[Libwebsockets] Interaction of external threads with libwebsockets server

"Andy Green (林安廸)" andy at warmcat.com
Wed Dec 18 03:07:10 CET 2013


On 18/12/13 06:40, the mail apparently from Andy Green included:
>
>
> 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.

Well... there's another thing to take care about... you need to manage 
your private list of live wsis carefully.  It means managing that list 
(with any of your locking needed) at the ESTABLISHED / CLOSED callback 
for the WSI.  That should be enough I think.

> 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.

I added a patch

http://git.libwebsockets.org/cgi-bin/cgit/libwebsockets/commit/?id=7a1327977ac10bdbace0012274b8ae889219880e

that lets you do this in a lock and unlock callback in the user code, in 
one place.

If you're not using threading you don't need to do anything new.

-Andy

> -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
>> _______________________________________________ Libwebsockets
>> mailing list Libwebsockets at ml.libwebsockets.org
>> http://ml.libwebsockets.org/mailman/listinfo/libwebsockets
>
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