Introduce macros determining if a replication slot is physical or logical.

These make the code a bit easier to read, and make it easier to add a
more explicit notion of a slot's type at some point in the future.

Author: Gurjeet Singh
Discussion: CABwTF4Wh_dBCzTU=49pFXR6coR4NW1ynb+vBqT+Po=7fuq5iCw@mail.gmail.com

src/backend/replication/logical/logical.c

@@ -228,7 +228,7 @@ CreateInitDecodingContext(char *plugin,
 		elog(ERROR, "cannot initialize logical decoding without a specified plugin");
 
 	/* Make sure the passed slot is suitable. These are user facing errors. */
-	if (slot->data.database == InvalidOid)
+	if (SlotIsPhysical(slot))
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 		errmsg("cannot use physical replication slot for logical decoding")));
@@ -377,7 +377,7 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 		elog(ERROR, "cannot perform logical decoding without an acquired slot");
 
 	/* make sure the passed slot is suitable, these are user facing errors */
-	if (slot->data.database == InvalidOid)
+	if (SlotIsPhysical(slot))
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 (errmsg("cannot use physical replication slot for logical decoding"))));

src/backend/replication/slot.c

@@ -693,7 +693,7 @@ ReplicationSlotsComputeLogicalRestartLSN(void)
 			continue;
 
 		/* we're only interested in logical slots */
-		if (s->data.database == InvalidOid)
+		if (!SlotIsLogical(s))
 			continue;
 
 		/* read once, it's ok if it increases while we're checking */
@@ -740,8 +740,8 @@ ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
 		if (!s->in_use)
 			continue;
 
-		/* not database specific, skip */
+		/* only logical slots are database specific, skip */
-		if (s->data.database == InvalidOid)
+		if (!SlotIsLogical(s))
 			continue;
 
 		/* not our database, skip */

src/backend/replication/walsender.c

@@ -514,7 +514,7 @@ StartReplication(StartReplicationCmd *cmd)
 	if (cmd->slotname)
 	{
 		ReplicationSlotAcquire(cmd->slotname);
-		if (MyReplicationSlot->data.database != InvalidOid)
+		if (SlotIsLogical(MyReplicationSlot))
 			ereport(ERROR,
 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					 (errmsg("cannot use a logical replication slot for physical replication"))));
@@ -1564,7 +1564,7 @@ ProcessStandbyReplyMessage(void)
 	 */
 	if (MyReplicationSlot && flushPtr != InvalidXLogRecPtr)
 	{
-		if (MyReplicationSlot->data.database != InvalidOid)
+		if (SlotIsLogical(MyReplicationSlot))
 			LogicalConfirmReceivedLocation(flushPtr);
 		else
 			PhysicalConfirmReceivedLocation(flushPtr);

src/include/replication/slot.h

@@ -125,6 +125,9 @@ typedef struct ReplicationSlot
 	XLogRecPtr	candidate_restart_lsn;
 } ReplicationSlot;
 
+#define SlotIsPhysical(slot) (slot->data.database == InvalidOid)
+#define SlotIsLogical(slot) (slot->data.database != InvalidOid)
+
 /*
  * Shared memory control area for all of replication slots.
  */