[Libwebsockets] Interaction of external threads with libwebsockets server

Andy Green andy at warmcat.com
Tue Dec 17 23:40:56 CET 2013



Eugene Agafonov <e.a.agafonov at gmail.com> wrote:
>Hi!
>
>I've just implemented the similar scenario just like it is stated in
>document
>http://git.libwebsockets.org/cgi-bin/cgit/libwebsockets/tree/README.coding
>
>I have a mutex-protected queue of outgoing events (as a std::list of
>strings). 
>Each string is intended to be send in separated message.
>
>The writer thread posts data onto queue and calls 
>libwebsocket_callback_on_writable(context, wsi)
>
>Protocol callback gets LWS_CALLBACK_SERVER_WRITEABLE as soon as
>websocket 
>thread is ready to send data. Once it comes, the callback extracts
>strings 
>from the queue one by one and writes them to socket using
>libwebsocket_write.
>
>I'm quite newby in writing WS server with libwebsockets. Any comments
>from WS 
>gurus are welcome

What you're doing is a good way to interact with what lws needs and get what you want from threading.

There's one danger with it, but that is a very tightly defined race rather than corruption or whatever.

The issue is that the 'wait for writeable' is oneshot, so after it becomes true in the poll lws will clear the POLLOUT wait flag.  But because the threads are asynchronous, eventually the other thread will come at the wrong time and set it just as we're clearing it with indeterminate results.  Because it's read-modify-write and flow control code also modifies this, there's a low probability deadlock case where POLLIN or POLLOUT from one thread or another did not get set for wait as the thread thinks it is.

If the extpoll stuff is used the issue can be worse depending on what it is doing.

However the approach you're taking is otherwise really workable I think.  To make it rock solid you'll need to patch lws to mutex-protect the few places it changes poll wait flags.

-Andy

>BR, Eugene Agafonov.
>
>On Tuesday 17 December 2013 12:03:43 Thomas Spitz wrote:
>> Hello everyone,
>> 
>> I am trying to find a way for external threads to ask libwebsocket
>server
>> to send websocket message on their behalf (as they cannot do it
>directly).
>> The only way I see at the present is to make a loop with
>> libwebsocket_service (context, VERYSHORTTIME); and test potential
>incoming
>> messages from other threads. This is not very elegant as CPU computes
>all
>> the time and not very reliable as I should put VERYSHORTTIME at 0 if
>I
>> don't want to miss any message.
>> 
>> I tried the following modification in test-server.c :
>> 
>> void *connexionServeur(void *arg) {
>> 
>> > struct libwebsocket_context *context = arg;
>> > while (1) {
>> > if(libwebsocket_service(context, 60000)<0){
>> > 
>> >  printf("Problem!\n");
>> > 
>> > break;
>> > }
>> > }
>> > 
>> >  return NULL ;
>> > 
>> > }
>> > int statut_Thread;
>> > pthread_t Thread;
>> > statut_Thread = pthread_create(&Thread, NULL, connexionServeur,
>context
>> > );
>> > 
>> >  if (statut_Thread != 0) {
>> > 
>> > printf("Problem with thread creation \n");
>> > }
>> > while (n >= 0 && !force_exit) {
>> > struct timeval tv;
>> > gettimeofday(&tv, NULL);
>> > if (((unsigned int)tv.tv_usec - oldus) > 50000) {
>> > 
>> >
>libwebsocket_callback_on_writable_all_protocol(&protocols[PROTOCOL_DUMB_I
>> > NCREMENT]); oldus = tv.tv_usec;
>> > libwebsocket_service(context, 0);
>> > 
>> >  }
>> > 
>> > usleep(10000);
>> > }
>> 
>> but it crashes quickly as soon as I try drawing . I suppose that
>> libwebsocket_service is called twice at the same time and it creates
>> conflicts that lead to stop crash the websocket.
>> 
>> I have seen in the mailing list that Thomas Koeper has already had a
>> similar question
>>
>http://ml.libwebsockets.org/pipermail/libwebsockets/2013-April/000403.html
>> . At that time, he was thinking of creating a mutex on
>> libwebsocket_service. Do you think this is the right solution?
>> 
>> Thanks in advance for your help
>> Best regards,
>> Thomas
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