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, 10 insertions, 7 deletions

diff --git a/src/backend/utils/adt/ri_triggers.c b/src/backend/utils/adt/ri_triggers.c
index 4cd84876532..59f7e497955 100644
--- a/src/backend/utils/adt/ri_triggers.c
+++ b/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
 	{