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It looks like pthread_mutex_lock() and pthread_mutex_unlock() are not reentrant on some platforms (in some implementations of the pthread API).
Antoine: if I understand correctly your patch, if we have a pending signal, all next signals will be simply ignored.
I think that this issue is not specific to Mac OS X: signal_handler() can be called twice at the same time in two different threads or in the same thread (reentrant call).
pthread_sigmask() can be used to avoid reentrant call, but it has no effect on the second case: "signal_handler() called twice at the same time in two different threads".
Note: SA_NODEFER flag of sigaction() has no effect on this issue because signal_handler() is called twice to handle two different signals (SIGUSR1 and SIGUSR2).