Persist slot invalidation correctly

We failed to save slot to disk after invalidating it, so the state was
lost in case of server restart or crash.  Fix by marking it dirty and
flushing.

Also, if the slot is known invalidated we don't need to reason about the
LSN at all -- it's known invalidated.  Only test the LSN if the slot is
known not invalidated.

Author: Fujii Masao <masao.fujii@oss.nttdata.com>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/17a69cfe-f1c1-a416-ee25-ae15427c69eb@oss.nttdata.com

src/backend/replication/slot.c

@@ -1157,6 +1157,7 @@ restart:
 		if (XLogRecPtrIsInvalid(restart_lsn) || restart_lsn >= oldestLSN)
 			continue;
 		LWLockRelease(ReplicationSlotControlLock);
+		CHECK_FOR_INTERRUPTS();
 
 		/* Get ready to sleep on the slot in case it is active */
 		ConditionVariablePrepareToSleep(&s->active_cv);
@@ -1214,10 +1215,7 @@ restart:
 		 * already been dropped.
 		 */
 		if (wspid == -1)
-		{
-			CHECK_FOR_INTERRUPTS();
 			goto restart;
-		}
 
 		ereport(LOG,
 				(errmsg("invalidating slot \"%s\" because its restart_lsn %X/%X exceeds max_slot_wal_keep_size",
@@ -1229,10 +1227,13 @@ restart:
 		s->data.invalidated_at = s->data.restart_lsn;
 		s->data.restart_lsn = InvalidXLogRecPtr;
 		SpinLockRelease(&s->mutex);
+
+		/* Make sure the invalidated state persists across server restart */
+		ReplicationSlotMarkDirty();
+		ReplicationSlotSave();
 		ReplicationSlotRelease();
 
 		/* if we did anything, start from scratch */
-		CHECK_FOR_INTERRUPTS();
 		goto restart;
 	}
 	LWLockRelease(ReplicationSlotControlLock);

src/backend/replication/slotfuncs.c

@@ -283,7 +283,6 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		bool		nulls[PG_GET_REPLICATION_SLOTS_COLS];
 		WALAvailability walstate;
 		XLogSegNo	last_removed_seg;
-		XLogRecPtr	targetLSN;
 		int			i;
 
 		if (!slot->in_use)
@@ -344,14 +343,15 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 			nulls[i++] = true;
 
 		/*
-		 * Report availability from invalidated_at when the slot has been
-		 * invalidated; otherwise slots would appear as invalid without any
-		 * more clues as to what happened.
+		 * If invalidated_at is valid and restart_lsn is invalid, we know for
+		 * certain that the slot has been invalidated.  Otherwise, test
+		 * availability from restart_lsn.
 		 */
-		targetLSN = XLogRecPtrIsInvalid(slot_contents.data.restart_lsn) ?
-			slot_contents.data.invalidated_at :
-			slot_contents.data.restart_lsn;
-		walstate = GetWALAvailability(targetLSN);
+		if (XLogRecPtrIsInvalid(slot_contents.data.restart_lsn) &&
+			!XLogRecPtrIsInvalid(slot_contents.data.invalidated_at))
+			walstate = WALAVAIL_REMOVED;
+		else
+			walstate = GetWALAvailability(slot_contents.data.restart_lsn);
 
 		switch (walstate)
 		{