Simplify some logic in CreateReplicationSlot()

This refactoring reduces the code in charge of creating replication
slots from two "if" block to a single one, making it slightly cleaner.

This change is possible since 1d04a59, that has removed the
intermediate code that existed between the two "if" blocks in charge of
initializing the output message buffer.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+PtnJzqKT41Zt8pChRzba=QgCqjtfYvcf84NMj3VFJoKfw@mail.gmail.com

Author: michaelpq

src/backend/replication/walsender.c

@@ -1061,9 +1061,25 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 		ReplicationSlotCreate(cmd->slotname, false,
 							  cmd->temporary ? RS_TEMPORARY : RS_PERSISTENT,
 							  false);
+
+		if (reserve_wal)
+		{
+			ReplicationSlotReserveWal();
+
+			ReplicationSlotMarkDirty();
+
+			/* Write this slot to disk if it's a permanent one. */
+			if (!cmd->temporary)
+				ReplicationSlotSave();
+		}
 	}
 	else
 	{
+		LogicalDecodingContext *ctx;
+		bool		need_full_snapshot = false;
+
+		Assert(cmd->kind == REPLICATION_KIND_LOGICAL);
+
 		CheckLogicalDecodingRequirements();
 
 		/*
@@ -1076,12 +1092,6 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 		ReplicationSlotCreate(cmd->slotname, true,
 							  cmd->temporary ? RS_TEMPORARY : RS_EPHEMERAL,
 							  two_phase);
-	}
-
-	if (cmd->kind == REPLICATION_KIND_LOGICAL)
-	{
-		LogicalDecodingContext *ctx;
-		bool		need_full_snapshot = false;
 
 		/*
 		 * Do options check early so that we can bail before calling the
@@ -1175,16 +1185,6 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 		if (!cmd->temporary)
 			ReplicationSlotPersist();
 	}
-	else if (cmd->kind == REPLICATION_KIND_PHYSICAL && reserve_wal)
-	{
-		ReplicationSlotReserveWal();
-
-		ReplicationSlotMarkDirty();
-
-		/* Write this slot to disk if it's a permanent one. */
-		if (!cmd->temporary)
-			ReplicationSlotSave();
-	}
 
 	snprintf(xloc, sizeof(xloc), "%X/%X",
 			 LSN_FORMAT_ARGS(MyReplicationSlot->data.confirmed_flush));