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F_SETLK is looking worse and worse...

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: F_SETLK is looking worse and worse...
Date: 2000-11-29 00:16:28
Message-ID: 25154.975456988@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
While testing interlocking of multiple postmasters, I discovered that
the HAVE_FCNTL_SETLK interlock code we have in StreamServerPort()
does not work at all on HPUX 10.20.  This platform has F_SETLK according
to configure, but:

1. The lock is never applied to a socket, because the open() on the
newly-created socket (at line 303 of pqcomm.c) fails with EOPNOTSUPP,
Operation not supported.

2. If a postmaster finds a socket file in its way, it is unable to
remove it despite the lack of any lock, because the open() at line
230 fails with EADDRINUSE, Address already in use.

I have no idea whether the fcntl(F_SETLK) call would succeed if control
did get to it, but these results don't leave me very hopeful.

Between this and the already-known result that F_SETLK doesn't work on
sockets in shipping Linux kernels, I'm pretty unimpressed with the
usefulness of this interlock method.

We talked before about flushing the F_SETLK technique and using good
old interlock files containing PIDs, same method that we use for
interlocking the data directory.  That is, if the socket file name is
/tmp/.s.PGSQL.5432, we'd create a plain file /tmp/.s.PGSQL.5432.lock
containing the owning process's PID.  The code would insist on getting
this interlock file first, and if successful would just unconditionally
remove any existing socket file before doing the bind().

I can only think of one scenario where this is worse than what we have
now: if someone is running a /tmp-directory-sweeper that is bright
enough not to remove socket files, it would still zap the interlock
file, thus potentially allowing a second postmaster to take over the
socket file.  This doesn't seem like a mainstream problem though.

BTW, it also seems like a good idea to reorder the postmaster's
startup operations so that the data-directory lockfile is checked
before trying to acquire the port lockfile, instead of after.  That
way, in the common scenario where you're trying to start a second
postmaster in the same directory + same port, it'd fail cleanly
even if /tmp/.s.PGSQL.5432.lock had disappeared.

Comments?

			regards, tom lane

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Subject: LOCK Fixes/Break on FreeBSD 4.2-STABLE
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