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nmischnmisch
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Lock before setting relhassubclass on RELKIND_PARTITIONED_INDEX.
Commit 5b56264 added a comment that SetRelationHasSubclass() callers must hold this lock. When commit 17f206f extended use of this column to partitioned indexes, it didn't take the lock. As the latter commit message mentioned, we currently never reset a partitioned index to relhassubclass=f. That largely avoids harm from the lock omission. The cause for fixing this now is to unblock introducing a rule about locks required to heap_update() a pg_class row. This might cause more deadlocks. It gives minor user-visible benefits: - If an ALTER INDEX SET TABLESPACE runs concurrently with ALTER TABLE ATTACH PARTITION or CREATE PARTITION OF, one transaction blocks instead of failing with "tuple concurrently updated". (Many cases of DDL concurrency still fail that way.) - Match ALTER INDEX ATTACH PARTITION in choosing to lock the index. While not user-visible today, we'll need this if we ever make something set the flag to false for a partitioned index, like ANALYZE does today for tables. Back-patch to v12 (all supported versions), the plan for the commit relying on the new rule. In back branches, add LockOrStrongerHeldByMe() instead of adding a LockHeldByMe() parameter. Reviewed (in an earlier version) by Robert Haas. Discussion: https://postgr.es/m/20240611024525.9f.nmisch@google.com
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-30
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7 files changed

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lines changed

‎src/backend/catalog/index.c

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1014,6 +1014,7 @@ index_create(Relation heapRelation,
10141014
if (OidIsValid(parentIndexRelid))
10151015
{
10161016
StoreSingleInheritance(indexRelationId, parentIndexRelid, 1);
1017+
LockRelationOid(parentIndexRelid, ShareUpdateExclusiveLock);
10171018
SetRelationHasSubclass(parentIndexRelid, true);
10181019
}
10191020

‎src/backend/commands/indexcmds.c

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -4245,7 +4245,10 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid)
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42464246
/* set relhassubclass if an index partition has been added to the parent */
42474247
if (OidIsValid(parentOid))
4248+
{
4249+
LockRelationOid(parentOid, ShareUpdateExclusiveLock);
42484250
SetRelationHasSubclass(parentOid, true);
4251+
}
42494252

42504253
/* set relispartition correctly on the partition */
42514254
update_relispartition(partRelid, OidIsValid(parentOid));

‎src/backend/commands/tablecmds.c

Lines changed: 14 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -3196,8 +3196,15 @@ findAttrByName(const char *attributeName, List *schema)
31963196
* SetRelationHasSubclass
31973197
* Set the value of the relation's relhassubclass field in pg_class.
31983198
*
3199-
* NOTE: caller must be holding an appropriate lock on the relation.
3200-
* ShareUpdateExclusiveLock is sufficient.
3199+
* It's always safe to set this field to true, because all SQL commands are
3200+
* ready to see true and then find no children. On the other hand, commands
3201+
* generally assume zero children if this is false.
3202+
*
3203+
* Caller must hold any self-exclusive lock until end of transaction. If the
3204+
* new value is false, caller must have acquired that lock before reading the
3205+
* evidence that justified the false value. That way, it properly waits if
3206+
* another backend is simultaneously concluding no need to change the tuple
3207+
* (new and old values are true).
32013208
*
32023209
* NOTE: an important side-effect of this operation is that an SI invalidation
32033210
* message is sent out to all backends --- including me --- causing plans
@@ -3212,6 +3219,11 @@ SetRelationHasSubclass(Oid relationId, bool relhassubclass)
32123219
HeapTuple tuple;
32133220
Form_pg_class classtuple;
32143221

3222+
Assert(CheckRelationOidLockedByMe(relationId,
3223+
ShareUpdateExclusiveLock, false) ||
3224+
CheckRelationOidLockedByMe(relationId,
3225+
ShareRowExclusiveLock, true));
3226+
32153227
/*
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* Fetch a modifiable copy of the tuple, modify it, update pg_class.
32173229
*/

‎src/backend/storage/lmgr/lmgr.c

Lines changed: 17 additions & 23 deletions
Original file line numberDiff line numberDiff line change
@@ -308,32 +308,26 @@ CheckRelationLockedByMe(Relation relation, LOCKMODE lockmode, bool orstronger)
308308
relation->rd_lockInfo.lockRelId.dbId,
309309
relation->rd_lockInfo.lockRelId.relId);
310310

311-
if (LockHeldByMe(&tag, lockmode))
312-
return true;
311+
return (orstronger ?
312+
LockOrStrongerHeldByMe(&tag, lockmode) :
313+
LockHeldByMe(&tag, lockmode));
314+
}
313315

314-
if (orstronger)
315-
{
316-
LOCKMODE slockmode;
316+
/*
317+
* CheckRelationOidLockedByMe
318+
*
319+
* Like the above, but takes an OID as argument.
320+
*/
321+
bool
322+
CheckRelationOidLockedByMe(Oid relid, LOCKMODE lockmode, bool orstronger)
323+
{
324+
LOCKTAG tag;
317325

318-
for (slockmode = lockmode + 1;
319-
slockmode <= MaxLockMode;
320-
slockmode++)
321-
{
322-
if (LockHeldByMe(&tag, slockmode))
323-
{
324-
#ifdef NOT_USED
325-
/* Sometimes this might be useful for debugging purposes */
326-
elog(WARNING, "lock mode %s substituted for %s on relation %s",
327-
GetLockmodeName(tag.locktag_lockmethodid, slockmode),
328-
GetLockmodeName(tag.locktag_lockmethodid, lockmode),
329-
RelationGetRelationName(relation));
330-
#endif
331-
return true;
332-
}
333-
}
334-
}
326+
SetLocktagRelationOid(&tag, relid);
335327

336-
return false;
328+
return (orstronger ?
329+
LockOrStrongerHeldByMe(&tag, lockmode) :
330+
LockHeldByMe(&tag, lockmode));
337331
}
338332

339333
/*

‎src/backend/storage/lmgr/lock.c

Lines changed: 39 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -578,11 +578,17 @@ DoLockModesConflict(LOCKMODE mode1, LOCKMODE mode2)
578578
}
579579

580580
/*
581-
* LockHeldByMe -- test whether lock 'locktag' is held with mode 'lockmode'
582-
* by the current transaction
581+
* LockHeldByMeExtended -- test whether lock 'locktag' is held by the current
582+
* transaction
583+
*
584+
* Returns true if current transaction holds a lock on 'tag' of mode
585+
* 'lockmode'. If 'orstronger' is true, a stronger lockmode is also OK.
586+
* ("Stronger" is defined as "numerically higher", which is a bit
587+
* semantically dubious but is OK for the purposes we use this for.)
583588
*/
584-
bool
585-
LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
589+
static bool
590+
LockHeldByMeExtended(const LOCKTAG *locktag,
591+
LOCKMODE lockmode, bool orstronger)
586592
{
587593
LOCALLOCKTAG localtag;
588594
LOCALLOCK *locallock;
@@ -598,7 +604,35 @@ LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
598604
(void *) &localtag,
599605
HASH_FIND, NULL);
600606

601-
return (locallock && locallock->nLocks > 0);
607+
if (locallock && locallock->nLocks > 0)
608+
return true;
609+
610+
if (orstronger)
611+
{
612+
LOCKMODE slockmode;
613+
614+
for (slockmode = lockmode + 1;
615+
slockmode <= MaxLockMode;
616+
slockmode++)
617+
{
618+
if (LockHeldByMeExtended(locktag, slockmode, false))
619+
return true;
620+
}
621+
}
622+
623+
return false;
624+
}
625+
626+
bool
627+
LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
628+
{
629+
return LockHeldByMeExtended(locktag, lockmode, false);
630+
}
631+
632+
bool
633+
LockOrStrongerHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
634+
{
635+
return LockHeldByMeExtended(locktag, lockmode, true);
602636
}
603637

604638
#ifdef USE_ASSERT_CHECKING

‎src/include/storage/lmgr.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -47,6 +47,8 @@ extern bool ConditionalLockRelation(Relation relation, LOCKMODE lockmode);
4747
extern void UnlockRelation(Relation relation, LOCKMODE lockmode);
4848
extern bool CheckRelationLockedByMe(Relation relation, LOCKMODE lockmode,
4949
bool orstronger);
50+
extern bool CheckRelationOidLockedByMe(Oid relid, LOCKMODE lockmode,
51+
bool orstronger);
5052
extern bool LockHasWaitersRelation(Relation relation, LOCKMODE lockmode);
5153

5254
extern void LockRelationIdForSession(LockRelId *relid, LOCKMODE lockmode);

‎src/include/storage/lock.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -561,6 +561,7 @@ extern void LockReleaseSession(LOCKMETHODID lockmethodid);
561561
extern void LockReleaseCurrentOwner(LOCALLOCK **locallocks, int nlocks);
562562
extern void LockReassignCurrentOwner(LOCALLOCK **locallocks, int nlocks);
563563
extern bool LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode);
564+
extern bool LockOrStrongerHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode);
564565
#ifdef USE_ASSERT_CHECKING
565566
extern HTAB *GetLockMethodLocalHash(void);
566567
#endif

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