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Doc: Fix pg_copy_logical_replication_slot description.

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This commit documents that the failover option is not copied when using
the pg_copy_logical_replication_slot function.

In passing, we modify the comments in the function clarifying the reason
for this behavior.

Reported-by: <duffieldzane@gmail.com>
Author: Hou Zhijie <houzj.fnst@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/173976850802.682632.11315364077431550250@wrigleys.postgresql.org
This commit is contained in:
Amit Kapila 2025-02-25 09:42:07 +05:30
parent 15601fa21a
commit 5b8f2ccc0a
2 changed files with 16 additions and 7 deletions
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4
doc/src/sgml/func.sgml
View File

@ -29374,6 +29374,10 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
from the same <acronym>LSN</acronym> as the source logical slot. Both
<parameter>temporary</parameter> and <parameter>plugin</parameter> are
optional; if they are omitted, the values of the source slot are used.
The <literal>failover</literal> option of the source logical slot
is not copied and is set to <literal>false</literal> by default. This
is to avoid the risk of being unable to continue logical replication
after failover to standby where the slot is being synchronized.
</para></entry>
</row>

19
src/backend/replication/slotfuncs.c
View File

@ -695,13 +695,18 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
* hence pass find_startpoint false. confirmed_flush will be set
* below, by copying from the source slot.
*
* To avoid potential issues with the slot synchronization where the
* restart_lsn of a replication slot can go backward, we set the
* failover option to false here. This situation occurs when a slot
* on the primary server is dropped and immediately replaced with a
* new slot of the same name, created by copying from another existing
* slot. However, the slot synchronization will only observe the
* restart_lsn of the same slot going backward.
* We don't copy the failover option to prevent potential issues with
* slot synchronization. For instance, if a slot was synchronized to
* the standby, then dropped on the primary, and immediately recreated
* by copying from another existing slot with much earlier restart_lsn
* and confirmed_flush_lsn, the slot synchronization would only
* observe the LSN of the same slot moving backward. As slot
* synchronization does not copy the restart_lsn and
* confirmed_flush_lsn backward (see update_local_synced_slot() for
* details), if a failover happens before the primary's slot catches
* up, logical replication cannot continue using the synchronized slot
* on the promoted standby because the slot retains the restart_lsn
* and confirmed_flush_lsn that are much later than expected.
*/
create_logical_replication_slot(NameStr(*dst_name),
plugin,