Horton

Re: Synchronizing slots from primary to standby

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Ajin Cherian <itsajin@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2023-11-17T11:38:14Z
Lists: pgsql-hackers 
Hi,

On 11/16/23 1:03 PM, shveta malik wrote:
> On Thu, Nov 16, 2023 at 3:43 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> PFA v35. It has below changes:

Thanks for the update!

> 6) shutdown the slotsync worker on promotion.

+   /*
+    * Shutdown the slot sync workers to prevent potential conflicts between
+    * user processes and slotsync workers after a promotion. Additionally,
+    * drop any slots that have initiated but not yet completed the sync
+    * process.
+    */
+   ShutDownSlotSync();
+   slotsync_drop_initiated_slots();

I think there is a corner case here.

If there is promotion while slot creation is in progress (slot has just
been created and is in 'i' state), then when we shutdown the sync slot worker
in ShutDownSlotSync() we'll set slot->in_use = false in ReplicationSlotDropPtr().

Indeed, when we shut the sync worker down:

(gdb) bt
#0  ReplicationSlotDropPtr (slot=0x7f25af5c9bb0) at slot.c:734
#1  0x000056266c8106a7 in ReplicationSlotDropAcquired () at slot.c:725
#2  0x000056266c810170 in ReplicationSlotRelease () at slot.c:583
#3  0x000056266c80f420 in ReplicationSlotShmemExit (code=1, arg=0) at slot.c:189
#4  0x000056266c86213b in shmem_exit (code=1) at ipc.c:243
#5  0x000056266c861fdf in proc_exit_prepare (code=1) at ipc.c:198
#6  0x000056266c861f23 in proc_exit (code=1) at ipc.c:111

So later on, when we'll want to drop this slot in slotsync_drop_initiated_slots()
we'll get things like:

2023-11-17 11:22:08.526 UTC [2195486] FATAL:  replication slot "logical_slot4" does not exist

Reason is that slotsync_drop_initiated_slots() does call SearchNamedReplicationSlot():

(gdb) bt
#0  SearchNamedReplicationSlot (name=0x7f743f5c9ab8 "logical_slot4", need_lock=false) at slot.c:388
#1  0x0000556ef0974ec1 in ReplicationSlotAcquire (name=0x7f743f5c9ab8 "logical_slot4", nowait=true) at slot.c:484
#2  0x0000556ef09754e7 in ReplicationSlotDrop (name=0x7f743f5c9ab8 "logical_slot4", nowait=true, user_cmd=false) at slot.c:668
#3  0x0000556ef095f0a3 in slotsync_drop_initiated_slots () at slotsync.c:369

that returns a NULL slot if slot->in_use = false.

One option could be to make sure slot->in_use = true before calling ReplicationSlotDrop() here?

+   foreach(lc, slots)
+   {
+       ReplicationSlot *s = (ReplicationSlot *) lfirst(lc);
+
+       ReplicationSlotDrop(NameStr(s->data.name), true, false);

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

Commits

  1. Doc: Add the new section "Logical Replication Failover".

  2. Fix the review comments and a bug in the slot sync code.

  3. Fix a test in failover slots regression test.

  4. Fix the intermittent buildfarm failures in 040_standby_failover_slots_sync.

  5. Split XLogCtl->LogwrtResult into separate struct members

  6. Ensure that the sync slots reach a consistent state after promotion without losing data.

  7. Introduce a new GUC 'standby_slot_names'.

  8. Fix BF failure introduced by commit b3f6b14cf4.

  9. Fixups for commit 93db6cbda0.

  10. Fix BF failure in commit 93db6cbda0.

  11. Add a new slot sync worker to synchronize logical slots.

  12. Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

  13. Disable autovacuum on primary in 040_standby_failover_slots_sync test.

  14. Fix the incorrect format specifier used in commit 7a424ece48.

  15. Change the LOG level in 040_standby_failover_slots_sync.pl to DEBUG2.

  16. Add more LOG and DEBUG messages for slot synchronization.

  17. Another try to fix BF failure introduced in commit ddd5f4f54a.

  18. Fix BF introduced in commit ddd5f4f54a.

  19. Add a slot synchronization function.

  20. Clean-ups for 776621a5e4 and 7329240437.

  21. Enhance libpqrcv APIs to support slot synchronization.

  22. Add a failover option to subscriptions.

  23. Allow setting failover property in the replication command.

  24. Allow to enable failover property for replication slots via SQL API.

  25. Add macros for looping through a List without a ListCell.

  26. Track conflict_reason in pg_replication_slots.

  27. Allow upgrades to preserve the full subscription's state.

  28. Add support for incremental backup.

  29. Simplify some logic in CreateReplicationSlot()

  30. Fix uninitialized access to InitialRunningXacts during decoding after ERROR.

  31. Flexible options for CREATE_REPLICATION_SLOT.

  32. Empty search_path in logical replication apply worker and walsender.

  33. Fix oldest xmin and LSN computation across repslots after advancing

  34. Require C99 (and thus MSCV 2013 upwards).