[Live-devel] Possible concurrency issue with triggerEvent()

[Hansen Jan Rørgård]

Hi,

In the liveMedia FAQ section it is written:
"(Note that "triggerEvent()" is the only LIVE555 function that may be called from an external (i.e., non-LIVE555) thread.)".

We use this possibility and trigger an event from a non-LIVE555 thread. Sometimes an the event "disappears" ie. it is signalled using triggerEvent() but the eventHandler is not called. The LIVE555 changelog has the following entry:

2015.03.16:
- Made a small change to the "BasicTaskScheduler" implementation to reduce the likelihood of
  a race condition with external thread(s) calling "triggerEvent()".

Is it correct that there is a non-zero possibility, that the event will disappear due to the variables BasicTaskScheduler0::fTriggersAwaitingHandling and BasicTaskScheduler0::fTriggeredEventClientDatas being accessed concurrently by the LIVE555 thread and a non-LIVE555 thread?

The behaviour is not immediately obvious from the FAQ and the function description, but we think it is what we "see" and a workaround to re-trigger the event at a later time seems to work for now.
Do you also see an issue here?

  virtual void triggerEvent(EventTriggerId eventTriggerId, void* clientData = NULL) = 0;
      // Causes the (previously-registered) handler function for the specified event to be handled (from the event loop).
      // The handler function is called with "clientData" as parameter.
      // Note: This function (unlike other library functions) may be called from an external thread
      // - to signal an external event.  (However, "triggerEvent()" should not be called with the
      // same 'event trigger id' from different threads.)

Best Regards

Jan Rørgaard Hansen
Lead Software Developer
Communication Solutions
Business Area Surveillance

Tel.:  + 45 3638 3000
Call me on: Skype<skype:jan.r.hansen at dk.saabgroup.com?call>
E-mail: jan.r.hansen at dk.saabgroup.com<mailto:jan.r.hansen at dk.saabgroup.com>


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