"Cyril VELTER" <cyril.velter@libertysurf.fr> writes:
> Are the PGShmemHeader fields only used by PGSharedMemoryCreate ?
Other than totalsize and freeoffset, I believe so. I see no reason
that a particular port couldn't stick different fields in there if it
had a mind to.
>> How does that solve the problem of determining whether a *previously*
>> created shmem block is still in use?
> Ok, I overlooked that, my proposal for PGSharedMemoryIsInUse doesn't
> make sense (and it doesn't matter on Beos because shared mem segments are
> automaticaly reaped at the end of the process).
Well, SharedMemoryIsInUse is *not* just about ensuring that the shared
memory gets reaped. The point is to ensure that you can't start a new
postmaster until the last old backend is gone. (Consider situations
where the parent postmaster process crashes, or perhaps is kill -9'd
by a careless DBA, but there are still active backends. We want to
detect that situation and ensure that a new postmaster will refuse to
start.)
>> How so? If those calls were needed before, why won't all three still
>> be needed?
> In the current hack, I've to iterate over all sharedmem segments (system
> wide) to find the original one by name. There is a race condition here if
> several backend are starting at the same time. beos_before_backend_startup
> beos_backend_startup_failed acquire / release a semaphore which prevent
> several fork at the same time.
Does keeping the shmem segment name around solve that? Seems like you
don't need a PGShmemHeader field for that; just store it in a static
variable.
regards, tom lane
