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.

3 files changed, 15 insertions, 10 deletions

diff --git a/src/backend/commands/portalcmds.c b/src/backend/commands/portalcmds.c
index 17192341739..d31fde65b6b 100644
--- a/src/backend/commands/portalcmds.c
+++ b/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 1.79.2.1 2009/10/02 17:58:21 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");
@@ -119,9 +120,17 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 	}
 
 	/*
+	 * 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);
 
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 40f43ee011b..3cffc2ad72b 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.208 2009/06/11 14:48:57 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.208.2.1 2009/10/02 17:58:21 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1211,10 +1211,7 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
 		}
 	}
 
-	/*
-	 * Set up the snapshot to use.	(PortalStart will do PushActiveSnapshot,
-	 * so we skip that here.)
-	 */
+	/* Set up the snapshot to use. */
 	if (read_only)
 		snapshot = GetActiveSnapshot();
 	else
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 73d597d7b37..12ca81616d1 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -19,7 +19,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.10 2009/06/11 14:49:06 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.10.2.1 2009/10/02 17:58:21 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -104,7 +104,6 @@ bool		FirstSnapshotSet = false;
 static bool registered_serializable = false;
 
 
-static Snapshot CopySnapshot(Snapshot snapshot);
 static void FreeSnapshot(Snapshot snapshot);
 static void SnapshotResetXmin(void);
 
@@ -192,7 +191,7 @@ SnapshotSetCommandId(CommandId curcid)
  * The copy is palloc'd in TopTransactionContext and has initial refcounts set
  * to 0.  The returned snapshot has the copied flag set.
  */
-static Snapshot
+Snapshot
 CopySnapshot(Snapshot snapshot)
 {
 	Snapshot	newsnap;