Prevent use of invalidated logical slot in CreateDecodingContext()

Previously we had checks for this in multiple places. Support for logical
decoding on standbys will add other forms of invalidation, making it worth
while to centralize the checks.

This slightly changes the error message for both the walsender and SQL
interface. Particularly the SQL interface error was inaccurate, as the "This
slot has never previously reserved WAL" portion was unreachable.

Reviewed-by: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/20230407075009.igg7be27ha2htkbt@awork3.anarazel.de

Author: anarazel

src/backend/replication/logical/logical.c

@@ -518,6 +518,22 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 				 errmsg("replication slot \"%s\" was not created in this database",
 						NameStr(slot->data.name))));
 
+	/*
+	 * Check if slot has been invalidated due to max_slot_wal_keep_size. Avoid
+	 * "cannot get changes" wording in this errmsg because that'd be
+	 * confusingly ambiguous about no changes being available when called from
+	 * pg_logical_slot_get_changes_guts().
+	 */
+	if (MyReplicationSlot->data.invalidated == RS_INVAL_WAL_REMOVED)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("can no longer get changes from replication slot \"%s\"",
+						NameStr(MyReplicationSlot->data.name)),
+				 errdetail("This slot has been invalidated because it exceeded the maximum reserved size.")));
+
+	Assert(MyReplicationSlot->data.invalidated == RS_INVAL_NONE);
+	Assert(MyReplicationSlot->data.restart_lsn != InvalidXLogRecPtr);
+
 	if (start_lsn == InvalidXLogRecPtr)
 	{
 		/* continue from last position */

src/backend/replication/logical/logicalfuncs.c

@@ -214,19 +214,6 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 									LogicalOutputPrepareWrite,
 									LogicalOutputWrite, NULL);
 
-		/*
-		 * After the sanity checks in CreateDecodingContext, make sure the
-		 * restart_lsn is valid.  Avoid "cannot get changes" wording in this
-		 * errmsg because that'd be confusingly ambiguous about no changes
-		 * being available.
-		 */
-		if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
-			ereport(ERROR,
-					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-					 errmsg("can no longer get changes from replication slot \"%s\"",
-							NameStr(*name)),
-					 errdetail("This slot has never previously reserved WAL, or it has been invalidated.")));
-
 		MemoryContextSwitchTo(oldcontext);
 
 		/*

src/backend/replication/walsender.c

@@ -1253,13 +1253,6 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 
 	ReplicationSlotAcquire(cmd->slotname, true);
 
-	if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("cannot read from logical replication slot \"%s\"",
-						cmd->slotname),
-				 errdetail("This slot has been invalidated because it exceeded the maximum reserved size.")));
-
 	/*
 	 * Force a disconnect, so that the decoding code doesn't need to care
 	 * about an eventual switch from running in recovery, to running in a