Re: [HACKERS] logical decoding of two-phase transactions

On Mon, Dec 14, 2020 at 2:59 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>

Today, I looked at one of the issues discussed earlier in this thread
[1] which is that decoding can block (or deadlock can happen) when the
user explicitly locks the catalog relation (like Lock pg_class) or
perform Cluster on non-relmapped catalog relations (like Cluster
pg_trigger using pg_class_oid_index; and the user_table on which we
have performed any operation has a trigger) in the prepared xact. As
discussed previously, we don't have a problem when user tables are
exclusively locked because during decoding we don't acquire any lock
on those and in fact, we have a test case for the same in the patch.

In the previous discussion, most people seem to be of opinion that we
should document it in a category "don't do that", or prohibit to
prepare transactions that lock system tables in the exclusive mode as
any way that can block the entire system. The other possibility could
be that the plugin can allow enabling lock_timeout when it wants to
allow decoding of two-phase xacts and if the timeout occurs it tries
to fetch by disabling two-phase option provided by the patch.

I think it is better to document this as there is no realistic
scenario where it can happen. I also think separately (not as part of
this patch) we can investigate whether it is a good idea to prohibit
prepare for transactions that acquire exclusive locks on catalog
relations.

Thoughts?

[1] - https://www.postgresql.org/message-id/CAMGcDxf83P5SGnGH52%3D_0wRP9pO6uRWCMRwAA0nxKtZvir2_vQ%40mail.gmail.com

--
With Regards,
Amit Kapila.