Ensure that a cursor has an immutable snapshot throughout its lifespan.

The old coding was using a regular snapshot, referenced elsewhere, that was
subject to having its command counter updated.  Fix by creating a private copy
of the snapshot exclusively for the cursor.

Backpatch to 8.4, which is when the bug was introduced during the snapshot
management rewrite.

src/backend/commands/portalcmds.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.79 2009/06/11 14:48:56 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.80 2009/10/02 17:57:29 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -47,6 +47,7 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 	DeclareCursorStmt *cstmt = (DeclareCursorStmt *) stmt->utilityStmt;
 	Portal		portal;
 	MemoryContext oldContext;
+	Snapshot	snapshot;
 
 	if (cstmt == NULL || !IsA(cstmt, DeclareCursorStmt))
 		elog(ERROR, "PerformCursorOpen called for non-cursor query");
@@ -118,10 +119,18 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 			portal->cursorOptions |= CURSOR_OPT_NO_SCROLL;
 	}
 
+	/*
+	 * Set up snapshot for portal.  Note that we need a fresh, independent copy
+	 * of the snapshot because we don't want it to be modified by future
+	 * CommandCounterIncrement calls.  We do not register it, because
+	 * portalmem.c will take care of that internally.
+	 */
+	snapshot = CopySnapshot(GetActiveSnapshot());
+
 	/*
 	 * Start execution, inserting parameters if any.
 	 */
-	PortalStart(portal, params, GetActiveSnapshot());
+	PortalStart(portal, params, snapshot);
 
 	Assert(portal->strategy == PORTAL_ONE_SELECT);