@@ -1018,6 +1018,12 @@ vac_truncate_clog(TransactionId frozenXID,
10181018/*
10191019 * Scan pg_database to compute the minimum datfrozenxid/datminmxid
10201020 *
1021+ * Since vac_update_datfrozenxid updates datfrozenxid/datminmxid in-place,
1022+ * the values could change while we look at them. Fetch each one just
1023+ * once to ensure sane behavior of the comparison logic. (Here, as in
1024+ * many other places, we assume that fetching or updating an XID in shared
1025+ * storage is atomic.)
1026+ *
10211027 * Note: we need not worry about a race condition with new entries being
10221028 * inserted by CREATE DATABASE. Any such entry will have a copy of some
10231029 * existing DB's datfrozenxid, and that source DB cannot be ours because
@@ -1033,10 +1039,12 @@ vac_truncate_clog(TransactionId frozenXID,
10331039
10341040while ((tuple = heap_getnext (scan ,ForwardScanDirection ))!= NULL )
10351041{
1036- Form_pg_database dbform = (Form_pg_database )GETSTRUCT (tuple );
1042+ volatile FormData_pg_database * dbform = (Form_pg_database )GETSTRUCT (tuple );
1043+ TransactionId datfrozenxid = dbform -> datfrozenxid ;
1044+ TransactionId datminmxid = dbform -> datminmxid ;
10371045
1038- Assert (TransactionIdIsNormal (dbform -> datfrozenxid ));
1039- Assert (MultiXactIdIsValid (dbform -> datminmxid ));
1046+ Assert (TransactionIdIsNormal (datfrozenxid ));
1047+ Assert (MultiXactIdIsValid (datminmxid ));
10401048
10411049/*
10421050 * If things are working properly, no database should have a
@@ -1047,21 +1055,21 @@ vac_truncate_clog(TransactionId frozenXID,
10471055 * databases have been scanned and cleaned up. (We will issue the
10481056 * "already wrapped" warning if appropriate, though.)
10491057 */
1050- if (TransactionIdPrecedes (lastSaneFrozenXid ,dbform -> datfrozenxid )||
1051- MultiXactIdPrecedes (lastSaneMinMulti ,dbform -> datminmxid ))
1058+ if (TransactionIdPrecedes (lastSaneFrozenXid ,datfrozenxid )||
1059+ MultiXactIdPrecedes (lastSaneMinMulti ,datminmxid ))
10521060bogus = true;
10531061
1054- if (TransactionIdPrecedes (nextXID ,dbform -> datfrozenxid ))
1062+ if (TransactionIdPrecedes (nextXID ,datfrozenxid ))
10551063frozenAlreadyWrapped = true;
1056- else if (TransactionIdPrecedes (dbform -> datfrozenxid ,frozenXID ))
1064+ else if (TransactionIdPrecedes (datfrozenxid ,frozenXID ))
10571065{
1058- frozenXID = dbform -> datfrozenxid ;
1066+ frozenXID = datfrozenxid ;
10591067oldestxid_datoid = HeapTupleGetOid (tuple );
10601068}
10611069
1062- if (MultiXactIdPrecedes (dbform -> datminmxid ,minMulti ))
1070+ if (MultiXactIdPrecedes (datminmxid ,minMulti ))
10631071{
1064- minMulti = dbform -> datminmxid ;
1072+ minMulti = datminmxid ;
10651073minmulti_datoid = HeapTupleGetOid (tuple );
10661074}
10671075}