Debugging zombies: pthread_sigmask and sigwait

[Ian Lepore]

On Wed, 2012-04-11 at 16:11 +0200, Mel Flynn wrote:
> Hi,
> 
> I'm currently stuck on a bug in Zarafa-spooler that creates zombies. and
> working around it by claiming that our pthread library isn't "normal"
> which uses standard signals rather then a signal thread.
> 
> My limited understanding of these facilities is however not enough to
> see the actual problem here and reading of related manpages did not lead
> me to a solution either. A test case reproducing the problem is attached.
> 
> What happens is that SIGCHLD is never received by the signal thread and
> the child processes turn to zombies. Signal counters never go up, not
> even for SIGINFO, which I added specifically to see if anything gets
> through at all.
> 
> The signal thread shows being stuck in sigwait. It's reproducible on
> 8.3-PRERELEASE of a few days ago (r233768). I'm not able to test it on
> anything newer unfortunately, but I suspect this is a bug/linuxism in
> the code not in FreeBSD.
> 
> Thanks in advance for any insights.
> _______________________________________________
> freebsd-hackers at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
> To unsubscribe, send any mail to "freebsd-hackers-unsubscribe at freebsd.org"

The signal mask for a new thread is inherited from the parent thread.
In your example code, the signal handling thread inherits the blocked
status of the signals as set up in main().  Try adding this line to
signal_handler() before it goes into its while() loop:

 pthread_sigmask(SIG_UNBLOCK, &signal_mask, NULL);

-- Ian