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

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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.
This commit is contained in:
Alvaro Herrera
2009-10-02 17:58:21 +00:00
parent d4bd8423c9
commit ae35363dce
6 changed files with 44 additions and 11 deletions
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13
src/backend/commands/portalcmds.c
View File

@@ -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");
@@ -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);

7
src/backend/executor/spi.c
View File

@@ -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

5
src/backend/utils/time/snapmgr.c
View File

@@ -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;