[mythtv] more on dvb record failure (race condition in signalmonitor.cpp?)

[Mark Weaver]

I am also having trouble with DVB (livetv|recording) -- sometimes 
getting the PMT timeout.  I have tried latest SVN but the backend seems 
very unstable for me.  I've done a little debugging, so far I think I 
have found a race condition in signalmonitor.cpp:


In signalmonitor.cpp there is a race condition:

void SignalMonitor::Start()
{
     VERBOSE(VB_CHANNEL, "SignalMonitor::Start() -- begin");
     if (!running)
         pthread_create(&monitor_thread, NULL, SpawnMonitorLoop, this);
     while (!running)
         usleep(50);

and

void SignalMonitor::MonitorLoop()
{
     //signal(SIGALRM, ALRMhandler);

     running = true;
     exit = false;

and

void SignalMonitor::Stop()
{
     if (running)
     {
         exit = true;
         pthread_join(monitor_thread, NULL);
     }

If you Start/Stop rapidly then it might be the case that 'exit' can be 
set to true by the line in Stop(), then set to false again by 
MonitorLoop() leading to a thread that doesn't die.

Admittedly it doesn't look very likely to occur but I have an instance 
in gdb that is stuck in exactly this way (one thread in MonitorLoop() 
with this->exit = false, and one thread in pthread_join from 
SignalMonitor::Stop() with the same 'this' pointer.

I think it would be reasonable to remove the exit = false; in 
MonitorLoop() to correct this -- it is set to false by the c'tor.