Lock before setting relhassubclass on RELKIND_PARTITIONED_INDEX.

Commit 5b562644fec696977df4a82790064e8287927891 added a comment that
SetRelationHasSubclass() callers must hold this lock.  When commit
17f206fbc824d2b4b14480199ca9ff7dea417eda 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

diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index bab02e16b528ff7b48b82cc8a37dbe3a82c14071..e5a6d867597096753093e95c30f87424e2bbbbe0 100644 (file)
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -1034,6 +1034,7 @@ index_create(Relation heapRelation,
    if (OidIsValid(parentIndexRelid))
    {
        StoreSingleInheritance(indexRelationId, parentIndexRelid, 1);
+       LockRelationOid(parentIndexRelid, ShareUpdateExclusiveLock);
        SetRelationHasSubclass(parentIndexRelid, true);
    }
 

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index c6234326206b25e3b62fc1408c8543e968ce701f..53e38f807e69fc2da56dc9f5fe89f90d54bd621d 100644 (file)
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -4233,7 +4233,10 @@ IndexSetParentIndex(Relation partitionIdx, Oid parentOid)
 
    /* set relhassubclass if an index partition has been added to the parent */
    if (OidIsValid(parentOid))
+   {
+       LockRelationOid(parentOid, ShareUpdateExclusiveLock);
        SetRelationHasSubclass(parentOid, true);
+   }
 
    /* set relispartition correctly on the partition */
    update_relispartition(partRelid, OidIsValid(parentOid));

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index a6cb24d17a5db969368bf806a48f9cca25b984f1..b393bd9497f2af1da78c757d960cf7dbd4107674 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -3228,8 +3228,15 @@ findAttrByName(const char *attributeName, List *schema)
  * SetRelationHasSubclass
  *     Set the value of the relation's relhassubclass field in pg_class.
  *
- * NOTE: caller must be holding an appropriate lock on the relation.
- * ShareUpdateExclusiveLock is sufficient.
+ * It's always safe to set this field to true, because all SQL commands are
+ * ready to see true and then find no children.  On the other hand, commands
+ * generally assume zero children if this is false.
+ *
+ * Caller must hold any self-exclusive lock until end of transaction.  If the
+ * new value is false, caller must have acquired that lock before reading the
+ * evidence that justified the false value.  That way, it properly waits if
+ * another backend is simultaneously concluding no need to change the tuple
+ * (new and old values are true).
  *
  * NOTE: an important side-effect of this operation is that an SI invalidation
  * message is sent out to all backends --- including me --- causing plans
@@ -3244,6 +3251,11 @@ SetRelationHasSubclass(Oid relationId, bool relhassubclass)
    HeapTuple   tuple;
    Form_pg_class classtuple;
 
+   Assert(CheckRelationOidLockedByMe(relationId,
+                                     ShareUpdateExclusiveLock, false) ||
+          CheckRelationOidLockedByMe(relationId,
+                                     ShareRowExclusiveLock, true));
+
    /*
     * Fetch a modifiable copy of the tuple, modify it, update pg_class.
     */

diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2acba3934b9599b52b52b968830a1adf6787fb17..47d29a17ab82b907ade0a66da8d27b0ff13d067e 100644 (file)
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -336,32 +336,26 @@ CheckRelationLockedByMe(Relation relation, LOCKMODE lockmode, bool orstronger)
                         relation->rd_lockInfo.lockRelId.dbId,
                         relation->rd_lockInfo.lockRelId.relId);
 
-   if (LockHeldByMe(&tag, lockmode))
-       return true;
+   return (orstronger ?
+           LockOrStrongerHeldByMe(&tag, lockmode) :
+           LockHeldByMe(&tag, lockmode));
+}
 
-   if (orstronger)
-   {
-       LOCKMODE    slockmode;
+/*
+ *     CheckRelationOidLockedByMe
+ *
+ * Like the above, but takes an OID as argument.
+ */
+bool
+CheckRelationOidLockedByMe(Oid relid, LOCKMODE lockmode, bool orstronger)
+{
+   LOCKTAG     tag;
 
-       for (slockmode = lockmode + 1;
-            slockmode <= MaxLockMode;
-            slockmode++)
-       {
-           if (LockHeldByMe(&tag, slockmode))
-           {
-#ifdef NOT_USED
-               /* Sometimes this might be useful for debugging purposes */
-               elog(WARNING, "lock mode %s substituted for %s on relation %s",
-                    GetLockmodeName(tag.locktag_lockmethodid, slockmode),
-                    GetLockmodeName(tag.locktag_lockmethodid, lockmode),
-                    RelationGetRelationName(relation));
-#endif
-               return true;
-           }
-       }
-   }
+   SetLocktagRelationOid(&tag, relid);
 
-   return false;
+   return (orstronger ?
+           LockOrStrongerHeldByMe(&tag, lockmode) :
+           LockHeldByMe(&tag, lockmode));
 }
 
 /*

diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 86fc5576af4e391fb44bc7535946ccb6809d3ffc..692904dbf1b4c074e37631554230796a99518dd5 100644 (file)
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -579,11 +579,17 @@ DoLockModesConflict(LOCKMODE mode1, LOCKMODE mode2)
 }
 
 /*
- * LockHeldByMe -- test whether lock 'locktag' is held with mode 'lockmode'
- *     by the current transaction
+ * LockHeldByMeExtended -- test whether lock 'locktag' is held by the current
+ *     transaction
+ *
+ * Returns true if current transaction holds a lock on 'tag' of mode
+ * 'lockmode'.  If 'orstronger' is true, a stronger lockmode is also OK.
+ * ("Stronger" is defined as "numerically higher", which is a bit
+ * semantically dubious but is OK for the purposes we use this for.)
  */
-bool
-LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
+static bool
+LockHeldByMeExtended(const LOCKTAG *locktag,
+                    LOCKMODE lockmode, bool orstronger)
 {
    LOCALLOCKTAG localtag;
    LOCALLOCK  *locallock;
@@ -599,7 +605,35 @@ LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
                                          (void *) &localtag,
                                          HASH_FIND, NULL);
 
-   return (locallock && locallock->nLocks > 0);
+   if (locallock && locallock->nLocks > 0)
+       return true;
+
+   if (orstronger)
+   {
+       LOCKMODE    slockmode;
+
+       for (slockmode = lockmode + 1;
+            slockmode <= MaxLockMode;
+            slockmode++)
+       {
+           if (LockHeldByMeExtended(locktag, slockmode, false))
+               return true;
+       }
+   }
+
+   return false;
+}
+
+bool
+LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
+{
+   return LockHeldByMeExtended(locktag, lockmode, false);
+}
+
+bool
+LockOrStrongerHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode)
+{
+   return LockHeldByMeExtended(locktag, lockmode, true);
 }
 
 #ifdef USE_ASSERT_CHECKING

diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2c0c1f346fa51b349d6be1558affe7cf491d94bb..31281623bf5a9780f544cb4d9022bfcb18d0d826 100644 (file)
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -48,6 +48,8 @@ extern bool ConditionalLockRelation(Relation relation, LOCKMODE lockmode);
 extern void UnlockRelation(Relation relation, LOCKMODE lockmode);
 extern bool CheckRelationLockedByMe(Relation relation, LOCKMODE lockmode,
                                    bool orstronger);
+extern bool CheckRelationOidLockedByMe(Oid relid, LOCKMODE lockmode,
+                                      bool orstronger);
 extern bool LockHasWaitersRelation(Relation relation, LOCKMODE lockmode);
 
 extern void LockRelationIdForSession(LockRelId *relid, LOCKMODE lockmode);

diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index e4e1495b245e270a7f799532b98075a3f167acf6..e14815b236e58264132481b5c016e56916b9a33c 100644 (file)
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -561,6 +561,7 @@ extern void LockReleaseSession(LOCKMETHODID lockmethodid);
 extern void LockReleaseCurrentOwner(LOCALLOCK **locallocks, int nlocks);
 extern void LockReassignCurrentOwner(LOCALLOCK **locallocks, int nlocks);
 extern bool LockHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode);
+extern bool LockOrStrongerHeldByMe(const LOCKTAG *locktag, LOCKMODE lockmode);
 #ifdef USE_ASSERT_CHECKING
 extern HTAB *GetLockMethodLocalHash(void);
 #endif