From 0f45d1eb08d0916f8569644223fe0c11390aae0a Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 16 Apr 2008 18:23:04 +0000
Subject: [PATCH] Fix LOAD_CRIT_INDEX() macro to take out AccessShareLock on
the system index it is trying to build a relcache entry for. This is an
oversight in my 8.2 patch that tried to ensure we always took a lock on a
relation before trying to build its relcache entry. The implication is that
if someone committed a reindex of a critical system index at about the same
time that some other backend were starting up without a valid
pg_internal.init file, the second one might PANIC due to not seeing any valid
version of the index's pg_class row. Improbable case, but definitely not
impossible.
---
src/backend/utils/cache/relcache.c | 24 ++++++++++++++++++------
1 file changed, 18 insertions(+), 6 deletions(-)
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 9a32fd5845e..b199a478dfe 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/utils/cache/relcache.c,v 1.270 2008/04/01 00:48:33 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/cache/relcache.c,v 1.271 2008/04/16 18:23:04 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -220,8 +220,11 @@ static OpClassCacheEnt *LookupOpclassInfo(Oid operatorClassOid,
/*
* ScanPgRelation
*
- * this is used by RelationBuildDesc to find a pg_class
- * tuple matching targetRelId.
+ * This is used by RelationBuildDesc to find a pg_class
+ * tuple matching targetRelId. The caller must hold at least
+ * AccessShareLock on the target relid to prevent concurrent-update
+ * scenarios --- else our SnapshotNow scan might fail to find any
+ * version that it thinks is live.
*
* NB: the returned tuple has been copied into palloc'd storage
* and must eventually be freed with heap_freetuple.
@@ -773,18 +776,18 @@ equalRuleLocks(RuleLock *rlock1, RuleLock *rlock2)
}
-/* ----------------------------------
+/*
* RelationBuildDesc
*
* Build a relation descriptor --- either a new one, or by
* recycling the given old relation object. The latter case
* supports rebuilding a relcache entry without invalidating
- * pointers to it.
+ * pointers to it. The caller must hold at least
+ * AccessShareLock on the target relid.
*
* Returns NULL if no pg_class row could be found for the given relid
* (suggesting we are trying to access a just-deleted relation).
* Any other error is reported via elog.
- * --------------------------------
*/
static Relation
RelationBuildDesc(Oid targetRelId, Relation oldrelation)
@@ -1608,6 +1611,10 @@ RelationClose(Relation relation)
* RelationClearRelation just marks the entry as invalid by setting
* rd_isvalid to false. This routine is called to fix the entry when it
* is next needed.
+ *
+ * We assume that at the time we are called, we have at least AccessShareLock
+ * on the target index. (Note: in the calls from RelationClearRelation,
+ * this is legitimate because we know the rel has positive refcount.)
*/
static void
RelationReloadIndexInfo(Relation relation)
@@ -1692,6 +1699,9 @@ RelationReloadIndexInfo(Relation relation)
* usually used when we are notified of a change to an open relation
* (one with refcount > 0). However, this routine just does whichever
* it's told to do; callers must determine which they want.
+ *
+ * NB: when rebuilding, we'd better hold some lock on the relation.
+ * In current usages this is presumed true because it has refcnt > 0.
*/
static void
RelationClearRelation(Relation relation, bool rebuild)
@@ -2542,12 +2552,14 @@ RelationCacheInitializePhase2(void)
#define LOAD_CRIT_INDEX(indexoid) \
do { \
+ LockRelationOid(indexoid, AccessShareLock); \
ird = RelationBuildDesc(indexoid, NULL); \
if (ird == NULL) \
elog(PANIC, "could not open critical system index %u", \
indexoid); \
ird->rd_isnailed = true; \
ird->rd_refcnt = 1; \
+ UnlockRelationOid(indexoid, AccessShareLock); \
} while (0)
LOAD_CRIT_INDEX(ClassOidIndexId);
--
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