[mythtv] Backend crashes (related to EIT) - possible fix attached

David david at unsolicited.net
Thu Feb 21 20:53:21 UTC 2008

I've been looking into this issue further, and have a patch.

The problem is quite subtle and connected to the way Qt caches regular 
expression engine instances. MythTV creates threads directly using 
pthread primitives, bypassing Qt's QThread class. This is a problem, 
because QThread sets up thread local data structures that it expects to 
be able to use in QRegExp methods. These are uninitialised when the 
thread is created outside of Qt.

The actual (mis)behaviour caused by this is rather random. QRegExp may 
decide to create and destroy an engine during each regexp call, in which 
case everything works OK (if more slowly that it could). At other times 
threads end up sharing regexp engines and the associated control 
structures. Sometimes, if two threads execute inside QRegExp 
simultaneously then heap corruption occurs and the backend crashes.

Applying the attached change has seen my crashy backend (with 3 DTV 
tuners) work reliably for 21 hours so far. (This is unheard of).

Please consider applying this patch (diff is against trunk earlier this 
week). It changes the main thread constructs over to using QThread. 
There are still some pthread calls left, but these are in areas that are 
unlikely to exercise Qt. There is also a belt and braces change to 
pespacket.cpp that I put in after getting uncomfortable with the 
existing code. I'm not sure this fixes anything so feel free to ignore it.

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