Improve snapshot manager by keeping explicit track of snapshots.

There are two ways to track a snapshot: there's the "registered" list, which
is used for arbitrary long-lived snapshots; and there's the "active stack",
which is used for the snapshot that is considered "active" at any time.
This also allows users of snapshots to stop worrying about snapshot memory
allocation and freeing, and about using PG_TRY blocks around ActiveSnapshot
assignment.  This is all done automatically now.

As a consequence, this allows us to reset MyProc->xmin when there are no
more snapshots registered in the current backend, reducing the impact that
long-running transactions have on VACUUM.

3 files changed, 22 insertions, 10 deletions

diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index 160b6c9670e..ea0aa999441 100644
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/procarray.h,v 1.21 2008/03/26 16:20:48 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/storage/procarray.h,v 1.22 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,7 +26,7 @@ extern void ProcArrayRemove(PGPROC *proc, TransactionId latestXid);
 extern void ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid);
 extern void ProcArrayClearTransaction(PGPROC *proc);
 
-extern Snapshot GetSnapshotData(Snapshot snapshot, bool serializable);
+extern Snapshot GetSnapshotData(Snapshot snapshot);
 
 extern bool TransactionIdIsInProgress(TransactionId xid);
 extern bool TransactionIdIsActive(TransactionId xid);
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index c76c70feaac..8f3cc44c57b 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/snapmgr.h,v 1.1 2008/03/26 18:48:59 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/utils/snapmgr.h,v 1.2 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -16,9 +16,7 @@
 #include "utils/snapshot.h"
 
 
-extern PGDLLIMPORT Snapshot SerializableSnapshot;
-extern PGDLLIMPORT Snapshot LatestSnapshot;
-extern PGDLLIMPORT Snapshot ActiveSnapshot;
+extern bool FirstSnapshotSet;
 
 extern TransactionId TransactionXmin;
 extern TransactionId RecentXmin;
@@ -26,8 +24,19 @@ extern TransactionId RecentGlobalXmin;
 
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);
-extern Snapshot CopySnapshot(Snapshot snapshot);
-extern void FreeSnapshot(Snapshot snapshot);
-extern void FreeXactSnapshot(void);
+extern void SnapshotSetCommandId(CommandId curcid);
+
+extern void PushActiveSnapshot(Snapshot snapshot);
+extern void PushUpdatedSnapshot(Snapshot snapshot);
+extern void PopActiveSnapshot(void);
+extern Snapshot GetActiveSnapshot(void);
+extern bool ActiveSnapshotSet(void);
+
+extern Snapshot RegisterSnapshot(Snapshot snapshot);
+extern void UnregisterSnapshot(Snapshot snapshot);
+
+extern void AtSubCommit_Snapshot(int level);
+extern void AtSubAbort_Snapshot(int level);
+extern void AtEOXact_Snapshot(bool isCommit);
 
 #endif /* SNAPMGR_H */
diff --git a/src/include/utils/snapshot.h b/src/include/utils/snapshot.h
index 20d971a102a..52f9dd677a2 100644
--- a/src/include/utils/snapshot.h
+++ b/src/include/utils/snapshot.h
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/snapshot.h,v 1.2 2008/03/26 21:10:39 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/utils/snapshot.h,v 1.3 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -57,6 +57,9 @@ typedef struct SnapshotData
 	 * out any that are >= xmax
 	 */
 	CommandId	curcid;			/* in my xact, CID < curcid are visible */
+	uint32		active_count;	/* refcount on ActiveSnapshot stack */
+	uint32		regd_count;		/* refcount on RegisteredSnapshotList */
+	bool		copied;			/* false if it's a static snapshot */
 } SnapshotData;
 
 /*