From caa4cfa3697472a6673eb817eb34681684cba14f Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Fri, 2 Oct 2009 17:57:30 +0000
Subject: [PATCH] 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 | 13 +++++++++++--
src/backend/executor/spi.c | 7 ++-----
src/backend/utils/time/snapmgr.c | 5 ++---
src/include/utils/snapmgr.h | 3 ++-
src/test/regress/expected/portals.out | 15 +++++++++++++++
src/test/regress/sql/portals.sql | 12 ++++++++++++
6 files changed, 44 insertions(+), 11 deletions(-)
diff --git a/src/backend/commands/portalcmds.c b/src/backend/commands/portalcmds.c
index 17192341739..3ffdbff4695 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 1.80 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);
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 40f43ee011b..8fc45360414 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 1.209 2009/10/02 17:57:30 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..f8531276a47 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 1.11 2009/10/02 17:57:30 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;
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 979a52c6224..a82e53bb17a 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -6,7 +6,7 @@
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * $PostgreSQL: pgsql/src/include/utils/snapmgr.h,v 1.5 2009/06/11 14:49:13 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/snapmgr.h,v 1.6 2009/10/02 17:57:30 alvherre Exp $
*
*-------------------------------------------------------------------------
*/
@@ -26,6 +26,7 @@ extern TransactionId RecentGlobalXmin;
extern Snapshot GetTransactionSnapshot(void);
extern Snapshot GetLatestSnapshot(void);
extern void SnapshotSetCommandId(CommandId curcid);
+extern Snapshot CopySnapshot(Snapshot snapshot);
extern void PushActiveSnapshot(Snapshot snapshot);
extern void PushUpdatedSnapshot(Snapshot snapshot);
diff --git a/src/test/regress/expected/portals.out b/src/test/regress/expected/portals.out
index 95dcea5a1d9..be7348d6b2a 100644
--- a/src/test/regress/expected/portals.out
+++ b/src/test/regress/expected/portals.out
@@ -1242,3 +1242,18 @@ FETCH FROM c1;
DELETE FROM ucview WHERE CURRENT OF c1; -- fail, views not supported
ERROR: WHERE CURRENT OF on a view is not implemented
ROLLBACK;
+-- Make sure snapshot management works okay, per bug report in
+-- 235395b90909301035v7228ce63q392931f15aa74b31@mail.gmail.com
+BEGIN;
+SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+CREATE TABLE cursor (a int);
+INSERT INTO cursor VALUES (1);
+DECLARE c1 NO SCROLL CURSOR FOR SELECT * FROM cursor FOR UPDATE;
+UPDATE cursor SET a = 2;
+FETCH ALL FROM c1;
+ a
+---
+(0 rows)
+
+COMMIT;
+DROP TABLE cursor;
diff --git a/src/test/regress/sql/portals.sql b/src/test/regress/sql/portals.sql
index 4265aaa43cf..585a7c25ea7 100644
--- a/src/test/regress/sql/portals.sql
+++ b/src/test/regress/sql/portals.sql
@@ -458,3 +458,15 @@ DECLARE c1 CURSOR FOR SELECT * FROM ucview;
FETCH FROM c1;
DELETE FROM ucview WHERE CURRENT OF c1; -- fail, views not supported
ROLLBACK;
+
+-- Make sure snapshot management works okay, per bug report in
+-- 235395b90909301035v7228ce63q392931f15aa74b31@mail.gmail.com
+BEGIN;
+SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+CREATE TABLE cursor (a int);
+INSERT INTO cursor VALUES (1);
+DECLARE c1 NO SCROLL CURSOR FOR SELECT * FROM cursor FOR UPDATE;
+UPDATE cursor SET a = 2;
+FETCH ALL FROM c1;
+COMMIT;
+DROP TABLE cursor;
--
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