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Initialize the minimum frozen Xid in vac_update_datfrozenxid using

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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
This commit is contained in:
Alvaro Herrera
2008-09-11 14:01:10 +00:00
parent b8646012d5
commit d53a56687f
6 changed files with 43 additions and 35 deletions
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8
src/backend/utils/time/snapmgr.c
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@ -22,7 +22,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.4 2008/07/11 02:10:14 alvherre Exp $
* $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.5 2008/09/11 14:01:10 alvherre Exp $
*
*-------------------------------------------------------------------------
*/
@ -59,10 +59,14 @@ static Snapshot SecondarySnapshot = NULL;
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
*
* RecentGlobalXmin is initialized to InvalidTransactionId, to ensure that no
* one tries to use a stale value. Readers should ensure that it has been set
* to something else before using it.
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
TransactionId RecentGlobalXmin = FirstNormalTransactionId;
TransactionId RecentGlobalXmin = InvalidTransactionId;
/*
* Elements of the list of registered snapshots.