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Update comments atop ReplicationSlotCreate.

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Since commit 1462aad2e4, which introduced the ability to modify the
two_phase property of a slot, the comments above ReplicationSlotCreate
have become outdated. We have now added a cautionary note in the comments
above ReplicationSlotAlter explaining when it is safe to modify the
two_phase property of a slot.

Author: Daniil Davydov <3danissimo@gmail.com>
Author: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CAJDiXggZXQZ7bD0QcTizDt6us9aX6ZKK4dWxzgb5x3+TsVHjqQ@mail.gmail.com
This commit is contained in:
Amit Kapila
2026-01-06 05:02:25 +00:00
parent 6ca8506ea5
commit c970bdc037
1 changed files with 11 additions and 7 deletions
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18
src/backend/replication/slot.c
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@@ -369,13 +369,7 @@ IsSlotForConflictCheck(const char *name)
* name: Name of the slot
* db_specific: logical decoding is db specific; if the slot is going to
* be used for that pass true, otherwise false.
* two_phase: Allows decoding of prepared transactions. We allow this option
* to be enabled only at the slot creation time. If we allow this option
* to be changed during decoding then it is quite possible that we skip
* prepare first time because this option was not enabled. Now next time
* during getting changes, if the two_phase option is enabled it can skip
* prepare because by that time start decoding point has been moved. So the
* user will only get commit prepared.
* two_phase: If enabled, allows decoding of prepared transactions.
* failover: If enabled, allows the slot to be synced to standbys so
* that logical replication can be resumed after failover.
* synced: True if the slot is synchronized from the primary server.
@@ -940,6 +934,16 @@ ReplicationSlotDrop(const char *name, bool nowait)
/*
* Change the definition of the slot identified by the specified name.
*
* Altering the two_phase property of a slot requires caution on the
* client-side. Enabling it at any random point during decoding has the
* risk that transactions prepared before this change may be skipped by
* the decoder, leading to missing prepare records on the client. So, we
* enable it for subscription related slots only once the initial tablesync
* is finished. See comments atop worker.c. Disabling it is safe only when
* there are no pending prepared transaction, otherwise, the changes of
* already prepared transactions can be replicated again along with their
* corresponding commit leading to duplicate data or errors.
*/
void
ReplicationSlotAlter(const char *name, const bool *failover,