Thread

Re: [PATCH] Release replication slot on error in SQL-callable slot functions

Fujii Masao <masao.fujii@gmail.com> — 2026-05-11T03:01:33Z

On Sun, May 10, 2026 at 5:45 AM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Hi Hackers,
>
> SQL-callable replication slot functions acquire a slot (setting
> the process-global MyReplicationSlot) but can then ERROR before reaching
> ReplicationSlotRelease(). If such an error is caught by a PL/pgSQL
> EXCEPTION block (which uses a subtransaction), MyReplicationSlot remains
> set because there is no subtransaction-level cleanup hook for replication
> slots.
>
> Any subsequent slot operation in the same session then hits
> Assert(MyReplicationSlot == NULL) and crashes the backend on assert
> enabled builds. In release builds the stale MyReplicationSlot is silently overwritten,
> permanently orphaning the old slot as "active." The orphaned slot blocks any other
> session from acquiring it, vacuum and WAL deletion.
>
> Repro:
>
> SELECT pg_create_logical_replication_slot('adv_test', 'test_decoding');
>
> DO $$ BEGIN
>     PERFORM pg_replication_slot_advance('adv_test', '0/1'::pg_lsn);
> EXCEPTION WHEN others THEN
>     RAISE NOTICE 'caught: %', SQLERRM;
> END $$;
>
> SELECT count(*) FROM pg_logical_slot_get_changes('adv_test', NULL, NULL);
>
> 2026-05-09 19:45:06.619 UTC [1096805] STATEMENT:  SELECT pg_create_logical_replication_slot('adv_test', 'test_decoding');
> TRAP: failed Assert("MyReplicationSlot == NULL"), File: "slot.c", Line: 638, PID: 1096805
>
>
> Attached a patch to address this by wrapping error-prone paths in PG_TRY/PG_CATCH blocks
> and call ReplicationSlotRelease().

Thanks for the report and the patch!

I think wrapping the slot-processing code with PG_TRY()/PG_CATCH() seems
a good direction for addressing the issue you reported.


+   PG_CATCH();
+   {
+       ReplicationSlotRelease();

When create_logical_replication_slot() is called with temporary = true,
the created logical replication slot has RS_TEMPORARY persistency. Such a slot
is not dropped by ReplicationSlotRelease(), whereas an RS_EPHEMERAL slot is
dropped via ReplicationSlotDropAcquired().

So even with the v1 patch, a temporary logical replication slot can remain
unexpectedly if pg_create_logical_replication_slot() throws an error.
In this case, should create_logical_replication_slot() explicitly drop the slot
with ReplicationSlotDropAcquired(), or temporarily change the slot persistency
to RS_EPHEMERAL before calling ReplicationSlotRelease()?


Does a newly created logical replication slot created by
pg_copy_logical_replication_slot() have the same issue?


+ PG_CATCH();
+ {
+ ReplicationSlotRelease();

Should ReplicationSlotRelease() be called only when MyReplicationSlot
is not NULL?


    /* Acquire the slot so we "own" it */
    ReplicationSlotAcquire(NameStr(*slotname), true, true);

-   /* A slot whose restart_lsn has never been reserved cannot be advanced */
-   if (!XLogRecPtrIsValid(MyReplicationSlot->data.restart_lsn))
-       ereport(ERROR,
-               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-                errmsg("replication slot \"%s\" cannot be advanced",
-                       NameStr(*slotname)),
-                errdetail("This slot has never previously reserved
WAL, or it has been invalidated.")));
+   PG_TRY();
+   {
+       /* A slot whose restart_lsn has never been reserved cannot be
advanced */
+       if (!XLogRecPtrIsValid(MyReplicationSlot->data.restart_lsn))

Shouldn't ReplicationSlotAcquire() also be moved inside the PG_TRY() block?
Because it can throw an error after setting MyReplicationSlot.

Regards,

-- 
Fujii Masao