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, 12 insertions, 10 deletions

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index aab0a1f83c9..62a17003c1c 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/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;
 }