Re: [HACKERS] logical replication and PANIC during shutdowncheckpoint in publisher

On 02/05/17 05:35, Michael Paquier wrote:
> On Tue, May 2, 2017 at 7:07 AM, Peter Eisentraut
> <peter.eisentraut@2ndquadrant.com> wrote:
>> On 4/25/17 21:47, Michael Paquier wrote:
>>> Attached is an updated patch to reflect that.
>>
>> I edited this a bit, here is a new version.
> 
> Thanks, looks fine for me.
> 
>> A variant approach would be to prohibit *all* new commands after
>> entering the "stopping" state, just let running commands run.  That way
>> we don't have to pick which individual commands are at risk.  I'm not
>> sure that we have covered everything here.
> 
> It seems to me that everything is covered. We are taking about
> creation and dropping of slots here, where standby snapshots can be
> created and SQL queries can be run when doing a tablesync meaning that
> FPWs could be taken in the context of the WAL sender. Blocking all
> commands would be surely safer I agree, but I see no reason to block
> things more than necessary.
> 

I don't think the code covers all because a) the SQL queries are not
covered at all that I can see and b) logical decoding can theoretically
do HOT pruning (even if the chance is really small) so it's not safe to
start logical replication either.

I wonder if this whole prevent thing should just be called
unconditionally on walsender that's connected to database
(am_db_walsender), because in the WAL logging will only happen there -
CREATE_REPLICATION_SLOT PHYSICAL will not WAL log and
CREATE_REPLICATION_SLOT LOGICAL can't be run without being connected to
db, neither can logical decoding and SQL queries, so that coves all.

--  Petr Jelinek                  http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training &
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