Re: file-locking and postmaster.pid

korry wrote:

> > I think the next question is -- how would the lock interface be used?
> > We could acquire an exclusive lock on postmaster start (to make sure no
> > backend is running), then reduce it to a shared lock.  Every backend
> > would inherit the shared lock.  But the lock exchange is not guaranteed
> > to be atomic so a new postmaster could start just after we acquire the
> > lock and acquire the shared lock.  It'd need to be complemented with
> > another lock.
> 
> 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.

> 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 ...

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.