Re: PATCH: Keep one postmaster monitoring pipe per process

On Fri, Sep 16, 2016 at 2:23 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> Every read is an event, and that's what PostmasterIsAlive does.
>
> But in most places we only do a PostmasterIsAlive if WaitLatch returns
> WL_POSTMASTER_DEATH. The only walreceiver related place that doesn't is
> WalRcvWaitForStartPosition(). If that's indeed the cause of your issues
> this quite possibly could be fixed by doing the
> if (!PostmasterIsAlive())
> exit(1);
> check not unconditionally, but only after the WaitLatch at the end of
> the loop, and only if WL_POSTMASTER_DEATH is returned by WaitLatch()?
> That'll be a minor behaviour change for the WALRCV_RESTARTING, but that
> seems fine, we'll just loop once more outside (after a quick glance at
> least).

At least some of the latch implementations already check
PostmasterIsAlive() internally to avoid returning spurious events; and
secure_read() at least assumes that the WL_POSTMASTER_DEATH return is
reliable and doesn't need a double-check.

So we can probably just remove the check altogether and instead bail
out if it returns WL_POSTMASTER_DEATH. That probably saves a system
call per loop iteration even on platforms where the kernel doesn't
exhibit any surprising behavior.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company