Re: file-locking and postmaster.pid

[Alvaro Herrera <alvherre(at)commandprompt(dot)com>]

korry wrote:
> > > 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.

Well, you are wrong here.  We _want_ every backend to hold a shared
lock.  We need to stop a postmaster from starting if there is a backend
running that was started by a no-longer-running postmaster.

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