Date: Wed, 11 Sep 2013 11:32:01 -0400
From: Stephen Frost <sfrost@snowman.net>
To: Noah Misch <noah@leadboat.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Weaker shmem interlock w/o postmaster.pid
Message-ID: <20130911153200.GD2706@tamriel.snowman.net>
References: <20130911033341.GD225735@tornado.leadboat.com>
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* Noah Misch (noah@leadboat.com) wrote:
> Shouldn't it instead check the same things as PGSharedMemoryIsInUse()?

Offhand, I tend to agree that we should really be doing a very careful
job of looking at if an existing segment is still in use.

> The concrete situation in which I encountered this involved PostgreSQL 9.=
2 and
> an immediate shutdown with a backend that had blocked SIGQUIT.  The backe=
nd
> survived the immediate shutdown as one would expect. =20

Well..  We expect this now because of the analysis you did in the
adjacent thread showing how it can happen.

> The postmaster
> nonetheless removed postmaster.pid before exiting, and I could immediately
> restart PostgreSQL despite the survival of the SIGQUIT-blocked backend.  =
If I
> instead SIGKILL the postmaster, postmaster.pid remains, and I must kill s=
tray
> backends before restarting.  The postmaster should not remove postmaster.=
pid
> unless it has verified that its children have exited.

This makes sense, however..

> Concretely, that means
> not removing postmaster.pid on immediate shutdown in 9.3 and earlier.  Th=
at's
> consistent with the rough nature of an immediate shutdown, anyway.

I don't like leaving the postmaster.pid file around, even on an
immediate shutdown.  I don't have any great suggestions regarding what
to do, given what we try to do wrt 'immediate', so perhaps it's
acceptable for future releases.

> I'm thinking to preserve postmaster.pid at immediate shutdown in all rele=
ased
> versions, but I'm less sure about back-patching a change to make
> PGSharedMemoryCreate() pickier.  On the one hand, allowing startup to pro=
ceed
> with backends still active in the same data directory is a corruption haz=
ard.

The corruption risk, imv anyway, is sufficient to backpatch the change
and overrides the concerns around very fast shutdown/restarts.

	Thanks,

		Stephen

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