From 2ca19aa8165c6bec84c1d527fc5f3100c2161b1a Mon Sep 17 00:00:00 2001
From: Noah Misch <noah@leadboat.com>
Date: Mon, 29 Apr 2024 10:24:56 -0700
Subject: [PATCH] Close race condition between datfrozen and relfrozen updates.
vac_update_datfrozenxid() did multiple loads of relfrozenxid and
relminmxid from buffer memory, and it assumed each would get the same
value. Not so if a concurrent vac_update_relstats() did an inplace
update. Commit 2d2e40e3befd8b9e0d2757554537345b15fa6ea2 fixed the same
kind of bug in vac_truncate_clog(). Today's bug could cause the
rel-level field and XIDs in the rel's rows to precede the db-level
field. A cluster having such values should VACUUM affected tables.
Back-patch to v12 (all supported versions).
Discussion: https://postgr.es/m/20240423003956.e7.nmisch@google.com
---
src/backend/commands/vacuum.c | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 18018a4e8a3..8f24b885464 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -1480,6 +1480,8 @@ vac_update_datfrozenxid(void)
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
* index that can help us here.
+ *
+ * See vac_truncate_clog() for the race condition to prevent.
*/
relation = table_open(RelationRelationId, AccessShareLock);
@@ -1488,7 +1490,9 @@ vac_update_datfrozenxid(void)
while ((classTup = systable_getnext(scan)) != NULL)
{
- Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);
+ volatile FormData_pg_class *classForm = (Form_pg_class) GETSTRUCT(classTup);
+ TransactionId relfrozenxid = classForm->relfrozenxid;
+ TransactionId relminmxid = classForm->relminmxid;
/*
* Only consider relations able to hold unfrozen XIDs (anything else
@@ -1498,8 +1502,8 @@ vac_update_datfrozenxid(void)
classForm->relkind != RELKIND_MATVIEW &&
classForm->relkind != RELKIND_TOASTVALUE)
{
- Assert(!TransactionIdIsValid(classForm->relfrozenxid));
- Assert(!MultiXactIdIsValid(classForm->relminmxid));
+ Assert(!TransactionIdIsValid(relfrozenxid));
+ Assert(!MultiXactIdIsValid(relminmxid));
continue;
}
@@ -1518,34 +1522,34 @@ vac_update_datfrozenxid(void)
* before those relations have been scanned and cleaned up.
*/
- if (TransactionIdIsValid(classForm->relfrozenxid))
+ if (TransactionIdIsValid(relfrozenxid))
{
- Assert(TransactionIdIsNormal(classForm->relfrozenxid));
+ Assert(TransactionIdIsNormal(relfrozenxid));
/* check for values in the future */
- if (TransactionIdPrecedes(lastSaneFrozenXid, classForm->relfrozenxid))
+ if (TransactionIdPrecedes(lastSaneFrozenXid, relfrozenxid))
{
bogus = true;
break;
}
/* determine new horizon */
- if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
- newFrozenXid = classForm->relfrozenxid;
+ if (TransactionIdPrecedes(relfrozenxid, newFrozenXid))
+ newFrozenXid = relfrozenxid;
}
- if (MultiXactIdIsValid(classForm->relminmxid))
+ if (MultiXactIdIsValid(relminmxid))
{
/* check for values in the future */
- if (MultiXactIdPrecedes(lastSaneMinMulti, classForm->relminmxid))
+ if (MultiXactIdPrecedes(lastSaneMinMulti, relminmxid))
{
bogus = true;
break;
}
/* determine new horizon */
- if (MultiXactIdPrecedes(classForm->relminmxid, newMinMulti))
- newMinMulti = classForm->relminmxid;
+ if (MultiXactIdPrecedes(relminmxid, newMinMulti))
+ newMinMulti = relminmxid;
}
}
--
2.39.5