Re: [PATCH] Two remaining shmem attachment issues in single-user mode

On 19/09/2026 22:12, Ayush Tiwari wrote:
> Hi,
>
> I found two more shmem attachment issues in single-user mode after
> the recent fixes. Patches attached.
>
> With ShmemInitStruct(), a second call for the same name and size errors
> out with "already initialized". We only look for the old allocation if
> IsUnderPostmaster is true, so a standalone backend goes straight down
> the allocation path again.
>
> 0001 drops that condition. It fixes postmaster-startup reattachment
> too, which I did hesitate over at first. But AFAICS that was supported
> before the refactoring, and it's what the legacy API still promises.
> So I'd lean towards restoring that behaviour in both places. Is there
> a reason not to? (The size and initialization checks are still there.)

Hmm, so the scenario is:

1. Start postgres in single-user mode
2. Load an extension that calls ShmemInitStruct() to alloc a shmem area
3. The extension calls ShmemInitStruct() again, to get a pointer to the
already-initialized area.

I didn't think of that scenario. There's no reason to not support that,
although it's pretty weird for an extension to do that. One use case
might be to have a shared struct between two cooperating extensions, so
that they both call ShmemInitStruct() to get a pointer to the same area.
I'm not aware of any extensions actually doing that, though.

I'll commit that fix, thanks!

> The other one is a bit odd: ask for an existing area with
> SHMEM_ATTACH_UNKNOWN_SIZE after startup in single-user mode, and we
> tell you it "cannot be used during startup".
>
> IIUC, the distinction we need here is whether we're still working out
> the initial shmem requirements, not whether we have a postmaster.
> 0002 adds SRS_REQUESTING_AFTER_STARTUP for that. I couldn't see a clean
> way to reuse SRS_REQUESTING without mixing those cases up.
>
> I'm a bit on the fence about adding another state just for this.
> With the PG19 release getting close, I thought I'd send this out for
> feedback before spending more time iterating on it. Does the extra
> state seem like the right approach?

Extra state sounds reasonable. Thanks, I'll take a closer look, and I'll
double-check all the other places in shmem.c where we use
IsUnderPostmaster, too.

The IsUnderPostmaster variable is deceptive. It's easy to forget about
single-user code, and incorrectly assume that IsUnderPostmaster == true
means you're a backend and IsUnderPostmaster == false means you're
postmaster. I think that's what happened to me here and with the
previous single-user mode bugs. I remember I've struggled to keep that
in mind in the past too. We should perhaps replace IsUnderPostmaster
with a three-valued enum or something (postmaster, backend, single-user
backend).

We got that wrong in pg_get_shmem_pagesize(), too, also in previous
versions:

backend> select * from pg_get_shmem_allocations_numa();
1: name (typeid = 25, len = -1, typmod = -1, byval = f)
2: numa_node (typeid = 23, len = 4, typmod = -1, byval = t)
3: size (typeid = 20, len = 8, typmod = -1, byval = t)
----
TRAP: failed Assert("IsUnderPostmaster"), File:
"../src/backend/storage/ipc/shmem.c", Line: 731, PID: 121579

- Heikki