Fix creation of partition descriptor during concurrent detach+drop

If a partition undergoes DETACH CONCURRENTLY immediately followed by
DROP, this could cause a problem for a concurrent transaction
recomputing the partition descriptor when running a prepared statement,
because it tries to dereference a pointer to a tuple that's not found in
a catalog scan.

The existing retry logic added in commit dbca3469ebf8 is sufficient to
cope with the overall problem, provided we don't try to dereference a
non-existant heap tuple.

Arguably, the code in RelationBuildPartitionDesc() has been wrong all
along, since no check was added in commit 898e5e3290a7 against receiving
a NULL tuple from the catalog scan; that bug has only become
user-visible with DETACH CONCURRENTLY which was added in branch 14.
Therefore, even though there's no known mechanism to cause a crash
because of this, backpatch the addition of such a check to all supported
branches.  In branches prior to 14, this would cause the code to fail
with a "missing relpartbound for relation XYZ" error instead of
crashing; that's okay, because there are no reports of such behavior
anyway.

Author: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/18559-b48286d2eacd9a4e@postgresql.org

diff --git a/src/backend/partitioning/partdesc.c b/src/backend/partitioning/partdesc.c
index 5b1353cfc6c64ff1ed72ac331fca6678f7da2541..e2f47b0cd64496b1bfa5643304a2ead375145081 100644 (file)
--- a/src/backend/partitioning/partdesc.c
+++ b/src/backend/partitioning/partdesc.c
@@ -210,6 +210,10 @@ retry:
         * shared queue.  We solve this problem by reading pg_class directly
         * for the desired tuple.
         *
+        * If the partition recently detached is also dropped, we get no tuple
+        * from the scan.  In that case, we also retry, and next time through
+        * here, we don't see that partition anymore.
+        *
         * 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
@@ -224,8 +228,6 @@ retry:
            Relation    pg_class;
            SysScanDesc scan;
            ScanKeyData key[1];
-           Datum       datum;
-           bool        isnull;
 
            pg_class = table_open(RelationRelationId, AccessShareLock);
            ScanKeyInit(&key[0],
@@ -234,17 +236,29 @@ retry:
                        ObjectIdGetDatum(inhrelid));
            scan = systable_beginscan(pg_class, ClassOidIndexId, true,
                                      NULL, 1, key);
+
+           /*
+            * We could get one tuple from the scan (the normal case), or zero
+            * tuples if the table has been dropped meanwhile.
+            */
            tuple = systable_getnext(scan);
-           datum = heap_getattr(tuple, Anum_pg_class_relpartbound,
-                                RelationGetDescr(pg_class), &isnull);
-           if (!isnull)
-               boundspec = stringToNode(TextDatumGetCString(datum));
+           if (HeapTupleIsValid(tuple))
+           {
+               Datum       datum;
+               bool        isnull;
+
+               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);
 
            /*
-            * If we still don't get a relpartbound value, then it must be
-            * because of DETACH CONCURRENTLY.  Restart from the top, as
+            * If we still don't get a relpartbound value (either because
+            * boundspec is null or because there was no tuple), then it must
+            * be because of DETACH CONCURRENTLY.  Restart from the top, as
             * explained above.  We only do this once, for two reasons: first,
             * only one DETACH CONCURRENTLY session could affect us at a time,
             * since each of them would have to wait for the snapshot under