Horton

Re: locking [user] catalog tables vs 2pc vs logical rep

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>, Markus Wanner <markus.wanner@enterprisedb.com>
Date: 2021-02-23T03:26:39Z
Lists: pgsql-hackers 
On Tue, Feb 23, 2021 at 3:58 AM Andres Freund <andres@anarazel.de> wrote:
>
>
> At first this seems to be a significant issue. But on the other hand, if
> you were to shut the cluster down in this situation (or disconnect all
> sessions), you have broken cluster on your hand - without logical
> decoding being involved. As it turns out, we need to read pg_class to
> log in...  And I can't remember this being reported to be a problem?
>

I don't remember seeing such a report but I think that is a reason
enough (leaving aside logical decoding of 2PC) to either disallow
locking catalog tables or at least document it in some way.

>
> Perhaps all that we need to do is to disallow 2PC prepare if [user]
> catalog tables have been locked exclusively?
>

Right, and we have discussed this during development [1][2]. We
thought either we disallow this operation or will document it. I
thought of doing this along with a core-implementation of Prepare
waiting to get it logically replicated. But at this stage, I think if
the user wants he can do a similar thing in his application where
after prepare it can wait for the transaction to get logically
replicated (if they have their own replication solution based on
logical decoding) and then decide whether to rollback or commit. So,
maybe we should either disallow this operation or at least document
it. What do you think?

> Similar to how we're
> disallowing preparing tables with temp table access.
>

Yeah, we disallow other things like pg_export_snapshot as well in a
Prepared transaction, so we can probably disallow this operation as
well.

[1] - https://www.postgresql.org/message-id/CAA4eK1JeeXOwD6rYnhSOYk5YN-fUTmxe1GkTpN2-BvgnKN6gZg%40mail.gmail.com
[2] - https://www.postgresql.org/message-id/CAMGcDxf83P5SGnGH52=_0wRP9pO6uRWCMRwAA0nxKtZvir2_vQ@mail.gmail.com

-- 
With Regards,
Amit Kapila.

Commits

  1. Doc: Update caveats in synchronous logical replication.

  2. Document a few caveats in synchronous logical replication.

  3. Update the docs and comments for decoding of prepared xacts.

  4. Allow decoding at prepare time in ReorderBuffer.

  5. Documentation for logical decoding.