Re: crash with synchronized_standby_slots

On 2024-Nov-29, Amit Kapila wrote:

> I tried it on my Windows machine and noticed that ReplicationSlotCtl
> is NULL for syslogger, so the problem doesn't occur. The reason is
> that we don't attach to shared memory in syslogger, so ideally
> ReplicationSlotCtl should be NULL. Because we inherit everything
> through the fork for Linux systems and then later for processes that
> don't want to attach to shared memory, we call PGSharedMemoryDetach()
> from postmaster_child_launch(). The PGSharedMemoryDetach won't
> reinitialize the memory pointed to by ReplicationSlotCtl, so, it would
> be an invalid memory.

Heh, interesting.  I'm not sure if we should try to do something about
invalid pointers being left around after shmem initialization.  Also, is
this the first GUC check_hook that needs to take an LWLock?

Anyway, I have pushed this.

BTW it occurs to me that there might well be some sort of thundering
herd problem if every process needs to run the check_hook when a SIGHUP
is broadcast, and they'll all be waiting on that particular lwlock and
run the same validation locally again and again.  I bet if you have a
few thousand backends (hi Jakub! [1]) it's problematic.  Maybe we need a
different way to validate the GUC, but I don't know what that might be;
but doing the validation once and storing the result in shmem might be
better.

On 2024-Nov-29, Zhijie Hou (Fujitsu) wrote:

> I can also reproduce this bug and confirmed that the bug is fixed
> after applying the patch. In addition to the regression tests, I also
> manually tested the behavior of the postmaster, walsender, and user
> backend after reloading the configuration, and they all work as
> expected.

Many thanks for testing!

[1] https://postgr.es/m/CAKZiRmwrBjCbCJ433wV5zjvwt_OuY7BsVX12MBKiBu+eNZDm6g@mail.gmail.com

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Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
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