| From: | "Hayato Kuroda (Fujitsu)" <kuroda(dot)hayato(at)fujitsu(dot)com> |
|---|---|
| To: | 'Vitaly Davydov' <v(dot)davydov(at)postgrespro(dot)ru> |
| Cc: | suyu(dot)cmj <mengjuan(dot)cmj(at)alibaba-inc(dot)com>, aekorotkov <aekorotkov(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, tomas <tomas(at)vondra(dot)me>, michael <michael(at)paquier(dot)xyz>, bharath(dot)rupireddyforpostgres <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | RE: Newly created replication slot may be invalidated by checkpoint |
| Date: | 2025-09-25 05:43:16 |
| Message-ID: | OSCPR01MB14966FC74C1C617559345930BF51FA@OSCPR01MB14966.jpnprd01.prod.outlook.com |
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| Lists: | pgsql-hackers |
Dear Vitaly,
> I propose to apply this patch and then to think how to resolve this race
> condition, which seems to take place in 18 and master as well.
No, I think this invalidation can't happen in PG18/HEAD.
This is because in CheckpointGuts()->CheckPointReplicationSlots()->
ReplicationSlotsComputeRequiredLSN(), slots are examined one by one to determine
whether their restart_lsn has advanced since the last check. If any slot has
advanced, protection is applied starting from the oldest restart_lsn.
Crucially, this check is performed before WAL removal. The function call was
introduced in commit ca307d5cec.
Further analysis shows that it is also safe if a slot is being created and WAL
advances after CheckpointGuts() but before the removal segments are determined.
In this case the restart_lsn points the CHECKPOINT_REDO generated by the current
CHECKPOINT. This and later records won't be discarded.
Best regards,
Hayato Kuroda
FUJITSU LIMITED
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