Fix failure to set ActiveSnapshot while rewinding a cursor.

ActiveSnapshot needs to be set when we call ExecutorRewind because some
plan node types may execute user-defined functions during their ReScan
calls (nodeLimit.c does so, at least).  The wisdom of that is somewhat
debatable, perhaps, but for now the simplest fix is to make sure the
required context is valid.  Failure to do this typically led to a
null-pointer-dereference core dump, though it's possible that in more
complex cases a function could be executed with the wrong snapshot
leading to very subtle misbehavior.

Per report from Leif Jensen.  It's been broken for a long time, so
back-patch to all active branches.

src/backend/tcop/pquery.c

@@ -1623,6 +1623,9 @@ DoPortalRunFetch(Portal portal,
 static void
 DoPortalRewind(Portal portal)
 {
+	QueryDesc  *queryDesc;
+
+	/* Rewind holdStore, if we have one */
 	if (portal->holdStore)
 	{
 		MemoryContext oldcontext;
@@ -1631,8 +1634,15 @@ DoPortalRewind(Portal portal)
 		tuplestore_rescan(portal->holdStore);
 		MemoryContextSwitchTo(oldcontext);
 	}
-	if (PortalGetQueryDesc(portal))
-		ExecutorRewind(PortalGetQueryDesc(portal));
+
+	/* Rewind executor, if active */
+	queryDesc = PortalGetQueryDesc(portal);
+	if (queryDesc)
+	{
+		PushActiveSnapshot(queryDesc->snapshot);
+		ExecutorRewind(queryDesc);
+		PopActiveSnapshot();
+	}
 
 	portal->atStart = true;
 	portal->atEnd = false;

src/test/regress/expected/portals.out

@@ -1257,3 +1257,27 @@ FETCH ALL FROM c1;
 
 COMMIT; 
 DROP TABLE cursor;
+-- Check rewinding a cursor containing a stable function in LIMIT,
+-- per bug report in 8336843.9833.1399385291498.JavaMail.root@quick
+begin;
+create function nochange(int) returns int
+  as 'select $1 limit 1' language sql stable;
+declare c cursor for select * from int8_tbl limit nochange(3);
+fetch all from c;
+        q1        |        q2        
+------------------+------------------
+              123 |              456
+              123 | 4567890123456789
+ 4567890123456789 |              123
+(3 rows)
+
+move backward all in c;
+fetch all from c;
+        q1        |        q2        
+------------------+------------------
+              123 |              456
+              123 | 4567890123456789
+ 4567890123456789 |              123
+(3 rows)
+
+rollback;

src/test/regress/sql/portals.sql

@@ -470,3 +470,14 @@ UPDATE cursor SET a = 2;
 FETCH ALL FROM c1; 
 COMMIT; 
 DROP TABLE cursor;
+
+-- Check rewinding a cursor containing a stable function in LIMIT,
+-- per bug report in 8336843.9833.1399385291498.JavaMail.root@quick
+begin;
+create function nochange(int) returns int
+  as 'select $1 limit 1' language sql stable;
+declare c cursor for select * from int8_tbl limit nochange(3);
+fetch all from c;
+move backward all in c;
+fetch all from c;
+rollback;