Block replication slot advance for these not yet reserving WAL

Such replication slots are physical slots freshly created without WAL
being reserved, which is the default behavior, which have not been used
yet as WAL consumption resources to retain WAL.  This prevents advancing
a slot to a position older than any WAL available, which could falsify
calculations for WAL segment recycling.

This also cleans up a bit the code, as ReplicationSlotRelease() would be
called on ERROR, and improves error messages.

Reported-by: Kyotaro Horiguchi
Author: Michael Paquier
Reviewed-by: Andres Freund, Álvaro Herrera, Kyotaro Horiguchi
Discussion: https://postgr.es/m/20180626071305.GH31353@paquier.xyz

contrib/test_decoding/expected/slot.out

@@ -131,3 +131,20 @@ SELECT pg_drop_replication_slot('regression_slot1');
 ERROR:  replication slot "regression_slot1" does not exist
 SELECT pg_drop_replication_slot('regression_slot2');
 ERROR:  replication slot "regression_slot2" does not exist
+-- slot advance with physical slot, error with non-reserved slot
+SELECT slot_name FROM pg_create_physical_replication_slot('regression_slot3');
+    slot_name     
+------------------
+ regression_slot3
+(1 row)
+
+SELECT pg_replication_slot_advance('regression_slot3', '0/0'); -- invalid LSN
+ERROR:  invalid target wal lsn
+SELECT pg_replication_slot_advance('regression_slot3', '0/1'); -- error
+ERROR:  cannot advance replication slot that has not previously reserved WAL
+SELECT pg_drop_replication_slot('regression_slot3');
+ pg_drop_replication_slot 
+--------------------------
+ 
+(1 row)
+

contrib/test_decoding/sql/slot.sql

@@ -68,3 +68,9 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot1', 'test_
 -- both should error as they should be dropped on error
 SELECT pg_drop_replication_slot('regression_slot1');
 SELECT pg_drop_replication_slot('regression_slot2');
+
+-- slot advance with physical slot, error with non-reserved slot
+SELECT slot_name FROM pg_create_physical_replication_slot('regression_slot3');
+SELECT pg_replication_slot_advance('regression_slot3', '0/0'); -- invalid LSN
+SELECT pg_replication_slot_advance('regression_slot3', '0/1'); -- error
+SELECT pg_drop_replication_slot('regression_slot3');

doc/src/sgml/catalogs.sgml

@@ -9867,7 +9867,8 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       <entry></entry>
       <entry>The address (<literal>LSN</literal>) of oldest WAL which still
       might be required by the consumer of this slot and thus won't be
-      automatically removed during checkpoints.
+      automatically removed during checkpoints.  <literal>NULL</literal>
+      if the <literal>LSN</literal> of this slot has never been reserved.
       </entry>
      </row>
 

src/backend/replication/slotfuncs.c

@@ -483,6 +483,12 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 	/* Acquire the slot so we "own" it */
 	ReplicationSlotAcquire(NameStr(*slotname), true);
 
+	/* A slot whose restart_lsn has never been reserved cannot be advanced */
+	if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("cannot advance replication slot that has not previously reserved WAL")));
+
 	/*
 	 * Check if the slot is not moving backwards.  Physical slots rely simply
 	 * on restart_lsn as a minimum point, while logical slots have confirmed
@@ -495,14 +501,11 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 		minlsn = MyReplicationSlot->data.restart_lsn;
 
 	if (moveto < minlsn)
-	{
-		ReplicationSlotRelease();
 		ereport(ERROR,
-				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("cannot move slot to %X/%X, minimum is %X/%X",
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("cannot advance replication slot to %X/%X, minimum is %X/%X",
 						(uint32) (moveto >> 32), (uint32) moveto,
 						(uint32) (minlsn >> 32), (uint32) minlsn)));
-	}
 
 	/* Do the actual slot update, depending on the slot type */
 	if (OidIsValid(MyReplicationSlot->data.database))