Re: BUG #18559: Crash after detaching a partition concurrently from another session

On Tue, Jul 30, 2024 at 7:18 PM PG Bug reporting form
<noreply(at)postgresql(dot)org> wrote:
Adding Alvaro.
>
> The following code assumes that an pg_class entry for the detached partition
> will always be available which is wrong.
>
Referring to the following code.
/*
* Two problems are possible here. First, a concurrent ATTACH
* PARTITION might be in the process of adding a new partition, but
* the syscache doesn't have it, or its copy of it does not yet have
* its relpartbound set. We cannot just AcceptInvalidationMessages(),
* because the other process might have already removed itself from
* the ProcArray but not yet added its invalidation messages to the
* shared queue. We solve this problem by reading pg_class directly
* for the desired tuple.
*
* The other problem is that DETACH CONCURRENTLY is in the process of
* removing a partition, which happens in two steps: first it marks it
* as "detach pending", commits, then unsets relpartbound. If
* find_inheritance_children_extended included that partition but we
* below we see that DETACH CONCURRENTLY has reset relpartbound for
* it, we'd see an inconsistent view. (The inconsistency is seen
* because table_open below reads invalidation messages.) We protect
* against this by retrying find_inheritance_children_extended().
*/
if (boundspec == NULL)
{
Relation pg_class;
SysScanDesc scan;
ScanKeyData key[1];
Datum datum;
bool isnull;

pg_class = table_open(RelationRelationId, AccessShareLock);
ScanKeyInit(&key[0],
Anum_pg_class_oid,
BTEqualStrategyNumber, F_OIDEQ,
ObjectIdGetDatum(inhrelid));
scan = systable_beginscan(pg_class, ClassOidIndexId, true,
NULL, 1, key);
tuple = systable_getnext(scan);
datum = heap_getattr(tuple, Anum_pg_class_relpartbound,
RelationGetDescr(pg_class), &isnull);
if (!isnull)
boundspec = stringToNode(TextDatumGetCString(datum));
systable_endscan(scan);
table_close(pg_class, AccessShareLock);

IIUC, a simple fix would be to retry if an entry is not found. Attached a patch.

--
Thanks & Regards,
Kuntal Ghosh