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, 17 insertions, 28 deletions

diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index cd0ed39168..b248384af4 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/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();
 
 	/*