Re: Restrict copying of invalidated replication slots

On Mon, Mar 10, 2025 at 11:46 AM Shlok Kyal <shlok.kyal.oss@gmail.com> wrote:
>
>
> I tried to reproduce the scenario described using the following steps:
>
> Here are the steps:
>
> 1. set max_replication_slots = 2
>
> 2. create two logical replication slot, lets say slot1 and slot2. drop
> the first slot (slot1).
>
> 3. Now run pg_copy_logical_replication slot function on slot2. Lets
> say copied slot is 'slot_cp'.
> In function copy_replication_slot add breakpoint just after function
> 'create_logical_replication_slot'.
> slot_cp will be created. In array
> ReplicationSlotCtl->replication_slots, slot_cp will come before slot2.
>
> 4. Now invalidate the 'slot2'.
> Function 'InvalidateObsoleteReplicationSlots' will be called. Now it
> will loop through ReplicationSlotCtl->replication_slots and will get
> 'slot_cp' first. Since the slot is acquired by copy_replication_slot,
> It will wait for the slot to be released. Once slot is released,
> 'slot_cp' and 'slot2' will be invalidated.
>

It is also possible that InvalidateObsoleteReplicationSlots() has
already past slot_cp location in which case it doesn't need to wait
for the backend.

I have changed the comments for the master branch patch. Do let me
know what you think of the attached change. If you agree with this,
then please prepare the patches for back-branches that reflect this
change.

--
With Regards,
Amit Kapila.