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.

1 files changed, 20 insertions, 3 deletions

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 679c0126242..c5793231f22 100644
--- a/src/backend/commands/vacuum.c
+++ b/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();
 
 	/*