Обсуждение: Win32 latch implementation revisited

It just occurred to me that the Windows latch implementation goes
through a lot of trouble to dynamically assign the shared Windows event
handles to the latches in OwnLatch, but there's really no reason why
they can't be statically assigned in InitSharedLatch instead. We have to
allocate the same amount of event handles anyway.

That makes the implementation a lot simpler, eliminating the shared
memory block dedicated to latches altogether, and all the related
bookkeeping. We no longer need NumSharedLatches() function anymore
either, each InitSharedLatch call can allocate a new event handle directly.

It was the separation of InitSharedLatch and OwnLatch steps that made
this possible, which is probably why this didn't occur to me earlier.
Separating those steps was definitely a good move.

Unless someone sees a problem with this, I'll commit the attached patch
to do the simplification.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> It just occurred to me that the Windows latch implementation goes 
> through a lot of trouble to dynamically assign the shared Windows event 
> handles to the latches in OwnLatch, but there's really no reason why 
> they can't be statically assigned in InitSharedLatch instead. We have to 
> allocate the same amount of event handles anyway.

> That makes the implementation a lot simpler, eliminating the shared 
> memory block dedicated to latches altogether, and all the related 
> bookkeeping. We no longer need NumSharedLatches() function anymore 
> either, each InitSharedLatch call can allocate a new event handle directly.

That sounds real good.  The only possible downside I can see is this:

> + * InitSharedLatch needs to be called in postmaster before forking child
> + * processes, usually right after allocating the shared memory block
> + * containing the latch with ShmemInitStruct. The Unix implementation
> + * doesn't actually require that, but the Windows one does.

But realistically I think we have to insist on InitSharedLatch being
done during shared memory setup anyway, else there will be race
condition issues.  So no objection here.
        regards, tom lane