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Reduce the delay of ASYNC connection #1458

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Reduce the delay of ASYNC connection #1458

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9 changes: 5 additions & 4 deletions src/MQTTAsyncUtils.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1357,7 +1357,10 @@ static int MQTTAsync_processCommand(void)
make sure we check for writeability as well as readability, otherwise we wait around longer than we need to
in Socket_getReadySocket() */
if (rc == EINPROGRESS)
{
Socket_addPendingWrite(command->client->c->net.socket);
rc = MQTTAsync_connecting(command->client);
}
}
}
else if (command->command.type == SUBSCRIBE)
Expand Down Expand Up @@ -2031,7 +2034,7 @@ thread_return_type WINAPI MQTTAsync_receiveThread(void* n)

if (sock == 0)
continue;
timeout = 1000L;
timeout = 10L;
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I don't think this modification is correct.
The first time the function is entered, the call to MQTTAsync_cycle(&sock, timeout, &rc); uses timeout = 10.
If this loop has to be executed the second time around, then timeout is increased to 1000.
The question really is why was 1000 picked?

Let's consider the following:

  1. What is really this timeout value?
    Feel free to look at static MQTTPacket* MQTTAsync_cycle(SOCKET* sock, unsigned long timeout, int* rc).
    It's used in the Socket_getReadySocket(0, (int)timeout, socket_mutex, &rc1); call.
  2. In the body of SOCKET Socket_getReadySocket(int more_work, int timeout, mutex_type mutex, int* rc), timeout is used to set a local variable timeout_ms, which as a good programming practice is initialized at the top of the function (happens to be 1000).
  3. Now, there are two versions of the SOCKET Socket_getReadySocket(int more_work, int timeout, mutex_type mutex, int* rc) function. Although they are different, they seem to imply that a slowdown in operation might be desired if getting a socket did not succeed the first time around.

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I agree this change is not the best practice. This can be considered to be a temporary fix. The root cause of connection delay is that, not all socket operations are done in Socket_getReadySocket(). TCP connection is established in send thread. Even when it is completed, receive thread will continue wait until timeout. The best solution to avoid unexpected timeout for TCP connection is to move that logic (TCP connection) to receive Thread.


/* find client corresponding to socket */
if (ListFindItem(MQTTAsync_handles, &sock, clientSockCompare) == NULL)
Expand Down Expand Up @@ -3017,8 +3020,6 @@ static MQTTPacket* MQTTAsync_cycle(SOCKET* sock, unsigned long timeout, int* rc)
MQTTAsync_lock_mutex(mqttasync_mutex);
should_stop = MQTTAsync_tostop;
MQTTAsync_unlock_mutex(mqttasync_mutex);
if (!should_stop && *sock == 0 && (timeout > 0L))
MQTTAsync_sleep(100L);
#if defined(OPENSSL)
}
#endif
Expand All @@ -3032,7 +3033,7 @@ static MQTTPacket* MQTTAsync_cycle(SOCKET* sock, unsigned long timeout, int* rc)
{
Log(TRACE_MINIMUM, -1, "m->c->connect_state = %d", m->c->connect_state);
if (m->c->connect_state == TCP_IN_PROGRESS || m->c->connect_state == SSL_IN_PROGRESS || m->c->connect_state == WEBSOCKET_IN_PROGRESS)
*rc = MQTTAsync_connecting(m);
;
else
pack = MQTTPacket_Factory(m->c->MQTTVersion, &m->c->net, rc);
if (m->c->connect_state == WAIT_FOR_CONNACK && *rc == SOCKET_ERROR)
Expand Down