }
};


OutgoingMessage.prototype._implicitHeader = function _implicitHeader() {
  throw new Error('_implicitHeader() method is not implemented');
};

Object.defineProperty(OutgoingMessage.prototype, 'headersSent', {
  configurable: true,
  enumerable: true,
  get: function() { return !!this._header; }
});


const crlf_buf = Buffer.from('\r\n');
OutgoingMessage.prototype.write = function write(chunk, encoding, callback) {
  if (this.finished) {
    var err = new Error('write after end');
    process.nextTick(writeAfterEndNT, this, err, callback);

    return true;
  }

  if (!this._header) {
    this._implicitHeader();
  }

  if (!this._hasBody) {
    debug('This type of response MUST NOT have a body. ' +
          'Ignoring write() calls.');
    return true;
  }

  if (typeof chunk !== 'string' && !(chunk instanceof Buffer)) {
    throw new TypeError('First argument must be a string or Buffer');
  }


  // If we get an empty string or buffer, then just do nothing, and
  // signal the user to keep writing.
  if (chunk.length === 0) return true;

  var len, ret;
  if (this.chunkedEncoding) {
    if (typeof chunk === 'string' &&
        encoding !== 'hex' &&
        encoding !== 'base64' &&
        encoding !== 'latin1') {
      len = Buffer.byteLength(chunk, encoding);
      chunk = len.toString(16) + CRLF + chunk + CRLF;
      ret = this._send(chunk, encoding, callback);
    } else {
      // buffer, or a non-toString-friendly encoding
      if (typeof chunk === 'string')
        len = Buffer.byteLength(chunk, encoding);
      else
        len = chunk.length;

      if (this.connection && !this.connection.corked) {
        this.connection.cork();
        process.nextTick(connectionCorkNT, this.connection);
      }

      this._send(len.toString(16), 'latin1', null);
      this._send(crlf_buf, null, null);
      this._send(chunk, encoding, null);
      ret = this._send(crlf_buf, null, callback);
    }
  } else {
    ret = this._send(chunk, encoding, callback);
  }

  debug('write ret = ' + ret);
  return ret;
};


function writeAfterEndNT(self, err, callback) {
  self.emit('error', err);
  if (callback) callback(err);
}


function connectionCorkNT(conn) {
  conn.uncork();
}


function escapeHeaderValue(value) {
  // Protect against response splitting. The regex test is there to
  // minimize the performance impact in the common case.
  return /[\r\n]/.test(value) ? value.replace(/[\r\n]+[ \t]*/g, '') : value;
}


OutgoingMessage.prototype.addTrailers = function addTrailers(headers) {
  this._trailer = '';
  var keys = Object.keys(headers);
  var isArray = Array.isArray(headers);
  var field, value;
  for (var i = 0, l = keys.length; i < l; i++) {
    var key = keys[i];
    if (isArray) {
      field = headers[key][0];
      value = headers[key][1];
    } else {
      field = key;
      value = headers[key];
    }
    if (typeof field !== 'string' || !field || !checkIsHttpToken(field)) {
      throw new TypeError(
        'Trailer name must be a valid HTTP Token ["' + field + '"]');
    }
    if (checkInvalidHeaderChar(value)) {
      debug('Trailer "%s" contains invalid characters', field);
      throw new TypeError('The trailer content contains invalid characters');
    }
    this._trailer += field + ': ' + escapeHeaderValue(value) + CRLF;
  }
};

function onFinish(outmsg) {
  outmsg.emit('finish');
}

OutgoingMessage.prototype.end = function end(data, encoding, callback) {
  if (typeof data === 'function') {
    callback = data;
    data = null;
  } else if (typeof encoding === 'function') {
    callback = encoding;
    encoding = null;
  }

  if (data && typeof data !== 'string' && !(data instanceof Buffer)) {
    throw new TypeError('First argument must be a string or Buffer');
  }

  if (this.finished) {
    return false;
  }

  if (!this._header) {
    if (data) {
      if (typeof data === 'string')
        this._contentLength = Buffer.byteLength(data, encoding);
      else
        this._contentLength = data.length;
    } else {
      this._contentLength = 0;
    }
    this._implicitHeader();
  }

  if (data && !this._hasBody) {
    debug('This type of response MUST NOT have a body. ' +
          'Ignoring data passed to end().');
    data = null;
  }

  if (this.connection && data)
    this.connection.cork();

  if (data) {
    // Normal body write.
    this.write(data, encoding);
  }

  if (typeof callback === 'function')
    this.once('finish', callback);

  var finish = onFinish.bind(undefined, this);

  var ret;
  if (this._hasBody && this.chunkedEncoding) {
    ret = this._send('0\r\n' + this._trailer + '\r\n', 'latin1', finish);
  } else {
    // Force a flush, HACK.
    ret = this._send('', 'latin1', finish);
  }

  if (this.connection && data)
    this.connection.uncork();

  this.finished = true;

  // There is the first message on the outgoing queue, and we've sent
  // everything to the socket.
  debug('outgoing message end.');
  if (this.output.length === 0 &&
      this.connection &&
      this.connection._httpMessage === this) {
    this._finish();
  }

  return ret;
};


OutgoingMessage.prototype._finish = function _finish() {
  assert(this.connection);
  this.emit('prefinish');
};


// This logic is probably a bit confusing. Let me explain a bit:
//
// In both HTTP servers and clients it is possible to queue up several
// outgoing messages. This is easiest to imagine in the case of a client.
// Take the following situation:
//
//    req1 = client.request('GET', '/');
//    req2 = client.request('POST', '/');
//
// When the user does
//
//   req2.write('hello world\n');
//
// it's possible that the first request has not been completely flushed to
// the socket yet. Thus the outgoing messages need to be prepared to queue
// up data internally before sending it on further to the socket's queue.
//
// This function, outgoingFlush(), is called by both the Server and Client
// to attempt to flush any pending messages out to the socket.
OutgoingMessage.prototype._flush = function _flush() {
  var socket = this.socket;
  var ret;

  if (socket && socket.writable) {
    // There might be remaining data in this.output; write it out
    ret = this._flushOutput(socket);

    if (this.finished) {
      // This is a queue to the server or client to bring in the next this.
      this._finish();
    } else if (ret) {
      // This is necessary to prevent https from breaking
      this.emit('drain');
    }
  }
};

OutgoingMessage.prototype._flushOutput = function _flushOutput(socket) {
  var ret;
  var outputLength = this.output.length;
  if (outputLength <= 0)
    return ret;

  var output = this.output;
  var outputEncodings = this.outputEncodings;
  var outputCallbacks = this.outputCallbacks;
  socket.cork();
  for (var i = 0; i < outputLength; i++) {
    ret = socket.write(output[i], outputEncodings[i], outputCallbacks[i]);
  }
  socket.uncork();

  this.output = [];
  this.outputEncodings = [];
  this.outputCallbacks = [];
  if (typeof this._onPendingData === 'function')
    this._onPendingData(-this.outputSize);
  this.outputSize = 0;

  return ret;
};


OutgoingMessage.prototype.flushHeaders = function flushHeaders() {
  if (!this._header) {
    this._implicitHeader();
  }

  // Force-flush the headers.
  this._send('');
};

OutgoingMessage.prototype.flush = internalUtil.deprecate(function() {
  this.flushHeaders();
}, 'OutgoingMessage.flush is deprecated. Use flushHeaders instead.', 'DEP0001');