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Idea about fixing the lockfile issues in postgresql.init

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Cc: Lamar Owen <lowen(at)pari(dot)edu>
Subject: Idea about fixing the lockfile issues in postgresql.init
Date: 2004-09-30 21:42:27
Message-ID: 25834.1096580547@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
I've been getting some more flak lately about fixing the RPM-supplied
init scripts to avoid the problem where the postmaster fails to start
because there's a stale lockfile in the data directory.  See for
instance https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=134090
but it's hardly the first time the question has been brought up.

I've always resisted the standard solution of having the init script
itself remove the lock file, because I think that is a great way to
shoot yourself in the foot.  But I finally had an idea that would make
things better with no increased risk.

The postmaster does not abort simply because the lockfile is there.
It checks whether the PID mentioned in the lockfile exists, and belongs
to a postgres-owned process (the latter by seeing if "kill(pid, 0)"
succeeds), and is not the postmaster itself.  If any of these tests fail
then it knows the lockfile is stale.  So the scenario in which
it gets fooled is where the reboot has used just one or two more
processes than were used in the last boot cycle, so that the PID that
belonged to the postmaster in the last cycle now belongs to either
pg_ctl or the postgres-owned shell launched by "su".

So what occurred to me is that we could eliminate this scenario if we
could get rid of those two processes.  The init script is running as
root, and so if its PID is the one mentioned in the old lockfile,
the kill() test will fail and the postmaster will go on its merry way.
It's only the other processes launched by "su" that could fool the
kill() test.

After some experimentation, I have found that what will actually work
requires two changes:

1. In the init script, do not use pg_ctl to launch the postmaster, but
instead invoke it directly, ie something like
	su - postgres -c "/usr/bin/postmaster ...args... &"
pg_ctl is not really buying us any functionality or notational advantage
here, so this seems like no loss.  This brings us down to one extra
postgres-owned process, namely the shell that su launches which in
turn launches the postmaster.  (Depending on timing, the shell might or
might not still be around when the postmaster probes.)

2. In the postmaster, reject as bogus matches not only our own PID,
but our parent's PID from getppid().  This is perfectly safe since
whatever launched the postmaster is certainly not a competing
postmaster.  Now we cannot be fooled by the parent shell either.

AFAICS this is a bulletproof solution for typical users who only launch
one postmaster during system boot.  If you launch two or more postmasters
then it's still possible for the first-launched one to have the same PID
that belonged to the second one during the previous boot cycle.  But the
odds of this seem pretty low, since it would imply a much greater change
in the usage of PIDs during the boot cycle.  The known failure cases
involve a change of just one or two PIDs, whereas postmasters launched
by different init scripts will surely be separated by dozens if not
hundreds of PIDs.  (Besides, if you are truly paranoid you'd be running
separate postmasters under separate user IDs, which'd eliminate the
problem again.)

I'd also want to remove the existing code in the init scripts that zaps
the socket lockfile, since it would be unnecessary.  I've forebore to
remove it so far because it doesn't introduce a serious risk of data
corruption the way zapping the datadir lockfile would, but it is
definitely risky especially in multi-postmaster scenarios.

Anyone see any downsides to these changes?

			regards, tom lane

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