PATCH: backtraces for error messages

Hi folks

I recently needed a way to get backtraces from errors in a convenient,
non-interactive and indescriminate way. The attached patch is the result.
It teaches Pg to use libunwind to self-backtrace in elog/ereport.

Anyone think this is useful/interesting/worth pursuing?

(Patch is currently against pg10, so this is a PoC only).

As written it emits a backtrace when log_error_verbosity=verbose or,
unconditionally, on PANIC. A bt can be hidden by errhidestack() but is
otherwise shown. That's ridiculously, excessively spammy, so it's not
viable for core as-is. Before playing with it too much I thought I'd ask
for ideas on if anyone thinks it's useful, and if so, how it'd work best.

My goal is to allow capture of extra diagnostic info from key locations in
production without needing to attach gdb. It's imperfect since sometimes
there's no convenient message, and other times you can't afford to set up
logging with enough detail. So this would be most useful when combined with
one of the occasionally discussed patches to allow for selective logging
verbosity on a module- or file- level. But I think it's still handy without
that.

I briefly looked into Windows too. Roughly the same approach could be used
to plug in dbghelp.dll support for Windows self-backtracing, it's just
rather uglier; see
https://jpassing.com/2008/03/12/walking-the-stack-of-the-current-thread/ .

BTW, Álvaro posted a simpler patch at
https://www.postgresql.org/message-id/20180410213203.nl645o5vj5ap66sl@alvherre.pgsql.
It uses backtrace() from glibc, and requires each site you want to bt to be
annotated explicitly. I forgot about backtrace() completely when I wrote
mine, and I prefer the control libunwind gives me anyway, but the reduced
dependency would be nice. Especially since backtrace() is in FreeBSD too.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services