Robert Haas <robertmh...@gmail.com> writes:
> On Sun, Apr 10, 2011 at 5:03 PM, A.M. <age...@themactionfaction.com> wrote:
>> To ensure that no two postmasters can startup in the same data directory, I
>> use fcntl range locking on the data directory lock file, which also works
>> properly on (properly configured) NFS volumes. Whenever a postmaster or
>> postmaster child starts, it acquires a read (non-exclusive) lock on the data
>> directory's lock file. When a new postmaster starts, it queries if anything
>> would block a write (exclusive) lock on the lock file which returns a
>> lock-holding PID in the case when other postgresql processes are running.
> This seems a lot leakier than what we do now (imagine, for example,
> shared storage) and I'm not sure what the advantage is.
BTW, the above-described solution flat out doesn't work anyway, because
it has a race condition. Postmaster children have to reacquire the lock
after forking, because fcntl locks aren't inherited during fork(). And
that means you can't tell whether there's a just-started backend that
hasn't yet acquired the lock. It's really critical for our purposes
that SysV shmem segments are inherited at fork() and so there's no
window where a just-forked backend isn't visible to somebody checking
the state of the shmem segment.
regards, tom lane
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