On 2015/12/03 15:30, Amit Langote wrote:
> On 2015/12/03 13:09, Tom Lane wrote:
>> Amit Langote <[email protected]> writes:
>>> Currently find_inheritance_children() is smart enough to skip a child
>>> table that it finds has been dropped concurrently after it gets a lock on
>>> the same. It does so by looking up the child relid in syscache. It seems
>>> it should also check if the table is still in the list of children of the
>>> parent.
>
>> I wonder whether we could improve matters by rechecking validity of the
>> pg_inherits tuple (which we saw already and could presumably retain the
>> TID of). There is at least one place where we do something like that now,
>> IIRC.
>
> Not sure whether sane but how about performing ordered scan on pg_inherits
> (systable_getnext_ordered())and using systable_recheck_tuple() in step
> with it? Does using ordered catalog scan ensure safety against deadlocks
> that the existing approach of ordered locking of child tables does?
> Perhaps I'm missing something.
Just leaving here a patch that does this. It still returns the list in
order determined by qsort(), IOW, not in the pg_inherits_parent_index
order to avoid broken tests. I could not figure how to do it the way you
suggested.
Thanks,
Amit
diff --git a/src/backend/catalog/pg_inherits.c b/src/backend/catalog/pg_inherits.c
index 04687c1..bfef40d 100644
--- a/src/backend/catalog/pg_inherits.c
+++ b/src/backend/catalog/pg_inherits.c
@@ -50,9 +50,10 @@ find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
{
List *list = NIL;
Relation relation;
+ Relation index;
SysScanDesc scan;
ScanKeyData key[1];
- HeapTuple inheritsTuple;
+ HeapTuple tup;
Oid inhrelid;
Oid *oidarr;
int maxoids,
@@ -74,46 +75,30 @@ find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
numoids = 0;
relation = heap_open(InheritsRelationId, AccessShareLock);
+ index = index_open(InheritsParentIndexId, AccessShareLock);
ScanKeyInit(&key[0],
Anum_pg_inherits_inhparent,
BTEqualStrategyNumber, F_OIDEQ,
ObjectIdGetDatum(parentrelId));
- scan = systable_beginscan(relation, InheritsParentIndexId, true,
- NULL, 1, key);
+ scan = systable_beginscan_ordered(relation, index, NULL, 1, key);
- while ((inheritsTuple = systable_getnext(scan)) != NULL)
+ while ((tup = systable_getnext_ordered(scan, ForwardScanDirection)) != NULL)
{
- inhrelid = ((Form_pg_inherits) GETSTRUCT(inheritsTuple))->inhrelid;
+ inhrelid = ((Form_pg_inherits) GETSTRUCT(tup))->inhrelid;
if (numoids >= maxoids)
{
maxoids *= 2;
oidarr = (Oid *) repalloc(oidarr, maxoids * sizeof(Oid));
}
- oidarr[numoids++] = inhrelid;
- }
-
- systable_endscan(scan);
-
- heap_close(relation, AccessShareLock);
-
- /*
- * If we found more than one child, sort them by OID. This ensures
- * reasonably consistent behavior regardless of the vagaries of an
- * indexscan. This is important since we need to be sure all backends
- * lock children in the same order to avoid needless deadlocks.
- */
- if (numoids > 1)
- qsort(oidarr, numoids, sizeof(Oid), oid_cmp);
-
- /*
- * Acquire locks and build the result list.
- */
- for (i = 0; i < numoids; i++)
- {
- inhrelid = oidarr[i];
+ /*
+ * Following code assumes that inhrelids are returned in the same
+ * order by systable_getnext_ordered() in all backends. This is
+ * important since we need to be sure all backends lock children
+ * in the same order to avoid needless deadlocks.
+ */
if (lockmode != NoLock)
{
/* Get the lock to synchronize against concurrent drop */
@@ -124,7 +109,8 @@ find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
* really exists or not. If not, assume it was dropped while we
* waited to acquire lock, and ignore it.
*/
- if (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(inhrelid)))
+ if (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(inhrelid)) ||
+ !systable_recheck_tuple(scan, tup))
{
/* Release useless lock */
UnlockRelationOid(inhrelid, lockmode);
@@ -133,10 +119,22 @@ find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
}
}
- list = lappend_oid(list, inhrelid);
+ oidarr[numoids++] = inhrelid;
}
- pfree(oidarr);
+ /*
+ * Do not break regression tests - return the oids in the same order as
+ * would have been previously.
+ */
+ if (numoids > 1)
+ qsort(oidarr, numoids, sizeof(Oid), oid_cmp);
+
+ for (i = 0; i < numoids; i++)
+ list = lappend_oid(list, oidarr[i]);
+
+ systable_endscan_ordered(scan);
+ index_close(index, AccessShareLock);
+ heap_close(relation, AccessShareLock);
return list;
}
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