Re: Physical replication slot advance is not persistent

From: Alexey Kondratov <a(dot)kondratov(at)postgrespro(dot)ru>
To: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, michael(at)paquier(dot)xyz, simon(at)2ndquadrant(dot)com
Subject: Re: Physical replication slot advance is not persistent
Date: 2019-12-25 17:28:04
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On 25.12.2019 16:51, Alexey Kondratov wrote:
> On 25.12.2019 07:03, Kyotaro Horiguchi wrote:
>> As the result I think what is needed there is just checking if the
>> returned lsn is equal or larger than moveto. Doen't the following
>> change work?
>> -    if (XLogRecPtrIsInvalid(endlsn))
>> +    if (moveto <= endlsn)
> Yep, it helps with physical replication slot persistence after
> advance, but the whole validation (moveto <= endlsn) does not make
> sense for me. The value of moveto should be >= than minlsn ==
> confirmed_flush / restart_lsn, while endlsn == retlsn is also always
> initialized with confirmed_flush / restart_lsn. Thus, your condition
> seems to be true in any case, even if it was no-op one, which we were
> intended to catch.
> I will recheck everything again and try to come up with something
> during this week.

If I get it correctly, then we already keep previous slot position in
the minlsn, so we just have to compare endlsn with minlsn and treat
endlsn <= minlsn as a no-op without slot state flushing.

Attached is a patch that does this, so it fixes the bug without
affecting any user-facing behavior. Detailed comment section and DEBUG
output are also added. What do you think now?

I have also forgotten to mention that all versions down to 11.0 should
be affected with this bug.


Alexey Kondratov

Postgres Professional
Russian Postgres Company

Attachment Content-Type Size
v2-0001-Make-physical-replslot-advance-persistent.patch text/x-patch 1.4 KB

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