Re: Better shared data structure management and resizable shared data structures

On Thu, Mar 12, 2026 at 3:21 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> >> I'm currently leaning towards _PG_init(), except for allocations that
> >> depend on MaxBackends. For those, you can install a shmem_request_hook
> >> that sets the size in the descriptor. In other words, you can leave the
> >> 'size' as empty in _PG_init(), but set it later in the shmem_request_hook.
> >
> > Why can't you just do the whole thing later?
>
> shmem_request_hook won't work in EXEC_BACKEND mode, because in
> EXEC_BACKEND mode, ShmemRegisterStruct() also needs to be called at
> backend startup.
>
> One of my design goals is to avoid EXEC_BACKEND specific steps so that
> if you write your extension oblivious to EXEC_BACKEND mode, it will
> still usually work with EXEC_BACKEND. For example, if it was necessary
> to call a separate AttachShmem() function for every shmem struct in
> EXEC_BACKEND mode, but which was not needed on Unix, that would be bad.

That's *definitely* a good goal. A less important but still valuable
goal is to maximize the notational simplicity of the mechanism. Your
callback idea is elegant in theory but in practice it seems like it
might make it harder for people to get started quickly on a new
module, and having to create the object in one place and then fill in
the size in another sort of has the same problem. I don't really know
what to do about that, but it's something to think about. The
complexity of getting the details right is annoyingly high in this
area.

> > Yeah, I think RequestNamedLWLockTranche() might be fine if you just
> > need LWLocks, but if you need a bunch of resources, putting them all
> > into the same chunk of memory seems cleaner.
>
> Agreed. Then again, how often do you need just a LWLock (or multiple
> LWLocks)? Surely you have a struct you want to protect with the lock. I
> guess having shmem hash table but no struct would be pretty common, though.

Yeah, we've developed an annoying number of different ways to do this
stuff. I don't entirely know how to fix that.

--
Robert Haas
EDB: http://www.enterprisedb.com