On Tue, May 18, 2021 at 1:29 PM osumi.takamichi@fujitsu.com
<osumi.takamichi@fujitsu.com> wrote:
>
> On Monday, May 17, 2021 6:45 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > We allow taking locks on system catalogs, so why prohibit
> > user_catalog_tables? However, I agree that if we want plugins to acquire the
> > lock on user_catalog_tables then we should either prohibit decoding of such
> > relations or do something else to avoid deadlock hazards.
> OK.
>
> Although we have not concluded the range of logical decoding of user_catalog_table
> (like we should exclude TRUNCATE command only or all operations on that type of table),
> I'm worried that disallowing the logical decoding of user_catalog_table produces
> the deadlock still. It's because disabling it by itself does not affect the
> lock taken by TRUNCATE command. What I have in mind is an example below.
>
> (1) plugin (e.g. pgoutput) is designed to take a lock on user_catalog_table.
> (2) logical replication is set up in synchronous mode.
> (3) TRUNCATE command takes an access exclusive lock on the user_catalog_table.
> (4) This time, we don't do anything for the TRUNCATE decoding.
> (5) the plugin tries to take a lock on the truncated table
> but, it can't due to the lock by TRUNCATE command.
>
If you skip decoding of truncate then we won't invoke plugin API so
step 5 will be skipped.
> I was not sure that the place where the plugin takes the lock is in truncate_cb
> or somewhere else not directly related to decoding of the user_catalog_table itself,
> so I might be wrong. However, in this case,
> the solution would be not disabling the decoding of user_catalog_table
> but prohibiting TRUNCATE command on user_catalog_table in synchronous_mode.
> If this is true, I need to extend an output plugin and simulate the deadlock first
> and remove it by fixing the TRUNCATE side. Thoughts ?
>
I suggest not spending too much time reproducing this because it is
quite clear that it will lead to deadlock if the plugin acquires lock
on user_catalog_table and we allow decoding of truncate. But if you
want to see how that happens you can try as well.
--
With Regards,
Amit Kapila.
