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

On Mon, May 25, 2026 at 12:42 PM SATYANARAYANA NARLAPURAM
<satyanarlapuram(at)gmail(dot)com> wrote:
>
> Hi
>
> On Fri, May 22, 2026 at 2:16 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>>
>> Thanks for reporting the issue. I could reproduce the same issue with
>> all these as well:
>>
>> pg_logical_slot_peek_changes
>> pg_logical_slot_get_binary_changes
>> pg_logical_slot_peek_binary_changes
>
>
> Please find the attached v2 patch that addressed these three cases as well.
>

Thank You for addressuing these cases. A few comments:

1)

+-- Test 2: session remains usable after the error (MyReplicationSlot cleared)

It shoudl be part of 'Test 1' itself and thus should not be named as 'Test 2'

2)
--------
+-- Test 4: copy_replication_slot with max_replication_slots exceeded.
+-- We reduce max_replication_slots artificially by filling all remaining slots.
+-- Instead, trigger an error by copying to an already-existing name.
+DO $$
+BEGIN
+ PERFORM pg_copy_logical_replication_slot('regression_slot_t3',
'regression_slot_t3');
+EXCEPTION WHEN OTHERS THEN
+ RAISE NOTICE 'caught: %', SQLERRM;
+END;
+$$;
+-- The original slot must still exist and be usable
+SELECT count(*) = 1 AS orig_slot_ok FROM pg_replication_slots
+ WHERE slot_name = 'regression_slot_t3';
-----------

I don't think we can hit the Assert with above test (at-least I could
not). Since creation of slot itself will fail as the slot with
same-name already exists, MyReplicationSlot will never be set and thus
Assert will not be hit. A better testcase will be below which fails
during LoadOutputPlugin() after slot-creation and MyReplicationSlot is
set already.

SELECT pg_create_logical_replication_slot('src_slot', 'test_decoding');

DO $$
BEGIN
PERFORM pg_copy_logical_replication_slot('src_slot', 'dst_slot',
false, 'nonexistent_plugin');
EXCEPTION WHEN others THEN
RAISE NOTICE 'caught: %', SQLERRM;
END $$;

SELECT count(*) FROM pg_logical_slot_get_changes('src_slot', NULL, NULL);

3)
So overall these are the problematic APIs:

pg_create_logical_replication_slot
pg_replication_slot_advance
pg_copy_logical_replication_slot
pg_logical_slot_peek_binary_changes
pg_logical_slot_peek_changes
pg_logical_slot_get_changes
pg_logical_slot_get_binary_changes

First 3 are are mutually exclusive fixes fow which we have added
testcases. Last 4 are addressed by fixing common function
pg_logical_slot_get_changes_guts(). I think we should add a test case
for at-least any one of these APIs to cover
pg_logical_slot_get_changes_guts().

Thanks.
Shveta