amqplib/lib/channel.js

An AMQP 0-9-1 (e.g., RabbitMQ) library and client.

const defs = require('./defs');
const closeMsg = require('./format').closeMessage;
const inspect = require('./format').inspect;
const methodName = require('./format').methodName;
const assert = require('node:assert');
const EventEmitter = require('node:events');
const fmt = require('node:util').format;
const IllegalOperationError = require('./error').IllegalOperationError;
const stackCapture = require('./error').stackCapture;

class Channel extends EventEmitter {
  constructor(connection) {
    super();

    this.connection = connection;
    // for the presently outstanding RPC
    this.reply = null;
    // for the RPCs awaiting action
    this.pending = [];
    // for unconfirmed messages
    this.lwm = 1; // the least, unconfirmed deliveryTag
    this.unconfirmed = []; // rolling window of delivery callbacks
    this.on(
      'ack',
      this.handleConfirm.bind(this, (cb) => {
        if (cb) cb(null);
      }),
    );
    this.on(
      'nack',
      this.handleConfirm.bind(this, (cb) => {
        if (cb) cb(new Error('message nacked'));
      }),
    );
    this.on('close', function () {
      let cb = this.unconfirmed.shift();
      while (cb) {
        if (cb) cb(new Error('channel closed'));
        cb = this.unconfirmed.shift();
      }
    });
    // message frame state machine
    this.handleMessage = acceptDeliveryOrReturn;
  }

  setOptions(options) {
    this.options = options;
  }

  allocate() {
    this.ch = this.connection.freshChannel(this, this.options);
    return this;
  }

  // Incoming frames are either notifications of e.g., message delivery,
  // or replies to something we've sent. In general I deal with the
  // former by emitting an event, and with the latter by keeping a track
  // of what's expecting a reply.
  //
  // The AMQP specification implies that RPCs can't be pipelined; that
  // is, you can have only one outstanding RPC on a channel at a
  // time. Certainly that's what RabbitMQ and its clients assume. For
  // this reason, I buffer RPCs if the channel is already waiting for a
  // reply.
  // Just send the damn frame.
  sendImmediately(method, fields) {
    return this.connection.sendMethod(this.ch, method, fields);
  }

  // Invariant: !this.reply -> pending.length == 0. That is, whenever we
  // clear a reply, we must send another RPC (and thereby fill
  // this.reply) if there is one waiting. The invariant relevant here
  // and in `accept`.
  sendOrEnqueue(method, fields, reply) {
    if (!this.reply) {
      // if no reply waiting, we can go
      assert(this.pending.length === 0);
      this.reply = reply;
      this.sendImmediately(method, fields);
    } else {
      this.pending.push({
        method: method,
        fields: fields,
        reply: reply,
      });
    }
  }

  sendMessage(fields, properties, content) {
    return this.connection.sendMessage(this.ch, defs.BasicPublish, fields, defs.BasicProperties, properties, content);
  }

  // Internal, synchronously resolved RPC; the return value is resolved
  // with the whole frame.
  _rpc(method, fields, expect, cb) {
    const self = this;

    function reply(err, f) {
      if (err === null) {
        if (f.id === expect) {
          return cb(null, f);
        } else {
          // We have detected a problem, so it's up to us to close the
          // channel
          const expectedName = methodName(expect);

          const e = new Error(fmt('Expected %s; got %s', expectedName, inspect(f, false)));
          self.closeWithError(f.id, fmt('Expected %s; got %s', expectedName, methodName(f.id)), defs.constants.UNEXPECTED_FRAME, e);
          return cb(e);
        }
      }

      // An error will be given if, for example, this is waiting to be
      // sent and the connection closes
      else if (err instanceof Error) return cb(err);
      // A close frame will be given if this is the RPC awaiting reply
      // and the channel is closed by the server
      else {
        // otherwise, it's a close frame
        const closeReason = (err.fields.classId << 16) + err.fields.methodId;
        const e =
          method === closeReason
            ? fmt('Operation failed: %s; %s', methodName(method), closeMsg(err))
            : fmt('Channel closed by server: %s', closeMsg(err));
        const closeFrameError = new Error(e);
        closeFrameError.code = err.fields.replyCode;
        closeFrameError.classId = err.fields.classId;
        closeFrameError.methodId = err.fields.methodId;
        return cb(closeFrameError);
      }
    }

    this.sendOrEnqueue(method, fields, reply);
  }

  // Move to entirely closed state.
  toClosed(capturedStack) {
    this._rejectPending();
    invalidateSend(this, 'Channel closed', capturedStack);
    this.accept = invalidOp('Channel closed', capturedStack);
    this.connection.releaseChannel(this.ch);
    this.emit('close');
  }

  // Stop being able to send and receive methods and content. Used when
  // we close the channel. Invokes the continuation once the server has
  // acknowledged the close, but before the channel is moved to the
  // closed state.
  toClosing(capturedStack, k) {
    const send = this.sendImmediately.bind(this);
    invalidateSend(this, 'Channel closing', capturedStack);

    this.accept = function (f) {
      if (f.id === defs.ChannelCloseOk) {
        if (k) k();
        const s = stackCapture('ChannelCloseOk frame received');
        this.toClosed(s);
      } else if (f.id === defs.ChannelClose) {
        send(defs.ChannelCloseOk, {});
      }
      // else ignore frame
    };
  }

  _rejectPending() {
    function reject(r) {
      r(new Error('Channel ended, no reply will be forthcoming'));
    }
    if (this.reply !== null) reject(this.reply);
    this.reply = null;

    let discard = this.pending.shift();
    while (discard) {
      if (discard) reject(discard.reply);
      discard = this.pending.shift();
    }
    this.pending = null; // so pushes will break
  }

  closeBecause(reason, code, k) {
    this.sendImmediately(defs.ChannelClose, {
      replyText: reason,
      replyCode: code,
      methodId: 0,
      classId: 0,
    });
    const s = stackCapture(`closeBecause called: ${reason}`);
    this.toClosing(s, k);
  }

  // If we close because there's been an error, we need to distinguish
  // between what we tell the server (`reason`) and what we report as
  // the cause in the client (`error`).
  closeWithError(id, reason, code, error) {
    this.closeBecause(reason, code, () => {
      error.code = code;
      // content frames and consumer errors do not provide a method a class/method ID
      if (id) {
        error.classId = defs.info(id).classId;
        error.methodId = defs.info(id).methodId;
      }
      this.emit('error', error);
    });
  }

  // A trampolining state machine for message frames on a channel. A
  // message arrives in at least two frames: first, a method announcing
  // the message (either a BasicDeliver or BasicGetOk); then, a message
  // header with the message properties; then, zero or more content
  // frames.
  // Keep the try/catch localised, in an attempt to avoid disabling
  // optimisation
  acceptMessageFrame(f) {
    try {
      this.handleMessage = this.handleMessage(f);
    } catch (msg) {
      if (typeof msg === 'string') {
        this.closeWithError(f.id, msg, defs.constants.UNEXPECTED_FRAME, new Error(msg));
      } else if (msg instanceof Error) {
        this.closeWithError(f.id, 'Error while processing message', defs.constants.INTERNAL_ERROR, msg);
      } else {
        this.closeWithError(f.id, 'Internal error while processing message', defs.constants.INTERNAL_ERROR, new Error(msg.toString()));
      }
    }
  }

  handleConfirm(handle, f) {
    const tag = f.deliveryTag;
    const multi = f.multiple;

    if (multi) {
      const confirmed = this.unconfirmed.splice(0, tag - this.lwm + 1);
      this.lwm = tag + 1;
      confirmed.forEach(handle);
    } else {
      let c;
      if (tag === this.lwm) {
        c = this.unconfirmed.shift();
        this.lwm++;
        // Advance the LWM and the window to the next non-gap, or
        // possibly to the end
        while (this.unconfirmed[0] === null) {
          this.unconfirmed.shift();
          this.lwm++;
        }
      } else {
        c = this.unconfirmed[tag - this.lwm];
        this.unconfirmed[tag - this.lwm] = null;
      }
      // Technically, in the single-deliveryTag case, I should report a
      // protocol breach if it's already been confirmed.
      handle(c);
    }
  }

  pushConfirmCallback(cb) {
    // `null` is used specifically for marking already confirmed slots,
    // so I coerce `undefined` and `null` to false; functions are never
    // falsey.
    this.unconfirmed.push(cb || false);
  }

  onBufferDrain() {
    this.emit('drain');
  }

  accept(f) {
    switch (f.id) {
      // Message frames
      case undefined: // content frame!
      case defs.BasicDeliver:
      case defs.BasicReturn:
      case defs.BasicProperties:
        return this.acceptMessageFrame(f);

      // confirmations, need to do confirm.select first
      case defs.BasicAck:
        return this.emit('ack', f.fields);
      case defs.BasicNack:
        return this.emit('nack', f.fields);
      case defs.BasicCancel:
        // The broker can send this if e.g., the queue is deleted.
        return this.emit('cancel', f.fields);

      case defs.ChannelClose: {
        // Any remote closure is an error to us. Reject the pending reply
        // with the close frame, so it can see whether it was that
        // operation that caused it to close.
        if (this.reply) {
          const reply = this.reply;
          this.reply = null;
          reply(f);
        }
        const emsg = `Channel closed by server: ${closeMsg(f)}`;
        this.sendImmediately(defs.ChannelCloseOk, {});

        const error = new Error(emsg);
        error.code = f.fields.replyCode;
        error.classId = f.fields.classId;
        error.methodId = f.fields.methodId;
        this.emit('error', error);

        const s = stackCapture(emsg);
        this.toClosed(s);
        return;
      }

      case defs.BasicFlow:
        // RabbitMQ doesn't send this, it just blocks the TCP socket
        return this.closeWithError(f.id, 'Flow not implemented', defs.constants.NOT_IMPLEMENTED, new Error('Flow not implemented'));

      default: {
        // assume all other things are replies
        // Resolving the reply may lead to another RPC; to make sure we
        // don't hold that up, clear this.reply
        const reply = this.reply;
        this.reply = null;
        // however, maybe there's an RPC waiting to go? If so, that'll
        // fill this.reply again, restoring the invariant. This does rely
        // on any response being recv'ed after resolving the promise,
        // below; hence, I use synchronous defer.
        if (this.pending.length > 0) {
          const send = this.pending.shift();
          this.reply = send.reply;
          this.sendImmediately(send.method, send.fields);
        }
        return reply(null, f);
      }
    }
  }
}

// Shutdown protocol. There's three scenarios:
//
// 1. The application decides to shut the channel
// 2. The server decides to shut the channel, possibly because of
// something the application did
// 3. The connection is closing, so there won't be any more frames
// going back and forth.
//
// 1 and 2 involve an exchange of method frames (Close and CloseOk),
// while 3 doesn't; the connection simply says "shutdown" to the
// channel, which then acts as if it's closing, without going through
// the exchange.

function invalidOp(msg, stack) {
  return () => {
    throw new IllegalOperationError(msg, stack);
  };
}

function invalidateSend(ch, msg, stack) {
  ch.sendImmediately = ch.sendOrEnqueue = ch.sendMessage = invalidOp(msg, stack);
}

// Kick off a message delivery given a BasicDeliver or BasicReturn
// frame (BasicGet uses the RPC mechanism)
function acceptDeliveryOrReturn(f) {
  let event;
  if (f.id === defs.BasicDeliver) event = 'delivery';
  else if (f.id === defs.BasicReturn) event = 'return';
  else throw fmt('Expected BasicDeliver or BasicReturn; got %s', inspect(f));
  const fields = f.fields;
  return acceptMessage((message) => {
    message.fields = fields;
    this.emit(event, message);
  });
}

// Move to the state of waiting for message frames (headers, then
// one or more content frames)
function acceptMessage(continuation) {
  let totalSize = 0;
  let remaining = 0;
  let buffers = null;

  const message = {
    fields: null,
    properties: null,
    content: null,
  };

  return headers;

  // expect a headers frame
  function headers(f) {
    if (f.id === defs.BasicProperties) {
      message.properties = f.fields;
      totalSize = remaining = f.size;

      // for zero-length messages, content frames aren't required.
      if (totalSize === 0) {
        message.content = Buffer.alloc(0);
        continuation(message);
        return acceptDeliveryOrReturn;
      } else {
        return content;
      }
    } else {
      throw 'Expected headers frame after delivery';
    }
  }

  // expect a content frame
  // %%% TODO cancelled messages (sent as zero-length content frame)
  function content(f) {
    if (f.content) {
      const size = f.content.length;
      remaining -= size;
      if (remaining === 0) {
        if (buffers !== null) {
          buffers.push(f.content);
          message.content = Buffer.concat(buffers);
        } else {
          message.content = f.content;
        }
        continuation(message);
        return acceptDeliveryOrReturn;
      } else if (remaining < 0) {
        throw fmt('Too much content sent! Expected %d bytes', totalSize);
      } else {
        if (buffers !== null) buffers.push(f.content);
        else buffers = [f.content];
        return content;
      }
    } else throw 'Expected content frame after headers';
  }
}

// This adds just a bit more stuff useful for the APIs, but not
// low-level machinery.
class BaseChannel extends Channel {
  constructor(connection) {
    super(connection);
    this.consumers = new Map();
  }

  // Not sure I like the ff, it's going to be changing hidden classes
  // all over the place. On the other hand, whaddya do.
  registerConsumer(tag, callback) {
    this.consumers.set(tag, callback);
  }

  unregisterConsumer(tag) {
    this.consumers.delete(tag);
  }

  dispatchMessage(fields, message) {
    const consumerTag = fields.consumerTag;
    const consumer = this.consumers.get(consumerTag);
    if (consumer) {
      return consumer(message);
    } else {
      // %%% Surely a race here
      throw new Error(`Unknown consumer: ${consumerTag}`);
    }
  }

  handleDelivery(message) {
    return this.dispatchMessage(message.fields, message);
  }

  handleCancel(fields) {
    const result = this.dispatchMessage(fields, null);
    this.unregisterConsumer(fields.consumerTag);
    return result;
  }
}

module.exports.acceptMessage = acceptMessage;
module.exports.BaseChannel = BaseChannel;
module.exports.Channel = Channel;