_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 {
  ArrayIsArray,
  ObjectCreate,
  ObjectDefineProperty,
  ObjectKeys,
  ObjectPrototypeHasOwnProperty,
  ObjectSetPrototypeOf,
  Symbol,
} = primordials;

const { getDefaultHighWaterMark } = require('internal/streams/state');
const assert = require('internal/assert');
const EE = require('events');
const Stream = require('stream');
const internalUtil = require('internal/util');
const { kOutHeaders, utcDate, kNeedDrain } = require('internal/http');
const { Buffer } = require('buffer');
const common = require('_http_common');
const checkIsHttpToken = common._checkIsHttpToken;
const checkInvalidHeaderChar = common._checkInvalidHeaderChar;
const {
  defaultTriggerAsyncIdScope,
  symbols: { async_id_symbol }
} = require('internal/async_hooks');
const {
  codes: {
    ERR_HTTP_HEADERS_SENT,
    ERR_HTTP_INVALID_HEADER_VALUE,
    ERR_HTTP_TRAILER_INVALID,
    ERR_INVALID_HTTP_TOKEN,
    ERR_INVALID_ARG_TYPE,
    ERR_INVALID_CHAR,
    ERR_METHOD_NOT_IMPLEMENTED,
    ERR_STREAM_CANNOT_PIPE,
    ERR_STREAM_ALREADY_FINISHED,
    ERR_STREAM_WRITE_AFTER_END,
    ERR_STREAM_NULL_VALUES,
    ERR_STREAM_DESTROYED
  },
  hideStackFrames
} = require('internal/errors');
const { validateString } = require('internal/validators');

const HIGH_WATER_MARK = getDefaultHighWaterMark();
const { CRLF, debug } = common;

const kCorked = Symbol('corked');

function nop() {}

const RE_CONN_CLOSE = /(?:^|\W)close(?:$|\W)/i;
const RE_TE_CHUNKED = common.chunkExpression;

// isCookieField performs a case-insensitive comparison of a provided string
// against the word "cookie." As of V8 6.6 this is faster than handrolling or
// using a case-insensitive RegExp.
function isCookieField(s) {
  return s.length === 6 && s.toLowerCase() === 'cookie';
}

function noopPendingOutput(amount) {}

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.outputData = [];

  // `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.destroyed = false;

  this._last = 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[kNeedDrain] = false;

  this.finished = false;
  this._headerSent = false;
  this[kCorked] = 0;

  this.socket = null;
  this._header = null;
  this[kOutHeaders] = null;

  this._onPendingData = noopPendingOutput;
}
ObjectSetPrototypeOf(OutgoingMessage.prototype, Stream.prototype);
ObjectSetPrototypeOf(OutgoingMessage, Stream);

ObjectDefineProperty(OutgoingMessage.prototype, 'writableFinished', {
  get() {
    return (
      this.finished &&
      this.outputSize === 0 &&
      (!this.socket || this.socket.writableLength === 0)
    );
  }
});

ObjectDefineProperty(OutgoingMessage.prototype, 'writableObjectMode', {
  get() {
    return false;
  }
});

ObjectDefineProperty(OutgoingMessage.prototype, 'writableLength', {
  get() {
    return this.outputSize + (this.socket ? this.socket.writableLength : 0);
  }
});

ObjectDefineProperty(OutgoingMessage.prototype, 'writableHighWaterMark', {
  get() {
    return this.socket ? this.socket.writableHighWaterMark : HIGH_WATER_MARK;
  }
});

ObjectDefineProperty(OutgoingMessage.prototype, 'writableCorked', {
  get() {
    const corked = this.socket ? this.socket.writableCorked : 0;
    return corked + this[kCorked];
  }
});

ObjectDefineProperty(OutgoingMessage.prototype, '_headers', {
  get: internalUtil.deprecate(function() {
    return this.getHeaders();
  }, 'OutgoingMessage.prototype._headers is deprecated', 'DEP0066'),
  set: internalUtil.deprecate(function(val) {
    if (val == null) {
      this[kOutHeaders] = null;
    } else if (typeof val === 'object') {
      const headers = this[kOutHeaders] = ObjectCreate(null);
      const keys = ObjectKeys(val);
      // Retain for(;;) loop for performance reasons
      // Refs: https://github.com/nodejs/node/pull/30958
      for (let i = 0; i < keys.length; ++i) {
        const name = keys[i];
        headers[name.toLowerCase()] = [name, val[name]];
      }
    }
  }, 'OutgoingMessage.prototype._headers is deprecated', 'DEP0066')
});

ObjectDefineProperty(OutgoingMessage.prototype, 'connection', {
  get: function() {
    return this.socket;
  },
  set: function(val) {
    this.socket = val;
  }
});

ObjectDefineProperty(OutgoingMessage.prototype, '_headerNames', {
  get: internalUtil.deprecate(function() {
    const headers = this[kOutHeaders];
    if (headers !== null) {
      const out = ObjectCreate(null);
      const keys = ObjectKeys(headers);
      // Retain for(;;) loop for performance reasons
      // Refs: https://github.com/nodejs/node/pull/30958
      for (let i = 0; i < keys.length; ++i) {
        const key = keys[i];
        const val = headers[key][0];
        out[key] = val;
      }
      return out;
    }
    return null;
  }, 'OutgoingMessage.prototype._headerNames is deprecated', 'DEP0066'),
  set: internalUtil.deprecate(function(val) {
    if (typeof val === 'object' && val !== null) {
      const headers = this[kOutHeaders];
      if (!headers)
        return;
      const keys = ObjectKeys(val);
      // Retain for(;;) loop for performance reasons
      // Refs: https://github.com/nodejs/node/pull/30958
      for (let i = 0; i < keys.length; ++i) {
        const header = headers[keys[i]];
        if (header)
          header[0] = val[keys[i]];
      }
    }
  }, 'OutgoingMessage.prototype._headerNames is deprecated', 'DEP0066')
});


OutgoingMessage.prototype._renderHeaders = function _renderHeaders() {
  if (this._header) {
    throw new ERR_HTTP_HEADERS_SENT('render');
  }

  const headersMap = this[kOutHeaders];
  const headers = {};

  if (headersMap !== null) {
    const keys = ObjectKeys(headersMap);
    // Retain for(;;) loop for performance reasons
    // Refs: https://github.com/nodejs/node/pull/30958
    for (let i = 0, l = keys.length; i < l; i++) {
      const key = keys[i];
      headers[headersMap[key][0]] = headersMap[key][1];
    }
  }
  return headers;
};

OutgoingMessage.prototype.cork = function() {
  if (this.socket) {
    this.socket.cork();
  } else {
    this[kCorked]++;
  }
};

OutgoingMessage.prototype.uncork = function() {
  if (this.socket) {
    this.socket.uncork();
  } else if (this[kCorked]) {
    this[kCorked]--;
  }
};

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

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

  if (!this.socket) {
    this.once('socket', function socketSetTimeoutOnConnect(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.destroyed) {
    return;
  }
  this.destroyed = true;

  if (this.socket) {
    this.socket.destroy(error);
  } else {
    this.once('socket', function socketDestroyOnConnect(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 === 'utf8' || encoding === 'latin1' || !encoding)) {
      data = this._header + data;
    } else {
      const header = this._header;
      if (this.outputData.length === 0) {
        this.outputData = [{
          data: header,
          encoding: 'latin1',
          callback: null
        }];
      } else {
        this.outputData.unshift({
          data: header,
          encoding: 'latin1',
          callback: null
        });
      }
      this.outputSize += header.length;
      this._onPendingData(header.length);
    }
    this._headerSent = true;
  }
  return this._writeRaw(data, encoding, callback);
};


OutgoingMessage.prototype._writeRaw = _writeRaw;
function _writeRaw(data, encoding, callback) {
  const conn = this.socket;
  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) {
    // There might be pending data in the this.output buffer.
    if (this.outputData.length) {
      this._flushOutput(conn);
    }
    // Directly write to socket.
    return conn.write(data, encoding, callback);
  }
  // Buffer, as long as we're not destroyed.
  this.outputData.push({ data, encoding, callback });
  this.outputSize += data.length;
  this._onPendingData(data.length);
  return this.outputSize < HIGH_WATER_MARK;
}


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'
  const state = {
    connection: false,
    contLen: false,
    te: false,
    date: false,
    expect: false,
    trailer: false,
    header: firstLine
  };

  if (headers) {
    if (headers === this[kOutHeaders]) {
      for (const key in headers) {
        const entry = headers[key];
        processHeader(this, state, entry[0], entry[1], false);
      }
    } else if (ArrayIsArray(headers)) {
      for (const entry of headers) {
        processHeader(this, state, entry[0], entry[1], true);
      }
    } else {
      for (const key in headers) {
        if (ObjectPrototypeHasOwnProperty(headers, key)) {
          processHeader(this, state, key, headers[key], true);
        }
      }
    }
  }

  let { header } = state;

  // Date header
  if (this.sendDate && !state.date) {
    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.
  if (this.chunkedEncoding && (this.statusCode === 204 ||
                               this.statusCode === 304)) {
    debug(this.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) {
    const shouldSendKeepAlive = this.shouldKeepAlive &&
        (state.contLen || this.useChunkedEncodingByDefault || this.agent);
    if (shouldSendKeepAlive) {
      header += 'Connection: keep-alive\r\n';
    } else {
      this._last = true;
      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') {
      header += 'Content-Length: ' + this._contentLength + CRLF;
    } else if (!this._removedTE) {
      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');
    }
  }

  // Test non-chunked message does not have trailer header set,
  // message will be terminated by the first empty line after the
  // header fields, regardless of the header fields present in the
  // message, and thus cannot contain a message body or 'trailers'.
  if (this.chunkedEncoding !== true && state.trailer) {
    throw new ERR_HTTP_TRAILER_INVALID();
  }

  this._header = 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 processHeader(self, state, key, value, validate) {
  if (validate)
    validateHeaderName(key);
  if (ArrayIsArray(value)) {
    if (value.length < 2 || !isCookieField(key)) {
      // Retain for(;;) loop for performance reasons
      // Refs: https://github.com/nodejs/node/pull/30958
      for (let i = 0; i < value.length; i++)
        storeHeader(self, state, key, value[i], validate);
      return;
    }
    value = value.join('; ');
  }
  storeHeader(self, state, key, value, validate);
}

function storeHeader(self, state, key, value, validate) {
  if (validate)
    validateHeaderValue(key, value);
  state.header += key + ': ' + value + CRLF;
  matchHeader(self, state, key, value);
}

function matchHeader(self, state, field, value) {
  if (field.length < 4 || field.length > 17)
    return;
  field = field.toLowerCase();
  switch (field) {
    case 'connection':
      state.connection = true;
      self._removedConnection = false;
      if (RE_CONN_CLOSE.test(value))
        self._last = true;
      else
        self.shouldKeepAlive = true;
      break;
    case 'transfer-encoding':
      state.te = true;
      self._removedTE = false;
      if (RE_TE_CHUNKED.test(value)) self.chunkedEncoding = true;
      break;
    case 'content-length':
      state.contLen = true;
      self._removedContLen = false;
      break;
    case 'date':
    case 'expect':
    case 'trailer':
      state[field] = true;
      break;
  }
}

const validateHeaderName = hideStackFrames((name) => {
  if (typeof name !== 'string' || !name || !checkIsHttpToken(name)) {
    throw new ERR_INVALID_HTTP_TOKEN('Header name', name);
  }
});

const validateHeaderValue = hideStackFrames((name, value) => {
  if (value === undefined) {
    throw new ERR_HTTP_INVALID_HEADER_VALUE(value, name);
  }
  if (checkInvalidHeaderChar(value)) {
    debug('Header "%s" contains invalid characters', name);
    throw new ERR_INVALID_CHAR('header content', name);
  }
});

OutgoingMessage.prototype.setHeader = function setHeader(name, value) {
  if (this._header) {
    throw new ERR_HTTP_HEADERS_SENT('set');
  }
  validateHeaderName(name);
  validateHeaderValue(name, value);

  let headers = this[kOutHeaders];
  if (headers === null)
    this[kOutHeaders] = headers = ObjectCreate(null);

  headers[name.toLowerCase()] = [name, value];
};


OutgoingMessage.prototype.getHeader = function getHeader(name) {
  validateString(name, 'name');

  const headers = this[kOutHeaders];
  if (headers === null)
    return;

  const entry = headers[name.toLowerCase()];
  return entry && entry[1];
};


// Returns an array of the names of the current outgoing headers.
OutgoingMessage.prototype.getHeaderNames = function getHeaderNames() {
  return this[kOutHeaders] !== null ? ObjectKeys(this[kOutHeaders]) : [];
};


// Returns a shallow copy of the current outgoing headers.
OutgoingMessage.prototype.getHeaders = function getHeaders() {
  const headers = this[kOutHeaders];
  const ret = ObjectCreate(null);
  if (headers) {
    const keys = ObjectKeys(headers);
    // Retain for(;;) loop for performance reasons
    // Refs: https://github.com/nodejs/node/pull/30958
    for (let 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) {
  validateString(name, 'name');
  return this[kOutHeaders] !== null &&
    !!this[kOutHeaders][name.toLowerCase()];
};


OutgoingMessage.prototype.removeHeader = function removeHeader(name) {
  validateString(name, 'name');

  if (this._header) {
    throw new ERR_HTTP_HEADERS_SENT('remove');
  }

  const key = name.toLowerCase();

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

  if (this[kOutHeaders] !== null) {
    delete this[kOutHeaders][key];
  }
};


OutgoingMessage.prototype._implicitHeader = function _implicitHeader() {
  throw new ERR_METHOD_NOT_IMPLEMENTED('_implicitHeader()');
};

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

ObjectDefineProperty(OutgoingMessage.prototype, 'writableEnded', {
  get: function() { return this.finished; }
});


const crlf_buf = Buffer.from('\r\n');
OutgoingMessage.prototype.write = function write(chunk, encoding, callback) {
  if (typeof encoding === 'function') {
    callback = encoding;
    encoding = null;
  }

  const ret = write_(this, chunk, encoding, callback, false);
  if (!ret)
    this[kNeedDrain] = true;
  return ret;
};

function onError(msg, err, callback) {
  const triggerAsyncId = msg.socket ? msg.socket[async_id_symbol] : undefined;
  defaultTriggerAsyncIdScope(triggerAsyncId,
                             process.nextTick,
                             emitErrorNt,
                             msg,
                             err,
                             callback);
}

function emitErrorNt(msg, err, callback) {
  callback(err);
  if (typeof msg.emit === 'function') msg.emit('error', err);
}

function write_(msg, chunk, encoding, callback, fromEnd) {
  if (typeof callback !== 'function')
    callback = nop;

  let len;
  if (chunk === null) {
    throw new ERR_STREAM_NULL_VALUES();
  } else if (typeof chunk === 'string') {
    len = Buffer.byteLength(chunk, encoding);
  } else if (chunk instanceof Buffer) {
    len = chunk.length;
  } else {
    throw new ERR_INVALID_ARG_TYPE(
      'chunk', ['string', 'Buffer', 'Uint8Array'], chunk);
  }

  let err;
  if (msg.finished) {
    err = new ERR_STREAM_WRITE_AFTER_END();
  } else if (msg.destroyed) {
    err = new ERR_STREAM_DESTROYED('write');
  }

  if (err) {
    onError(msg, err, callback);
    return false;
  }

  if (!msg._header) {
    if (fromEnd) {
      msg._contentLength = len;
    }
    msg._implicitHeader();
  }

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

  if (!fromEnd && msg.socket && !msg.socket.writableCorked) {
    msg.socket.cork();
    process.nextTick(connectionCorkNT, msg.socket);
  }

  let ret;
  if (msg.chunkedEncoding && chunk.length !== 0) {
    msg._send(len.toString(16), 'latin1', null);
    msg._send(crlf_buf, null, null);
    msg._send(chunk, encoding, null);
    ret = msg._send(crlf_buf, null, callback);
  } else {
    ret = msg._send(chunk, encoding, callback);
  }

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


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


OutgoingMessage.prototype.addTrailers = function addTrailers(headers) {
  this._trailer = '';
  const keys = ObjectKeys(headers);
  const isArray = ArrayIsArray(headers);
  // Retain for(;;) loop for performance reasons
  // Refs: https://github.com/nodejs/node/pull/30958
  for (let i = 0, l = keys.length; i < l; i++) {
    let field, value;
    const 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 ERR_INVALID_HTTP_TOKEN('Trailer name', field);
    }
    if (checkInvalidHeaderChar(value)) {
      debug('Trailer "%s" contains invalid characters', field);
      throw new ERR_INVALID_CHAR('trailer content', field);
    }
    this._trailer += field + ': ' + value + CRLF;
  }
};

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

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

  if (this.socket) {
    this.socket.cork();
  }

  if (chunk) {
    write_(this, chunk, encoding, null, true);
  } else if (this.finished) {
    if (typeof callback === 'function') {
      if (!this.writableFinished) {
        this.on('finish', callback);
      } else {
        callback(new ERR_STREAM_ALREADY_FINISHED('end'));
      }
    }
    return this;
  } else if (!this._header) {
    this._contentLength = 0;
    this._implicitHeader();
  }

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

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

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

  if (this.socket) {
    // Fully uncork connection on end().
    this.socket._writableState.corked = 1;
    this.socket.uncork();
  }
  this[kCorked] = 0;

  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.outputData.length === 0 &&
      this.socket &&
      this.socket._httpMessage === this) {
    this._finish();
  }

  return this;
};


OutgoingMessage.prototype._finish = function _finish() {
  assert(this.socket);
  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() {
  const socket = this.socket;

  if (socket && socket.writable) {
    // There might be remaining data in this.output; write it out
    const 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[kNeedDrain]) {
      this[kNeedDrain] = false;
      this.emit('drain');
    }
  }
};

OutgoingMessage.prototype._flushOutput = function _flushOutput(socket) {
  while (this[kCorked]) {
    this[kCorked]--;
    socket.cork();
  }

  const outputLength = this.outputData.length;
  if (outputLength <= 0)
    return undefined;

  const outputData = this.outputData;
  socket.cork();
  let ret;
  // Retain for(;;) loop for performance reasons
  // Refs: https://github.com/nodejs/node/pull/30958
  for (let i = 0; i < outputLength; i++) {
    const { data, encoding, callback } = outputData[i];
    ret = socket.write(data, encoding, callback);
  }
  socket.uncork();

  this.outputData = [];
  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.pipe = function pipe() {
  // OutgoingMessage should be write-only. Piping from it is disabled.
  this.emit('error', new ERR_STREAM_CANNOT_PIPE());
};

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

module.exports = {
  OutgoingMessage
};