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

On Mon, May 8, 2023 at 8:09 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, May 5, 2023 at 9:14 AM Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > On Wednesday, May 3, 2023 3:17 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> >
> > Attach another patch to fix the problem that pa_shutdown will access invalid
> > MyLogicalRepWorker. I personally want to avoid introducing new static variable,
> > so I only reorder the callback registration in this version.
> >
> > When testing this, I notice a rare case that the leader is possible to receive
> > the worker termination message after the leader stops the parallel worker. This
> > is unnecessary and have a risk that the leader would try to access the detached
> > memory queue. This is more likely to happen and sometimes cause the failure in
> > regression tests after the registration reorder patch because the dsm is
> > detached earlier after applying the patch.
> >
>
> I think it is only possible for the leader apply can worker to try to
> receive the error message from an error queue after your 0002 patch.
> Because another place already detached from the queue before stopping
> the parallel apply workers. So, I combined both the patches and
> changed a few comments and a commit message. Let me know what you
> think of the attached.

I have one comment on the detaching error queue part:

+ /*
+ * Detach from the error_mq_handle for the parallel apply worker before
+ * stopping it. This prevents the leader apply worker from trying to
+ * receive the message from the error queue that might already
be detached
+ * by the parallel apply worker.
+ */
+ shm_mq_detach(winfo->error_mq_handle);
+ winfo->error_mq_handle = NULL;

In pa_detach_all_error_mq(), we try to detach error queues of all
workers in the pool. I think we should check if the queue is already
detached (i.e. is NULL) there. Otherwise, we will end up a SEGV if an
error happens after detaching the error queue and before removing the
worker from the pool.

Regards,

--
Masahiko Sawada
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