Re: Perform streaming logical transactions by background workers and parallel apply

On Fri, Apr 28, 2023 at 11:48 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> While investigating this issue, I've reviewed the code around
> callbacks and worker termination etc and I found a problem.
>
> A parallel apply worker calls the before_shmem_exit callbacks in the
> following order:
>
> 1. ShutdownPostgres()
> 2. logicalrep_worker_onexit()
> 3. pa_shutdown()
>
> Since the worker is detached during logicalrep_worker_onexit(),
> MyLogicalReplication->leader_pid is an invalid when we call
> pa_shutdown():
>
> static void
> pa_shutdown(int code, Datum arg)
> {
>     Assert(MyLogicalRepWorker->leader_pid != InvalidPid);
>     SendProcSignal(MyLogicalRepWorker->leader_pid,
>                    PROCSIG_PARALLEL_APPLY_MESSAGE,
>                    InvalidBackendId);
>
> Also, if the parallel apply worker fails shm_toc_lookup() during the
> initialization, it raises an error (because of noError = false) but
> ends up a SEGV as MyLogicalRepWorker is still NULL.
>
> I think that we should not use MyLogicalRepWorker->leader_pid in
> pa_shutdown() but instead store the leader's pid to a static variable
> before registering pa_shutdown() callback.
>

Why not simply move the registration of pa_shutdown() to someplace
after logicalrep_worker_attach()? BTW, it seems we don't have access
to MyLogicalRepWorker->leader_pid till we attach to the worker slot
via logicalrep_worker_attach(), so we anyway need to do what you are
suggesting after attaching to the worker slot.

--
With Regards,
Amit Kapila.