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

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.