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, 6 insertions, 7 deletions

diff --git a/src/backend/commands/portalcmds.c b/src/backend/commands/portalcmds.c
index 83998233b49..3e5e573c605 100644
--- a/src/backend/commands/portalcmds.c
+++ b/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,