The code path in charge of invalidating a replication slot includes a
call to pgstat_drop_replslot(), which would result in removing the
statistics of the slot once invalidated. However, there is no need to
remove the statistics of an invalidated slot as one could still be
interested in looking at them to understand the activity of the slot
until its actual removal.
The initial design of the feature committed in be87200efd used the
approach to drop the slots, which is likely why the statistics were
still removed during the invalidation.
Another problem with this operation is that it was done without holding
ReplicationSlotAllocationLock, leaving it unprotected on concurrent
activity. This part is arguably a bug, but that's a limited problem in
practice so no backpatch is done.
In passing, this commit adds a test to check this behavior. The only
remaining code path where slot statistics are dropped now related to the
slot getting dropped.
Author: Bertrand Drouvot
Discussion: https://postgr.es/m/ZermH08Eq6YydHpO@ip-10-97-1-34.eu-west-3.compute.internal
d6e171fed6