Re: Introduce XID age and inactive timeout based replication slot invalidation

Hi, here are some review comments for v45-0001 (excluding the test code)

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doc/src/sgml/config.sgml

1.
+ Note that the inactive timeout invalidation mechanism is not
+ applicable for slots on the standby server that are being synced
+ from primary server (i.e., standby slots having

nit - /from primary server/from the primary server/

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src/backend/replication/slot.c

2. ReplicationSlotAcquire

+ errmsg("can no longer get changes from replication slot \"%s\"",
+ NameStr(s->data.name)),
+ errdetail("This slot has been invalidated because it was inactive
for longer than the amount of time specified by \"%s\".",
+ "replication_slot_inactive_timeout.")));

nit - "replication_slot_inactive_timeout." - should be no period
inside that GUC name literal

~~~

3. ReportSlotInvalidation

I didn't understand why there was a hint for:
"You might need to increase \"%s\".", "max_slot_wal_keep_size"

But you don't have an equivalent hint for timeout invalidation:
"You might need to increase \"%s\".", "replication_slot_inactive_timeout"

Why aren't these similar cases consistent?

~~~

4. RestoreSlotFromDisk

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

Is the deferred assignment really necessary? Why not just
unconditionally assign the 'now' just before the for-loop? Or even at
the declaration? e.g. The 'replication_slot_inactive_timeout' is
measured in seconds so I don't think 'inactive_since' being wrong by a
millisecond here will make any difference.

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src/include/replication/slot.h

5. ReplicationSlotSetInactiveSince

+/*
+ * Set slot's inactive_since property unless it was previously invalidated due
+ * to inactive timeout.
+ */
+static inline void
+ReplicationSlotSetInactiveSince(ReplicationSlot *s, TimestampTz *now,
+ bool acquire_lock)
+{
+ if (acquire_lock)
+ SpinLockAcquire(&s->mutex);
+
+ if (s->data.invalidated != RS_INVAL_INACTIVE_TIMEOUT)
+ s->inactive_since = *now;
+
+ if (acquire_lock)
+ SpinLockRelease(&s->mutex);
+}

Is the logic correct? What if the slot was already invalid due to some
reason other than RS_INVAL_INACTIVE_TIMEOUT? Is an Assert needed?

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Kind Regards,
Peter Smith.
Fujitsu Australia