Re: PostmasterIsAlive() in recovery (non-USE_POST_MASTER_DEATH_SIGNAL builds)

On Wed, Mar 31, 2021 at 7:02 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Fri, Mar 12, 2021 at 7:55 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> > On Thu, Mar 11, 2021 at 7:34 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> > > Wow. This probably means that we would be able to get rid of
> > > USE_POSTMASTER_DEATH_SIGNAL?
> >
> > We'll still need it, because there'd still be systems with no signal:
> > NetBSD, OpenBSD, AIX, HPUX, illumos.
>
> Erm, and of course macOS.
>
> There is actually something we could do here: ioctl(I_SETSIG) for
> SysV-derived systems and fcntl(O_ASYNC) for BSD-derived systems seems
> like a promising way to get a SIGIO signal when the postmaster goes
> away and the pipe becomes readable. Previously I'd discounted this,
> because it's not in POSIX and I doubted it would work well on other
> systems. But I was flicking through Stevens' UNIX book while trying
> to figure out that POLLHUP stuff from a nearby thread (though it's
> useless for that purpose) and I learned from section 12.6 that SIGIO
> is fairly ancient, originating in 4.2BSD, adopted by SVR4, so it's
> likely present in quite a few systems, maybe even all of our support
> platforms if we're prepared to do it two different ways. Just a
> thought.

Alright, here's a proof-of-concept patch that does that. Adding to the next CF.

This seems to work on Linux, macOS, FreeBSD and OpenBSD (and I assume
any other BSD). Can anyone tell me if it works on illumos, AIX or
HPUX, and if not, how to fix it or disable the feature gracefully?
For now the patch assumes that if you have SIGIO then you can do this;
perhaps it should also test for O_ASYNC? Perhaps HPUX has the signal
but requires a different incantation with I_SETSIG?

Full disclosure: The reason for my interest in this subject is that I
have a work-in-progress patch set to make latches, locks and condition
variables more efficient using futexes on several OSes, but it needs a
signal to wake on postmaster death.