On 04/04/2026 16:51, Matthias van de Meent wrote:
+++ b/src/include/storage/shmem.h
+/*
+ * Shared memory is reserved and allocated in stages at postmaster startup,
+ * and in EXEC_BACKEND mode, there's some extra work done to "attach" to them
+ * at backend startup. ShmemCallbacks holds callback functions that are
+ * called at different stages.
+ */
+typedef struct ShmemCallbacks
Maybe this should also have the opportunity for a (before_)shmem_exit callback?
Hmm, yeah, perhaps, but I'm going to skip that for now. We already have
a mechanism for shmem-exit callbacks, and I'm not sure how that would
plug into this. I think we can do that later if it turns out to be a
good idea, and I don't think it changes the parts that's included in the
patches now.
+ * on-demaind in a backend. If a subsystem sets this flag, the callbacks are
+ * called immediately after registration, to initialize or attach to the
+ * requested shared memory areas.
Ideally we only immediately call the callbacks if we're under
postmaster, or in a standalone backend; we shouldn't allocate shmem
for some preloaded libraries that set this flag, at least not ahead of
loading all preload libraries.
Right, the SHMEM_CALLBACKS_ALLOW_AFTER_STARTUP flag doesn't do anything
if called during shared_preload_libraries processing. I'll re-word the
comment to clarify that.
While it's mostly mechanical changes, it did make me notice the rather
annoying allocation patterns by XLOGShmemRequest. It allocates various
types of data in one go (which, in principle, is fine) but in doing so
it adds its own alignment tricks etc, and I'm not super stoked about
that. If time allows, could we clean that up?
I'm not going to cram it into these patches, but +1.
- Heikki
