Bug: deadlock from msyslog

[Kurt Roeckx]

On Wed, Jan 20, 2016 at 02:31:29AM -0800, Hal Murray wrote:
> 
> A while (months) ago, we had troubles with msyslog scrambling output when 
> called from the DNS thread and the main thread at the same time.  That was 
> fixed by adding a lock.
> 
> That will deadlock if a signal handler calls msyslog when the signal goes off 
> in the middle of a msyslog call.
> 
> I poked around a bit.  There are a few signal handlers that call msyslog.  I 
> think they can be deferred.  It's probably worthwhile to study all the signal 
> usage and make some notes.

You should be careful with signal handers.  The only thing you can
really do do in standard C is access a volatile sig_atomic_t, or
call functions like abort(), _exit(), or signal().  Your OS might provide
more async safe function, but that's implementation defined.
POSIX has a relative long list of such functions.

OpenBSD also defines some others async safe functions, like snprint(),
vsprint() and syslog_r().


Kurt