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.

src/backend/access/transam/xact.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.263 2008/05/12 00:00:46 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.264 2008/05/12 20:01:58 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -622,12 +622,9 @@ CommandCounterIncrement(void)
 		}
 		currentCommandIdUsed = false;
 
-		/* Propagate new command ID into static snapshots, if set */
-		if (SerializableSnapshot)
-			SerializableSnapshot->curcid = currentCommandId;
-		if (LatestSnapshot)
-			LatestSnapshot->curcid = currentCommandId;
-
+		/* Propagate new command ID into static snapshots */
+		SnapshotSetCommandId(currentCommandId);
+		
 		/*
 		 * Make any catalog changes done by the just-completed command
 		 * visible in the local syscache.  We obviously don't need to do
@@ -1509,9 +1506,8 @@ StartTransaction(void)
 	s->transactionId = InvalidTransactionId;	/* until assigned */
 
 	/*
-	 * Make sure we've freed any old snapshot, and reset xact state variables
+	 * Make sure we've reset xact state variables
 	 */
-	FreeXactSnapshot();
 	XactIsoLevel = DefaultXactIsoLevel;
 	XactReadOnly = DefaultXactReadOnly;
 	forceSyncCommit = false;
@@ -1753,6 +1749,7 @@ CommitTransaction(void)
 	AtEOXact_ComboCid();
 	AtEOXact_HashTables(true);
 	AtEOXact_PgStat(true);
+	AtEOXact_Snapshot(true);
 	pgstat_report_xact_timestamp(0);
 
 	CurrentResourceOwner = NULL;
@@ -1985,6 +1982,7 @@ PrepareTransaction(void)
 	AtEOXact_ComboCid();
 	AtEOXact_HashTables(true);
 	/* don't call AtEOXact_PgStat here */
+	AtEOXact_Snapshot(true);
 
 	CurrentResourceOwner = NULL;
 	ResourceOwnerDelete(TopTransactionResourceOwner);
@@ -2129,6 +2127,7 @@ AbortTransaction(void)
 	AtEOXact_ComboCid();
 	AtEOXact_HashTables(false);
 	AtEOXact_PgStat(false);
+	AtEOXact_Snapshot(false);
 	pgstat_report_xact_timestamp(0);
 
 	/*
@@ -3844,6 +3843,7 @@ CommitSubTransaction(void)
 					  s->parent->subTransactionId);
 	AtEOSubXact_HashTables(true, s->nestingLevel);
 	AtEOSubXact_PgStat(true, s->nestingLevel);
+	AtSubCommit_Snapshot(s->nestingLevel);
 
 	/*
 	 * We need to restore the upper transaction's read-only state, in case the
@@ -3963,6 +3963,7 @@ AbortSubTransaction(void)
 						  s->parent->subTransactionId);
 		AtEOSubXact_HashTables(false, s->nestingLevel);
 		AtEOSubXact_PgStat(false, s->nestingLevel);
+		AtSubAbort_Snapshot(s->nestingLevel);
 	}
 
 	/*

src/backend/catalog/index.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/index.c,v 1.298 2008/05/12 00:00:47 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/index.c,v 1.299 2008/05/12 20:01:59 alvherre Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -1507,7 +1507,7 @@ IndexBuildHeapScan(Relation heapRelation,
 	}
 	else if (indexInfo->ii_Concurrent)
 	{
-		snapshot = CopySnapshot(GetTransactionSnapshot());
+		snapshot = RegisterSnapshot(GetTransactionSnapshot());
 		OldestXmin = InvalidTransactionId;		/* not used */
 	}
 	else
@@ -1517,10 +1517,7 @@ IndexBuildHeapScan(Relation heapRelation,
 		OldestXmin = GetOldestXmin(heapRelation->rd_rel->relisshared, true);
 	}
 
-	scan = heap_beginscan(heapRelation, /* relation */
-						  snapshot,		/* seeself */
-						  0,	/* number of keys */
-						  NULL);	/* scan key */
+	scan = heap_beginscan(heapRelation, snapshot, 0, NULL);
 
 	reltuples = 0;
 
@@ -1793,6 +1790,10 @@ IndexBuildHeapScan(Relation heapRelation,
 
 	heap_endscan(scan);
 
+	/* we can now forget our snapshot, if set */
+	if (indexInfo->ii_Concurrent)
+		UnregisterSnapshot(snapshot);
+
 	ExecDropSingleTupleTableSlot(slot);
 
 	FreeExecutorState(estate);

src/backend/commands/cluster.c

@@ -11,7 +11,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.175 2008/05/12 00:00:47 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/cluster.c,v 1.176 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -212,6 +212,7 @@ cluster(ClusterStmt *stmt, bool isTopLevel)
 		rvs = get_tables_to_cluster(cluster_context);
 
 		/* Commit to get out of starting transaction */
+		PopActiveSnapshot();
 		CommitTransactionCommand();
 
 		/* Ok, now that we've got them all, cluster them one by one */
@@ -222,8 +223,9 @@ cluster(ClusterStmt *stmt, bool isTopLevel)
 			/* Start a new transaction for each relation. */
 			StartTransactionCommand();
 			/* functions in indexes may want a snapshot set */
-			ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+			PushActiveSnapshot(GetTransactionSnapshot());
 			cluster_rel(rvtc, true);
+			PopActiveSnapshot();
 			CommitTransactionCommand();
 		}
 

src/backend/commands/copy.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/copy.c,v 1.298 2008/03/26 18:48:59 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/copy.c,v 1.299 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1044,21 +1044,18 @@ DoCopy(const CopyStmt *stmt, const char *queryString)
 		plan = planner(query, 0, NULL);
 
 		/*
-		 * Update snapshot command ID to ensure this query sees results of any
-		 * previously executed queries.  (It's a bit cheesy to modify
-		 * ActiveSnapshot without making a copy, but for the limited ways in
-		 * which COPY can be invoked, I think it's OK, because the active
-		 * snapshot shouldn't be shared with anything else anyway.)
+		 * Use a snapshot with an updated command ID to ensure this query sees
+		 * results of any previously executed queries.
 		 */
-		ActiveSnapshot->curcid = GetCurrentCommandId(false);
+		PushUpdatedSnapshot(GetActiveSnapshot());
 
 		/* Create dest receiver for COPY OUT */
 		dest = CreateDestReceiver(DestCopyOut, NULL);
 		((DR_copy *) dest)->cstate = cstate;
 
 		/* Create a QueryDesc requesting no output */
-		cstate->queryDesc = CreateQueryDesc(plan,
-											ActiveSnapshot, InvalidSnapshot,
+		cstate->queryDesc = CreateQueryDesc(plan, GetActiveSnapshot(),
+											InvalidSnapshot,
 											dest, NULL, false);
 
 		/*
@@ -1161,6 +1158,7 @@ DoCopy(const CopyStmt *stmt, const char *queryString)
 		/* Close down the query and free resources. */
 		ExecutorEnd(cstate->queryDesc);
 		FreeQueryDesc(cstate->queryDesc);
+		PopActiveSnapshot();
 	}
 
 	/* Clean up storage (probably not really necessary) */
@@ -1390,7 +1388,7 @@ CopyTo(CopyState cstate)
 		values = (Datum *) palloc(num_phys_attrs * sizeof(Datum));
 		nulls = (bool *) palloc(num_phys_attrs * sizeof(bool));
 
-		scandesc = heap_beginscan(cstate->rel, ActiveSnapshot, 0, NULL);
+		scandesc = heap_beginscan(cstate->rel, GetActiveSnapshot(), 0, NULL);
 
 		while ((tuple = heap_getnext(scandesc, ForwardScanDirection)) != NULL)
 		{

src/backend/commands/explain.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.173 2008/04/18 01:42:17 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.174 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -229,17 +229,14 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
 	int			eflags;
 
 	/*
-	 * Update snapshot command ID to ensure this query sees results of any
-	 * previously executed queries.  (It's a bit cheesy to modify
-	 * ActiveSnapshot without making a copy, but for the limited ways in which
-	 * EXPLAIN can be invoked, I think it's OK, because the active snapshot
-	 * shouldn't be shared with anything else anyway.)
+	 * Use a snapshot with an updated command ID to ensure this query sees
+	 * results of any previously executed queries.
 	 */
-	ActiveSnapshot->curcid = GetCurrentCommandId(false);
+	PushUpdatedSnapshot(GetActiveSnapshot());
 
 	/* Create a QueryDesc requesting no output */
 	queryDesc = CreateQueryDesc(plannedstmt,
-								ActiveSnapshot, InvalidSnapshot,
+								GetActiveSnapshot(), InvalidSnapshot,
 								None_Receiver, params,
 								stmt->analyze);
 
@@ -324,6 +321,8 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
 
 	FreeQueryDesc(queryDesc);
 
+	PopActiveSnapshot();
+
 	/* We need a CCI just in case query expanded to multiple plans */
 	if (stmt->analyze)
 		CommandCounterIncrement();

src/backend/commands/indexcmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/indexcmds.c,v 1.175 2008/05/12 00:00:47 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/indexcmds.c,v 1.176 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -484,6 +484,7 @@ DefineIndex(RangeVar *heapRelation,
 	 */
 	LockRelationIdForSession(&heaprelid, ShareUpdateExclusiveLock);
 
+	PopActiveSnapshot();
 	CommitTransactionCommand();
 	StartTransactionCommand();
 
@@ -542,7 +543,7 @@ DefineIndex(RangeVar *heapRelation,
 	indexRelation = index_open(indexRelationId, RowExclusiveLock);
 
 	/* Set ActiveSnapshot since functions in the indexes may need it */
-	ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+	PushActiveSnapshot(GetTransactionSnapshot());
 
 	/* We have to re-build the IndexInfo struct, since it was lost in commit */
 	indexInfo = BuildIndexInfo(indexRelation);
@@ -581,6 +582,9 @@ DefineIndex(RangeVar *heapRelation,
 
 	heap_close(pg_index, RowExclusiveLock);
 
+	/* we can do away with our snapshot */
+	PopActiveSnapshot();
+
 	/*
 	 * Commit this transaction to make the indisready update visible.
 	 */
@@ -616,8 +620,8 @@ DefineIndex(RangeVar *heapRelation,
 	 * We also set ActiveSnapshot to this snap, since functions in indexes may
 	 * need a snapshot.
 	 */
-	snapshot = CopySnapshot(GetTransactionSnapshot());
-	ActiveSnapshot = snapshot;
+	snapshot = RegisterSnapshot(GetTransactionSnapshot());
+	PushActiveSnapshot(snapshot);
 
 	/*
 	 * Scan the index and the heap, insert any missing index entries.
@@ -645,7 +649,7 @@ DefineIndex(RangeVar *heapRelation,
 	 * Also, GetCurrentVirtualXIDs never reports our own vxid, so we need not
 	 * check for that.
 	 */
-	old_snapshots = GetCurrentVirtualXIDs(ActiveSnapshot->xmax, false,
+	old_snapshots = GetCurrentVirtualXIDs(snapshot->xmax, false,
 										  PROC_IS_AUTOVACUUM | PROC_IN_VACUUM);
 
 	while (VirtualTransactionIdIsValid(*old_snapshots))
@@ -686,6 +690,12 @@ DefineIndex(RangeVar *heapRelation,
 	 */
 	CacheInvalidateRelcacheByRelid(heaprelid.relId);
 
+	/* we can now do away with our active snapshot */
+	PopActiveSnapshot();
+
+	/* And we can remove the validating snapshot too */
+	UnregisterSnapshot(snapshot);
+
 	/*
 	 * Last thing to do is release the session-level lock on the parent table.
 	 */
@@ -1453,6 +1463,7 @@ ReindexDatabase(const char *databaseName, bool do_system, bool do_user)
 	heap_close(relationRelation, AccessShareLock);
 
 	/* Now reindex each rel in a separate transaction */
+	PopActiveSnapshot();
 	CommitTransactionCommand();
 	foreach(l, relids)
 	{
@@ -1460,11 +1471,12 @@ ReindexDatabase(const char *databaseName, bool do_system, bool do_user)
 
 		StartTransactionCommand();
 		/* functions in indexes may want a snapshot set */
-		ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+		PushActiveSnapshot(GetTransactionSnapshot());
 		if (reindex_relation(relid, true))
 			ereport(NOTICE,
 					(errmsg("table \"%s\" was reindexed",
 							get_rel_name(relid))));
+		PopActiveSnapshot();
 		CommitTransactionCommand();
 	}
 	StartTransactionCommand();

src/backend/commands/portalcmds.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.73 2008/04/02 18:31:50 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.74 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -121,7 +121,7 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 	/*
 	 * Start execution, inserting parameters if any.
 	 */
-	PortalStart(portal, params, ActiveSnapshot);
+	PortalStart(portal, params, GetActiveSnapshot());
 
 	Assert(portal->strategy == PORTAL_ONE_SELECT);
 
@@ -293,7 +293,6 @@ PersistHoldablePortal(Portal portal)
 {
 	QueryDesc  *queryDesc = PortalGetQueryDesc(portal);
 	Portal		saveActivePortal;
-	Snapshot	saveActiveSnapshot;
 	ResourceOwner saveResourceOwner;
 	MemoryContext savePortalContext;
 	MemoryContext oldcxt;
@@ -334,18 +333,18 @@ PersistHoldablePortal(Portal portal)
 	 * Set up global portal context pointers.
 	 */
 	saveActivePortal = ActivePortal;
-	saveActiveSnapshot = ActiveSnapshot;
 	saveResourceOwner = CurrentResourceOwner;
 	savePortalContext = PortalContext;
 	PG_TRY();
 	{
 		ActivePortal = portal;
-		ActiveSnapshot = queryDesc->snapshot;
 		CurrentResourceOwner = portal->resowner;
 		PortalContext = PortalGetHeapMemory(portal);
 
 		MemoryContextSwitchTo(PortalContext);
 
+		PushActiveSnapshot(queryDesc->snapshot);
+
 		/*
 		 * Rewind the executor: we need to store the entire result set in the
 		 * tuplestore, so that subsequent backward FETCHs can be processed.
@@ -411,7 +410,6 @@ PersistHoldablePortal(Portal portal)
 
 		/* Restore global vars and propagate error */
 		ActivePortal = saveActivePortal;
-		ActiveSnapshot = saveActiveSnapshot;
 		CurrentResourceOwner = saveResourceOwner;
 		PortalContext = savePortalContext;
 
@@ -425,10 +423,11 @@ PersistHoldablePortal(Portal portal)
 	portal->status = PORTAL_READY;
 
 	ActivePortal = saveActivePortal;
-	ActiveSnapshot = saveActiveSnapshot;
 	CurrentResourceOwner = saveResourceOwner;
 	PortalContext = savePortalContext;
 
+	PopActiveSnapshot();
+
 	/*
 	 * We can now release any subsidiary memory of the portal's heap context;
 	 * we'll never use it again.  The executor already dropped its context,

src/backend/commands/prepare.c

@@ -10,7 +10,7 @@
  * Copyright (c) 2002-2008, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.86 2008/05/12 00:00:47 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.87 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -263,7 +263,7 @@ ExecuteQuery(ExecuteStmt *stmt, const char *queryString,
 	/*
 	 * Run the portal to completion.
 	 */
-	PortalStart(portal, paramLI, ActiveSnapshot);
+	PortalStart(portal, paramLI, GetActiveSnapshot());
 
 	(void) PortalRun(portal, FETCH_ALL, false, dest, dest, completionTag);
 

src/backend/commands/trigger.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.232 2008/05/12 00:00:48 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.233 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2976,6 +2976,7 @@ void
 AfterTriggerFireDeferred(void)
 {
 	AfterTriggerEventList *events;
+	bool		snap_pushed = false;
 
 	/* Must be inside a transaction */
 	Assert(afterTriggers != NULL);
@@ -2990,7 +2991,10 @@ AfterTriggerFireDeferred(void)
 	 */
 	events = &afterTriggers->events;
 	if (events->head != NULL)
-		ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+	{
+		PushActiveSnapshot(GetTransactionSnapshot());
+		snap_pushed = true;
+	}
 
 	/*
 	 * Run all the remaining triggers.	Loop until they are all gone, in case
@@ -3003,6 +3007,9 @@ AfterTriggerFireDeferred(void)
 		afterTriggerInvokeEvents(events, firing_id, NULL, true);
 	}
 
+	if (snap_pushed)
+		PopActiveSnapshot();
+
 	Assert(events->head == NULL);
 }
 

src/backend/commands/vacuum.c

@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.372 2008/05/12 00:00:48 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/vacuum.c,v 1.373 2008/05/12 20:02:00 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -409,6 +409,10 @@ vacuum(VacuumStmt *vacstmt, List *relids,
 	 */
 	if (use_own_xacts)
 	{
+		/* ActiveSnapshot is not set by autovacuum */
+		if (ActiveSnapshotSet())
+			PopActiveSnapshot();
+
 		/* matches the StartTransaction in PostgresMain() */
 		CommitTransactionCommand();
 	}
@@ -446,7 +450,7 @@ vacuum(VacuumStmt *vacstmt, List *relids,
 				{
 					StartTransactionCommand();
 					/* functions in indexes may want a snapshot set */
-					ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+					PushActiveSnapshot(GetTransactionSnapshot());
 				}
 				else
 					old_context = MemoryContextSwitchTo(anl_context);
@@ -454,7 +458,10 @@ vacuum(VacuumStmt *vacstmt, List *relids,
 				analyze_rel(relid, vacstmt, vac_strategy);
 
 				if (use_own_xacts)
+				{
+					PopActiveSnapshot();
 					CommitTransactionCommand();
+				}
 				else
 				{
 					MemoryContextSwitchTo(old_context);
@@ -981,7 +988,7 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
 	if (vacstmt->full)
 	{
 		/* functions in indexes may want a snapshot set */
-		ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+		PushActiveSnapshot(GetTransactionSnapshot());
 	}
 	else
 	{
@@ -1038,6 +1045,8 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
 
 	if (!onerel)
 	{
+		if (vacstmt->full)
+			PopActiveSnapshot();
 		CommitTransactionCommand();
 		return;
 	}
@@ -1068,6 +1077,8 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
 					(errmsg("skipping \"%s\" --- only table or database owner can vacuum it",
 							RelationGetRelationName(onerel))));
 		relation_close(onerel, lmode);
+		if (vacstmt->full)
+			PopActiveSnapshot();
 		CommitTransactionCommand();
 		return;
 	}
@@ -1082,6 +1093,8 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
 				(errmsg("skipping \"%s\" --- cannot vacuum indexes, views, or special system tables",
 						RelationGetRelationName(onerel))));
 		relation_close(onerel, lmode);
+		if (vacstmt->full)
+			PopActiveSnapshot();
 		CommitTransactionCommand();
 		return;
 	}
@@ -1096,6 +1109,8 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
 	if (isOtherTempNamespace(RelationGetNamespace(onerel)))
 	{
 		relation_close(onerel, lmode);
+		if (vacstmt->full)
+			PopActiveSnapshot();
 		CommitTransactionCommand();
 		return;
 	}
@@ -1143,6 +1158,8 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, char expected_relkind,
 	/*
 	 * Complete the transaction and free all temporary memory used.
 	 */
+	if (vacstmt->full)
+		PopActiveSnapshot();
 	CommitTransactionCommand();
 
 	/*

src/backend/commands/variable.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/variable.c,v 1.127 2008/03/26 18:48:59 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/variable.c,v 1.128 2008/05/12 20:02:00 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -550,7 +550,7 @@ show_log_timezone(void)
 const char *
 assign_XactIsoLevel(const char *value, bool doit, GucSource source)
 {
-	if (SerializableSnapshot != NULL)
+	if (FirstSnapshotSet)
 	{
 		ereport(GUC_complaint_elevel(source),
 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),

src/backend/executor/execMain.c

@@ -26,7 +26,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.308 2008/05/12 00:00:48 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.309 2008/05/12 20:02:00 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -54,6 +54,7 @@
 #include "utils/acl.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
+#include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
 
@@ -185,8 +186,8 @@ ExecutorStart(QueryDesc *queryDesc, int eflags)
 	/*
 	 * Copy other important information into the EState
 	 */
-	estate->es_snapshot = queryDesc->snapshot;
-	estate->es_crosscheck_snapshot = queryDesc->crosscheck_snapshot;
+	estate->es_snapshot = RegisterSnapshot(queryDesc->snapshot);
+	estate->es_crosscheck_snapshot = RegisterSnapshot(queryDesc->crosscheck_snapshot);
 	estate->es_instrument = queryDesc->doInstrument;
 
 	/*
@@ -313,6 +314,10 @@ ExecutorEnd(QueryDesc *queryDesc)
 	if (estate->es_select_into)
 		CloseIntoRel(queryDesc);
 
+	/* do away with our snapshots */
+	UnregisterSnapshot(estate->es_snapshot);
+	UnregisterSnapshot(estate->es_crosscheck_snapshot);
+
 	/*
 	 * Must switch out of context before destroying it
 	 */

src/backend/executor/functions.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/functions.c,v 1.124 2008/03/26 18:48:59 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/functions.c,v 1.125 2008/05/12 20:02:00 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -295,15 +295,14 @@ postquel_start(execution_state *es, SQLFunctionCachePtr fcache)
 	 * In a read-only function, use the surrounding query's snapshot;
 	 * otherwise take a new snapshot for each query.  The snapshot should
 	 * include a fresh command ID so that all work to date in this transaction
-	 * is visible.	We copy in both cases so that postquel_end can
-	 * unconditionally do FreeSnapshot.
+	 * is visible.
 	 */
 	if (fcache->readonly_func)
-		snapshot = CopySnapshot(ActiveSnapshot);
+		snapshot = GetActiveSnapshot();
 	else
 	{
 		CommandCounterIncrement();
-		snapshot = CopySnapshot(GetTransactionSnapshot());
+		snapshot = GetTransactionSnapshot();
 	}
 
 	if (IsA(es->stmt, PlannedStmt))
@@ -340,56 +339,44 @@ static TupleTableSlot *
 postquel_getnext(execution_state *es, SQLFunctionCachePtr fcache)
 {
 	TupleTableSlot *result;
-	Snapshot	saveActiveSnapshot;
 	long		count;
 
 	/* Make our snapshot the active one for any called functions */
-	saveActiveSnapshot = ActiveSnapshot;
-	PG_TRY();
-	{
-		ActiveSnapshot = es->qd->snapshot;
-
-		if (es->qd->utilitystmt)
-		{
-			/* ProcessUtility needs the PlannedStmt for DECLARE CURSOR */
-			ProcessUtility((es->qd->plannedstmt ?
-							(Node *) es->qd->plannedstmt :
-							es->qd->utilitystmt),
-						   fcache->src,
-						   es->qd->params,
-						   false,		/* not top level */
-						   es->qd->dest,
-						   NULL);
-			result = NULL;
-		}
-		else
-		{
-			/*
-			 * If it's the function's last command, and it's a SELECT, fetch
-			 * one row at a time so we can return the results. Otherwise just
-			 * run it to completion.  (If we run to completion then
-			 * ExecutorRun is guaranteed to return NULL.)
-			 */
-			if (LAST_POSTQUEL_COMMAND(es) &&
-				es->qd->operation == CMD_SELECT &&
-				es->qd->plannedstmt->utilityStmt == NULL &&
-				es->qd->plannedstmt->intoClause == NULL)
-				count = 1L;
-			else
-				count = 0L;
+	PushActiveSnapshot(es->qd->snapshot);
 
-			result = ExecutorRun(es->qd, ForwardScanDirection, count);
-		}
+	if (es->qd->utilitystmt)
+	{
+		/* ProcessUtility needs the PlannedStmt for DECLARE CURSOR */
+		ProcessUtility((es->qd->plannedstmt ?
+						(Node *) es->qd->plannedstmt :
+						es->qd->utilitystmt),
+					   fcache->src,
+					   es->qd->params,
+					   false,		/* not top level */
+					   es->qd->dest,
+					   NULL);
+		result = NULL;
 	}
-	PG_CATCH();
+	else
 	{
-		/* Restore global vars and propagate error */
-		ActiveSnapshot = saveActiveSnapshot;
-		PG_RE_THROW();
+		/*
+		 * If it's the function's last command, and it's a SELECT, fetch
+		 * one row at a time so we can return the results. Otherwise just
+		 * run it to completion.  (If we run to completion then
+		 * ExecutorRun is guaranteed to return NULL.)
+		 */
+		if (LAST_POSTQUEL_COMMAND(es) &&
+			es->qd->operation == CMD_SELECT &&
+			es->qd->plannedstmt->utilityStmt == NULL &&
+			es->qd->plannedstmt->intoClause == NULL)
+			count = 1L;
+		else
+			count = 0L;
+
+		result = ExecutorRun(es->qd, ForwardScanDirection, count);
 	}
-	PG_END_TRY();
 
-	ActiveSnapshot = saveActiveSnapshot;
+	PopActiveSnapshot();
 
 	return result;
 }
@@ -397,8 +384,6 @@ postquel_getnext(execution_state *es, SQLFunctionCachePtr fcache)
 static void
 postquel_end(execution_state *es)
 {
-	Snapshot	saveActiveSnapshot;
-
 	/* mark status done to ensure we don't do ExecutorEnd twice */
 	es->status = F_EXEC_DONE;
 
@@ -406,26 +391,15 @@ postquel_end(execution_state *es)
 	if (es->qd->utilitystmt == NULL)
 	{
 		/* Make our snapshot the active one for any called functions */
-		saveActiveSnapshot = ActiveSnapshot;
-		PG_TRY();
-		{
-			ActiveSnapshot = es->qd->snapshot;
+		PushActiveSnapshot(es->qd->snapshot);
 
-			if (es->qd->operation != CMD_SELECT)
-				AfterTriggerEndQuery(es->qd->estate);
-			ExecutorEnd(es->qd);
-		}
-		PG_CATCH();
-		{
-			/* Restore global vars and propagate error */
-			ActiveSnapshot = saveActiveSnapshot;
-			PG_RE_THROW();
-		}
-		PG_END_TRY();
-		ActiveSnapshot = saveActiveSnapshot;
+		if (es->qd->operation != CMD_SELECT)
+			AfterTriggerEndQuery(es->qd->estate);
+		ExecutorEnd(es->qd);
+
+		PopActiveSnapshot();
 	}
 
-	FreeSnapshot(es->qd->snapshot);
 	FreeQueryDesc(es->qd);
 	es->qd = NULL;
 }

src/backend/storage/ipc/procarray.c

@@ -23,7 +23,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.43 2008/03/26 18:48:59 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.44 2008/05/12 20:02:00 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -660,8 +660,7 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
  *
  * We also update the following backend-global variables:
  *		TransactionXmin: the oldest xmin of any snapshot in use in the
- *			current transaction (this is the same as MyProc->xmin).  This
- *			is just the xmin computed for the first, serializable snapshot.
+ *			current transaction (this is the same as MyProc->xmin).
  *		RecentXmin: the xmin computed for the most recent snapshot.  XIDs
  *			older than this are known not running any more.
  *		RecentGlobalXmin: the global xmin (oldest TransactionXmin across all
@@ -669,7 +668,7 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
  *			the same computation done by GetOldestXmin(true, true).
  */
 Snapshot
-GetSnapshotData(Snapshot snapshot, bool serializable)
+GetSnapshotData(Snapshot snapshot)
 {
 	ProcArrayStruct *arrayP = procArray;
 	TransactionId xmin;
@@ -681,11 +680,6 @@ GetSnapshotData(Snapshot snapshot, bool serializable)
 
 	Assert(snapshot != NULL);
 
-	/* Serializable snapshot must be computed before any other... */
-	Assert(serializable ?
-		   !TransactionIdIsValid(MyProc->xmin) :
-		   TransactionIdIsValid(MyProc->xmin));
-
 	/*
 	 * Allocating space for maxProcs xids is usually overkill; numProcs would
 	 * be sufficient.  But it seems better to do the malloc while not holding
@@ -806,7 +800,7 @@ GetSnapshotData(Snapshot snapshot, bool serializable)
 		}
 	}
 
-	if (serializable)
+	if (!TransactionIdIsValid(MyProc->xmin))
 		MyProc->xmin = TransactionXmin = xmin;
 
 	LWLockRelease(ProcArrayLock);
@@ -830,6 +824,14 @@ GetSnapshotData(Snapshot snapshot, bool serializable)
 
 	snapshot->curcid = GetCurrentCommandId(false);
 
+	/*
+	 * This is a new snapshot, so set both refcounts are zero, and mark it
+	 * as not copied in persistent memory.
+	 */
+	snapshot->active_count = 0;
+	snapshot->regd_count = 0;
+	snapshot->copied = false;
+
 	return snapshot;
 }
 

src/backend/storage/large_object/inv_api.c

@@ -24,7 +24,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/large_object/inv_api.c,v 1.132 2008/04/12 23:14:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/large_object/inv_api.c,v 1.133 2008/05/12 20:02:00 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -246,12 +246,8 @@ inv_open(Oid lobjId, int flags, MemoryContext mcxt)
 	}
 	else if (flags & INV_READ)
 	{
-		/* be sure to copy snap into mcxt */
-		MemoryContext oldContext = MemoryContextSwitchTo(mcxt);
-
-		retval->snapshot = CopySnapshot(ActiveSnapshot);
+		retval->snapshot = RegisterSnapshot(GetActiveSnapshot());
 		retval->flags = IFS_RDLOCK;
-		MemoryContextSwitchTo(oldContext);
 	}
 	else
 		elog(ERROR, "invalid flags: %d", flags);
@@ -274,7 +270,7 @@ inv_close(LargeObjectDesc *obj_desc)
 {
 	Assert(PointerIsValid(obj_desc));
 	if (obj_desc->snapshot != SnapshotNow)
-		FreeSnapshot(obj_desc->snapshot);
+		UnregisterSnapshot(obj_desc->snapshot);
 	pfree(obj_desc);
 }
 

src/backend/tcop/fastpath.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tcop/fastpath.c,v 1.99 2008/03/26 18:48:59 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tcop/fastpath.c,v 1.100 2008/05/12 20:02:01 alvherre Exp $
  *
  * NOTES
  *	  This cruft is the server side of PQfn.
@@ -309,7 +309,7 @@ HandleFunctionRequest(StringInfo msgBuf)
 	 * Now that we know we are in a valid transaction, set snapshot in case
 	 * needed by function itself or one of the datatype I/O routines.
 	 */
-	ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
+	PushActiveSnapshot(GetTransactionSnapshot());
 
 	/*
 	 * Begin parsing the buffer contents.
@@ -396,6 +396,9 @@ HandleFunctionRequest(StringInfo msgBuf)
 
 	SendFunctionResult(retval, fcinfo.isnull, fip->rettype, rformat);
 
+	/* We no longer need the snapshot */
+	PopActiveSnapshot();
+
 	/*
 	 * Emit duration logging if appropriate.
 	 */

src/backend/tcop/postgres.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tcop/postgres.c,v 1.551 2008/05/12 00:00:50 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tcop/postgres.c,v 1.552 2008/05/12 20:02:01 alvherre Exp $
  *
  * NOTES
  *	  this is the "main" module of the postgres backend and
@@ -732,49 +732,37 @@ List *
 pg_plan_queries(List *querytrees, int cursorOptions, ParamListInfo boundParams,
 				bool needSnapshot)
 {
-	List	   * volatile stmt_list = NIL;
-	Snapshot	saveActiveSnapshot = ActiveSnapshot;
+	List	   *stmt_list = NIL;
+	ListCell   *query_list;
+	bool		snapshot_set = false;
 
-	/* PG_TRY to ensure previous ActiveSnapshot is restored on error */
-	PG_TRY();
+	foreach(query_list, querytrees)
 	{
-		Snapshot	mySnapshot = NULL;
-		ListCell   *query_list;
+		Query	   *query = (Query *) lfirst(query_list);
+		Node	   *stmt;
 
-		foreach(query_list, querytrees)
+		if (query->commandType == CMD_UTILITY)
 		{
-			Query	   *query = (Query *) lfirst(query_list);
-			Node	   *stmt;
-
-			if (query->commandType == CMD_UTILITY)
-			{
-				/* Utility commands have no plans. */
-				stmt = query->utilityStmt;
-			}
-			else
+			/* Utility commands have no plans. */
+			stmt = query->utilityStmt;
+		}
+		else
+		{
+			if (needSnapshot && !snapshot_set)
 			{
-				if (needSnapshot && mySnapshot == NULL)
-				{
-					mySnapshot = CopySnapshot(GetTransactionSnapshot());
-					ActiveSnapshot = mySnapshot;
-				}
-				stmt = (Node *) pg_plan_query(query, cursorOptions,
-											  boundParams);
+				PushActiveSnapshot(GetTransactionSnapshot());
+				snapshot_set = true;
 			}
 
-			stmt_list = lappend(stmt_list, stmt);
+			stmt = (Node *) pg_plan_query(query, cursorOptions,
+										  boundParams);
 		}
 
-		if (mySnapshot)
-			FreeSnapshot(mySnapshot);
+		stmt_list = lappend(stmt_list, stmt);
 	}
-	PG_CATCH();
-	{
-		ActiveSnapshot = saveActiveSnapshot;
-		PG_RE_THROW();
-	}
-	PG_END_TRY();
-	ActiveSnapshot = saveActiveSnapshot;
+
+	if (snapshot_set)
+		PopActiveSnapshot();
 
 	return stmt_list;
 }

src/backend/utils/adt/ri_triggers.c

@@ -15,7 +15,7 @@
  *
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  *
- * $PostgreSQL: pgsql/src/backend/utils/adt/ri_triggers.c,v 1.107 2008/03/26 21:10:39 alvherre Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/adt/ri_triggers.c,v 1.108 2008/05/12 20:02:02 alvherre Exp $
  *
  * ----------
  */
@@ -2756,12 +2756,13 @@ RI_Initial_Check(Trigger *trigger, Relation fk_rel, Relation pk_rel)
 	/*
 	 * Run the plan.  For safety we force a current snapshot to be used. (In
 	 * serializable mode, this arguably violates serializability, but we
-	 * really haven't got much choice.)  We need at most one tuple returned,
-	 * so pass limit = 1.
+	 * really haven't got much choice.)  We don't need to register the
+	 * snapshot, because SPI_execute_snapshot will see to it.  We need at most
+	 * one tuple returned, so pass limit = 1.
 	 */
 	spi_result = SPI_execute_snapshot(qplan,
 									  NULL, NULL,
-									  CopySnapshot(GetLatestSnapshot()),
+									  GetLatestSnapshot(),
 									  InvalidSnapshot,
 									  true, false, 1);
 
@@ -3311,13 +3312,15 @@ ri_PerformCheck(RI_QueryKey *qkey, SPIPlanPtr qplan,
 	 * caller passes detectNewRows == false then it's okay to do the query
 	 * with the transaction snapshot; otherwise we use a current snapshot, and
 	 * tell the executor to error out if it finds any rows under the current
-	 * snapshot that wouldn't be visible per the transaction snapshot.
+	 * snapshot that wouldn't be visible per the transaction snapshot.  Note
+	 * that SPI_execute_snapshot will register the snapshots, so we don't need
+	 * to bother here.
 	 */
 	if (IsXactIsoLevelSerializable && detectNewRows)
 	{
 		CommandCounterIncrement();		/* be sure all my own work is visible */
-		test_snapshot = CopySnapshot(GetLatestSnapshot());
-		crosscheck_snapshot = CopySnapshot(GetTransactionSnapshot());
+		test_snapshot = GetLatestSnapshot();
+		crosscheck_snapshot = GetTransactionSnapshot();
 	}
 	else
 	{

src/backend/utils/adt/txid.c

@@ -14,7 +14,7 @@
  *	Author: Jan Wieck, Afilias USA INC.
  *	64-bit txids: Marko Kreen, Skype Technologies
  *
- *	$PostgreSQL: pgsql/src/backend/utils/adt/txid.c,v 1.6 2008/03/26 18:48:59 alvherre Exp $
+ *	$PostgreSQL: pgsql/src/backend/utils/adt/txid.c,v 1.7 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -362,9 +362,9 @@ txid_current_snapshot(PG_FUNCTION_ARGS)
 	TxidEpoch	state;
 	Snapshot	cur;
 
-	cur = ActiveSnapshot;
+	cur = GetActiveSnapshot();
 	if (cur == NULL)
-		elog(ERROR, "txid_current_snapshot: ActiveSnapshot == NULL");
+		elog(ERROR, "no active snapshot set");
 
 	load_xid_epoch(&state);
 

src/backend/utils/cache/plancache.c

@@ -33,7 +33,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/plancache.c,v 1.17 2008/03/26 18:48:59 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/plancache.c,v 1.18 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -71,7 +71,6 @@ static List *cached_plans_list = NIL;
 
 static void StoreCachedPlan(CachedPlanSource *plansource, List *stmt_list,
 				MemoryContext plan_context);
-static List *do_planning(List *querytrees, int cursorOptions);
 static void AcquireExecutorLocks(List *stmt_list, bool acquire);
 static void AcquirePlannerLocks(List *stmt_list, bool acquire);
 static void LockRelid(Oid relid, LOCKMODE lockmode, void *arg);
@@ -481,8 +480,15 @@ RevalidateCachedPlan(CachedPlanSource *plansource, bool useResOwner)
 
 		if (plansource->fully_planned)
 		{
-			/* Generate plans for queries */
-			slist = do_planning(slist, plansource->cursor_options);
+			/*
+			 * Generate plans for queries.
+			 *
+			 * If a snapshot is already set (the normal case), we can just use
+			 * that for planning.  But if it isn't, we have to tell
+			 * pg_plan_queries to make a snap if it needs one.
+			 */
+			slist = pg_plan_queries(slist, plansource->cursor_options,
+									NULL, !ActiveSnapshotSet());
 		}
 
 		/*
@@ -537,49 +543,6 @@ RevalidateCachedPlan(CachedPlanSource *plansource, bool useResOwner)
 	return plan;
 }
 
-/*
- * Invoke the planner on some rewritten queries.  This is broken out of
- * RevalidateCachedPlan just to avoid plastering "volatile" all over that
- * function's variables.
- */
-static List *
-do_planning(List *querytrees, int cursorOptions)
-{
-	List	   *stmt_list;
-
-	/*
-	 * If a snapshot is already set (the normal case), we can just use that
-	 * for planning.  But if it isn't, we have to tell pg_plan_queries to make
-	 * a snap if it needs one.	In that case we should arrange to reset
-	 * ActiveSnapshot afterward, to ensure that RevalidateCachedPlan has no
-	 * caller-visible effects on the snapshot.	Having to replan is an unusual
-	 * case, and it seems a really bad idea for RevalidateCachedPlan to affect
-	 * the snapshot only in unusual cases.	(Besides, the snap might have been
-	 * created in a short-lived context.)
-	 */
-	if (ActiveSnapshot != NULL)
-		stmt_list = pg_plan_queries(querytrees, cursorOptions, NULL, false);
-	else
-	{
-		PG_TRY();
-		{
-			stmt_list = pg_plan_queries(querytrees, cursorOptions, NULL, true);
-		}
-		PG_CATCH();
-		{
-			/* Restore global vars and propagate error */
-			ActiveSnapshot = NULL;
-			PG_RE_THROW();
-		}
-		PG_END_TRY();
-
-		ActiveSnapshot = NULL;
-	}
-
-	return stmt_list;
-}
-
-
 /*
  * ReleaseCachedPlan: release active use of a cached plan.
  *

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);

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 */

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;
 
 /*

src/pl/plpgsql/src/pl_exec.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.212 2008/05/12 00:00:54 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.213 2008/05/12 20:02:02 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4129,7 +4129,7 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 	CachedPlan *cplan;
 	ParamListInfo paramLI;
 	int			i;
-	Snapshot	saveActiveSnapshot;
+	MemoryContext oldcontext;
 
 	/*
 	 * Forget it if expression wasn't simple before.
@@ -4218,37 +4218,26 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
 	 * updates made so far by our own function.
 	 */
 	SPI_push();
-	saveActiveSnapshot = ActiveSnapshot;
 
-	PG_TRY();
+	oldcontext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
+	if (!estate->readonly_func)
 	{
-		MemoryContext oldcontext;
+		CommandCounterIncrement();
+		PushActiveSnapshot(GetTransactionSnapshot());
+	}
 
-		oldcontext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);
-		if (!estate->readonly_func)
-		{
-			CommandCounterIncrement();
-			ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
-		}
+	/*
+	 * Finally we can call the executor to evaluate the expression
+	 */
+	*result = ExecEvalExpr(expr->expr_simple_state,
+						   econtext,
+						   isNull,
+						   NULL);
+	MemoryContextSwitchTo(oldcontext);
 
-		/*
-		 * Finally we can call the executor to evaluate the expression
-		 */
-		*result = ExecEvalExpr(expr->expr_simple_state,
-							   econtext,
-							   isNull,
-							   NULL);
-		MemoryContextSwitchTo(oldcontext);
-	}
-	PG_CATCH();
-	{
-		/* Restore global vars and propagate error */
-		ActiveSnapshot = saveActiveSnapshot;
-		PG_RE_THROW();
-	}
-	PG_END_TRY();
+	if (!estate->readonly_func)
+		PopActiveSnapshot();
 
-	ActiveSnapshot = saveActiveSnapshot;
 	SPI_pop();
 
 	/*