Replace replication slot's invalidated_at LSN with an enum

This is mainly useful because the upcoming logical-decoding-on-standby feature
adds further reasons for invalidating slots, and we don't want to end up with
multiple invalidated_* fields, or check different attributes.

Eventually we should consider not resetting restart_lsn when invalidating a
slot due to max_slot_wal_keep_size. But that's a user visible change, so left
for later.

Increases SLOT_VERSION, due to the changed field (with a different alignment,
no less).

Reviewed-by: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/20230407075009.igg7be27ha2htkbt@awork3.anarazel.de

src/backend/replication/slot.c

@@ -89,7 +89,7 @@ typedef struct ReplicationSlotOnDisk
 	sizeof(ReplicationSlotOnDisk) - ReplicationSlotOnDiskConstantSize
 
 #define SLOT_MAGIC		0x1051CA1	/* format identifier */
-#define SLOT_VERSION	2		/* version for new files */
+#define SLOT_VERSION	3		/* version for new files */
 
 /* Control array for replication slot management */
 ReplicationSlotCtlData *ReplicationSlotCtl = NULL;
@@ -855,8 +855,7 @@ ReplicationSlotsComputeRequiredXmin(bool already_locked)
 		SpinLockAcquire(&s->mutex);
 		effective_xmin = s->effective_xmin;
 		effective_catalog_xmin = s->effective_catalog_xmin;
-		invalidated = (!XLogRecPtrIsInvalid(s->data.invalidated_at) &&
-					   XLogRecPtrIsInvalid(s->data.restart_lsn));
+		invalidated = s->data.invalidated != RS_INVAL_NONE;
 		SpinLockRelease(&s->mutex);
 
 		/* invalidated slots need not apply */
@@ -901,14 +900,20 @@ ReplicationSlotsComputeRequiredLSN(void)
 	{
 		ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 		XLogRecPtr	restart_lsn;
+		bool		invalidated;
 
 		if (!s->in_use)
 			continue;
 
 		SpinLockAcquire(&s->mutex);
 		restart_lsn = s->data.restart_lsn;
+		invalidated = s->data.invalidated != RS_INVAL_NONE;
 		SpinLockRelease(&s->mutex);
 
+		/* invalidated slots need not apply */
+		if (invalidated)
+			continue;
+
 		if (restart_lsn != InvalidXLogRecPtr &&
 			(min_required == InvalidXLogRecPtr ||
 			 restart_lsn < min_required))
@@ -946,6 +951,7 @@ ReplicationSlotsComputeLogicalRestartLSN(void)
 	{
 		ReplicationSlot *s;
 		XLogRecPtr	restart_lsn;
+		bool		invalidated;
 
 		s = &ReplicationSlotCtl->replication_slots[i];
 
@@ -960,8 +966,13 @@ ReplicationSlotsComputeLogicalRestartLSN(void)
 		/* read once, it's ok if it increases while we're checking */
 		SpinLockAcquire(&s->mutex);
 		restart_lsn = s->data.restart_lsn;
+		invalidated = s->data.invalidated != RS_INVAL_NONE;
 		SpinLockRelease(&s->mutex);
 
+		/* invalidated slots need not apply */
+		if (invalidated)
+			continue;
+
 		if (restart_lsn == InvalidXLogRecPtr)
 			continue;
 
@@ -1012,6 +1023,8 @@ ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
 		if (s->data.database != dboid)
 			continue;
 
+		/* NB: intentionally counting invalidated slots */
+
 		/* count slots with spinlock held */
 		SpinLockAcquire(&s->mutex);
 		(*nslots)++;
@@ -1069,6 +1082,8 @@ restart:
 		if (s->data.database != dboid)
 			continue;
 
+		/* NB: intentionally including invalidated slots */
+
 		/* acquire slot, so ReplicationSlotDropAcquired can be reused  */
 		SpinLockAcquire(&s->mutex);
 		/* can't change while ReplicationSlotControlLock is held */
@@ -1294,7 +1309,12 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlot *s, XLogRecPtr oldestLSN,
 		{
 			MyReplicationSlot = s;
 			s->active_pid = MyProcPid;
-			s->data.invalidated_at = restart_lsn;
+			s->data.invalidated = RS_INVAL_WAL_REMOVED;
+
+			/*
+			 * XXX: We should consider not overwriting restart_lsn and instead
+			 * just rely on .invalidated.
+			 */
 			s->data.restart_lsn = InvalidXLogRecPtr;
 
 			/* Let caller know */

src/backend/replication/slotfuncs.c

@@ -315,12 +315,10 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 			nulls[i++] = true;
 
 		/*
-		 * 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.
+		 * If the slot has not been invalidated, test availability from
+		 * restart_lsn.
 		 */
-		if (XLogRecPtrIsInvalid(slot_contents.data.restart_lsn) &&
-			!XLogRecPtrIsInvalid(slot_contents.data.invalidated_at))
+		if (slot_contents.data.invalidated != RS_INVAL_NONE)
 			walstate = WALAVAIL_REMOVED;
 		else
 			walstate = GetWALAvailability(slot_contents.data.restart_lsn);

src/include/replication/slot.h

@@ -37,6 +37,17 @@ typedef enum ReplicationSlotPersistency
 	RS_TEMPORARY
 } ReplicationSlotPersistency;
 
+/*
+ * Slots can be invalidated, e.g. due to max_slot_wal_keep_size. If so, the
+ * 'invalidated' field is set to a value other than _NONE.
+ */
+typedef enum ReplicationSlotInvalidationCause
+{
+	RS_INVAL_NONE,
+	/* required WAL has been removed */
+	RS_INVAL_WAL_REMOVED,
+} ReplicationSlotInvalidationCause;
+
 /*
  * On-Disk data of a replication slot, preserved across restarts.
  */
@@ -72,8 +83,8 @@ typedef struct ReplicationSlotPersistentData
 	/* oldest LSN that might be required by this replication slot */
 	XLogRecPtr	restart_lsn;
 
-	/* restart_lsn is copied here when the slot is invalidated */
-	XLogRecPtr	invalidated_at;
+	/* RS_INVAL_NONE if valid, or the reason for having been invalidated */
+	ReplicationSlotInvalidationCause invalidated;
 
 	/*
 	 * Oldest LSN that the client has acked receipt for.  This is used as the

src/tools/pgindent/typedefs.list

@@ -2339,6 +2339,7 @@ ReplicaIdentityStmt
 ReplicationKind
 ReplicationSlot
 ReplicationSlotCtlData
+ReplicationSlotInvalidationCause
 ReplicationSlotOnDisk
 ReplicationSlotPersistency
 ReplicationSlotPersistentData