Re: file-locking and postmaster.pid

[korry <korry(at)appx(dot)com>]

> > You never need to reduce it to a shared lock.  On postmaster startup,
> > try to lock the sentinel byte (one byte past the end-of-file).  If you
> > can lock it, you know that no other postmaster has that byte locked.  If
> > you can't lock it, another postmaster is running. It is an atomic
> > operation. 
>
> This doesn't work if the postmaster dies but a backend continues to run,
> which is arguably the most important case we need to protect against.

I may be confused here, but I don't see the problem - byte-range locks are not inherited across a fork.  A backend would never hold the lock, a backend would never even look for the lock.


> > However, Tom may be correct about NFS locking, but I guess I'm surprised
> > that anyone would care :-)
>
> Quite a lot of people run NFS-mounted data directories ...

I'm happy to take your word for that, and I agree that if NFS is important and locking is brain-dead on NFS, then relying solely on a lock is unacceptable.


            -- Korry