Allow signal handlers to optionally use SA_SIGINFO information?

Hi,

During benchmarking/debugging I just had the problem that autovacuum was
signalled at an insane rate - leading to more than one hundred autovac
workers being started per second. Leading to a overall slowdown of more
than 90% and the anti-wraparound vacuum not finishing.

The problem is that I couldn't easily figure out where all the SIGUSR2's
to the autovacuum launcher where coming from. This isn't the first time
that I had that kind of problem.

Posix provides information about the source of the signal when using
SA_SIGINFO style handlers via si_code/si_pid. That information has been
available for a *long* while
(c.f. http://pubs.opengroup.org/onlinepubs/7908799/xsh/signal.h.html)

I've now hacked up my development instance to log something like
"autovacuum: invoked by pid 18175". I personally find that quite
helpful. I can also imagine it being rather helpful to log information
about the sender of SIGINT/TERM interrupts.

The existing abstractions make are nearly sufficient to make it easy to
optionally use SA_SIGINFO style handlers. Just by redifining SIGNAL_ARGS
and pqsigfunc. There unfortunately is two things making it harder:
SIG_IGN and SIG_DFL - those unfortunately can't be specified for
SA_SIGINFO style handlers (as they have a different signature). So we'd
need to use a different function for those two.

Comments, ideas?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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