Re: Improve error handling for invalid slots and ensure a same 'inactive_since' time for inactive slots

Hi Nisha,

Some review comments for patch v1-0001.

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src/backend/replication/logical/slotsync.c

ReplSlotSyncWorkerMain:

1.
+ /* Use same inactive_since time for all slots */
+ now = GetCurrentTimestamp();
+
LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);

for (int i = 0; i < max_replication_slots; i++)
@@ -1537,10 +1540,6 @@ update_synced_slots_inactive_since(void)
/* The slot must not be acquired by any process */
Assert(s->active_pid == 0);

- /* Use the same inactive_since time for all the slots. */
- if (now == 0)
- now = GetCurrentTimestamp();
-

AFAICT, this code was *already* ensuring to use the same
'inactive_since' even before your patch. The only difference is now
you are getting the timestamp value up-front instead of a deferred
assignment.

So why did you change this (and the code of RestoreSlotFromDisk) to do
the up-front assignment? Instead, you could have chosen to just leave
this code as-is, and then modify the RestoreSlotFromDisk code to match
it.

FWIW, I do prefer what you have done here because it is simpler, but I
just wondered about the choice because I think some people worry about
GetCurrentTimestamp overheads and try to avoid calling that wherever
possible.

======
src/backend/replication/slot.c

2. What about other loops?

AFAICT there are still some other loops where the inactive_since
timestamps might differ.

e.g. How about this logic in slot.c:

InvalidateObsoleteReplicationSlots:

LOOP:
for (int i = 0; i < max_replication_slots; i++)
{
ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];

calls InvalidatePossiblyObsoleteSlot(...)
which calls ReplicationSlotRelease(...)
which assigns now = GetCurrentTimestamp();
then slot->inactive_since = now;
}

~

So, should you also assign a 'now' value outside this loop and pass
that timestamp down the calls so they eventually all get assigned the
same? I don't know, but I guess at least that would require much fewer
unnecessary calls to GetCurrentTimestamp so that may be a good thing.

======
Kind Regards,
Peter Smith.
Fujitsu Australia