On 30/03/2026 07:50, Ashutosh Bapat wrote:
On Sat, Mar 28, 2026 at 4:47 AM Heikki Linnakangas <[email protected]> wrote:
On 27/03/2026 09:01, Ashutosh Bapat wrote:
/* Restore basic shared memory pointers */
if (UsedShmemSegAddr != NULL)
+ {
InitShmemAllocator(UsedShmemSegAddr);
+ ShmemCallRequestCallbacks();
It's not clear how we keep the list of registered callbacks across the
backends and also after restart in-sync. How do we make sure that the
callbacks registered at this time are the same callbacks registered
before creating the shared memory? How do we make sure that the
callbacks registered after the startup are also registered after
restart?
On Unix systems, the registered callbacks are inherited by fork(), and
also survive over crash restart. With EXEC_BACKEND, the assumption is
that calling a library's _PG_init() function will register the same
callbacks every time. We make the same assumption today with the
shmem_startup hook.
RegisterShmemCallbacks() may be called after the startup, and it will
add new areas to the shared memory. How are those registries synced
across the backends? From your answer below, those registries are not
synced across backends. They will be wiped out by the restart and
won't be registered again. Is that right? I think we need to document
this fact and also the need to call RegisterShmemCallbacks() from all
the backends where the new areas are required after the startup.
Correct. Ok, I'll add a note to comment on RegisterShmemCallbacks() to
call that out more explicitly, hope it helps.
- Heikki
