Initialize the minimum frozen Xid in vac_update_datfrozenxid using

GetOldestXmin() instead of RecentGlobalXmin; this is safer because we do not
depend on the latter being correctly set elsewhere, and while it is more
expensive, this code path is not performance-critical.  This is a real
risk for autovacuum, because it can execute whole cycles without doing
a single vacuum, which would mean that RecentGlobalXmin would stay at its
initialization value, FirstNormalTransactionId, causing a bogus value to be
inserted in pg_database.  This bug could explain some recent reports of
failure to truncate pg_clog.

At the same time, change the initialization of RecentGlobalXmin to
InvalidTransactionId, and ensure that it's set to something else whenever
it's going to be used.  Using it as FirstNormalTransactionId in HOT page
pruning could incur in data loss.  InitPostgres takes care of setting it
to a valid value, but the extra checks are there to prevent "special"
backends from behaving in unusual ways.

Per Tom Lane's detailed problem dissection in 29544.1221061979@sss.pgh.pa.us

1 files changed, 8 insertions, 2 deletions

diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 82532a196b0..15226b92bc1 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.172 2006/11/05 22:42:09 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.172.2.1 2008/09/11 14:01:16 alvherre Exp $
  *
  *
  *-------------------------------------------------------------------------
@@ -42,6 +42,7 @@
 #include "utils/guc.h"
 #include "utils/portal.h"
 #include "utils/relcache.h"
+#include "utils/tqual.h"
 #include "utils/syscache.h"
 #include "pgstat.h"
 
@@ -380,10 +381,15 @@ InitPostgres(const char *dbname, const char *username)
 	on_shmem_exit(ShutdownPostgres, 0);
 
 	/*
-	 * Start a new transaction here before first access to db
+	 * Start a new transaction here before first access to db, and get a
+	 * snapshot.  We don't have a use for the snapshot itself, but we're
+	 * interested in the secondary effect that it sets RecentGlobalXmin.
 	 */
 	if (!bootstrap)
+	{
 		StartTransactionCommand();
+		(void) GetTransactionSnapshot();
+	}
 
 	/*
 	 * Now that we have a transaction, we can take locks.  Take a writer's