Re: RegisterShmemCallbacks() does nothing in single-user mode

On 21/08/2026 08:36, Ayush Tiwari wrote:
> Looking at the after-startup path from 283e823f9dc, I noticed that
> RegisterShmemCallbacks() does not do anything useful in single-user mode.
> The branch that does the work immediately is guarded by:
>
>     if (shmem_request_state == SRS_DONE && IsUnderPostmaster)
>
> IsUnderPostmaster is false in a standalone backend as well as in the
> postmaster, so a standalone backend always falls through to the "remember
> the callbacks for later" branch, and startup has already consumed that
> list.  An extension loaded at runtime there, e.g. with LOAD or CREATE
> EXTENSION, therefore never has its callbacks called, and
> RegisterShmemCallbacks() reports nothing:
>
>     LOG:  test_shmem module's _PG_init called
>     ERROR:  shmem area not attached or initialized in this process
>
> The LOG line is the only trace of the registration.  The ERROR is
> test_shmem's own guard when the area is later used, not the shmem code;
> an extension without such a guard would dereference a NULL pointer?
>
> I am unsure whether this was left out intentionally or simply missed, but
> it seems worth having: a standalone backend has shared memory and the
> same after-startup reserve, and a silent no-op is easy to mistake for
> success.

That was an oversight, thanks for the testing!

> The attached patch also checks !IsPostmasterEnvironment and adds a
> single-user case to the test_shmem TAP test.

Hmm, I think the check should be just "if (shmem_request_state ==
SRS_DONE)", with an assert that it's not called from the postmaster. The
reason it cannot be called from the postmaster after startup is that
postmaster cannot acquire lwlocks.

> skip 'single-user mode is not supported on this platform', 1
> if $windows_os;

Huh, we don't support single-user mode on Windows? /me looks around. We
do, but apparently there are some issues with it in the CI. All other
single-user tests are also skipped on Windows. Per commit 1f2e51e3c7:

> These tests are skipped on Windows, as direct calls of
> postgres --single would fail on permission failures. There is no
> platform-specific behavior that needs to be checked, so living with this
> restriction should be fine. The CI is OK with that, now let's see what
> the buildfarm tells.

Ok then I guess, but I'll copy the comment from that commit to explain
why it's not run on Windows.

See attached.

- Heikki