Fix assorted syscache lookup sloppiness in partition-related code.

heap_drop_with_catalog and ATExecDetachPartition neglected to check for
SearchSysCache failures, as noted in bugs #14927 and #14928 from Pan Bian.
Such failures are pretty unlikely, since we should already have some sort
of lock on the rel at these points, but it's neither a good idea nor
per project style to omit a check for failure.

Also, StorePartitionKey contained a syscache lookup that it never did
anything with, including never releasing the result.  Presumably the
reason why we don't see refcount-leak complaints is that the lookup
always fails; but in any case it's pretty useless, so remove it.

All of these errors were evidently introduced by the relation
partitioning feature.  Back-patch to v10 where that came in.

Amit Langote and Tom Lane

Discussion: https://postgr.es/m/20171127090105.1463.3962@wrigleys.postgresql.org
Discussion: https://postgr.es/m/20171127091341.1468.72696@wrigleys.postgresql.org

src/backend/catalog/heap.c

@@ -1772,6 +1772,8 @@ heap_drop_with_catalog(Oid relid)
 	 * shared-cache-inval notice that will make them update their index lists.
 	 */
 	tuple = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for relation %u", relid);
 	if (((Form_pg_class) GETSTRUCT(tuple))->relispartition)
 	{
 		parentOid = get_partition_parent(relid);
@@ -3131,9 +3133,6 @@ StorePartitionKey(Relation rel,
 
 	Assert(rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE);
 
-	tuple = SearchSysCache1(PARTRELID,
-							ObjectIdGetDatum(RelationGetRelid(rel)));
-
 	/* Copy the partition attribute numbers, opclass OIDs into arrays */
 	partattrs_vec = buildint2vector(partattrs, partnatts);
 	partopclass_vec = buildoidvector(partopclass, partnatts);

src/backend/commands/tablecmds.c

@@ -14111,6 +14111,9 @@ ATExecDetachPartition(Relation rel, RangeVar *name)
 	classRel = heap_open(RelationRelationId, RowExclusiveLock);
 	tuple = SearchSysCacheCopy1(RELOID,
 								ObjectIdGetDatum(RelationGetRelid(partRel)));
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for relation %u",
+			 RelationGetRelid(partRel));
 	Assert(((Form_pg_class) GETSTRUCT(tuple))->relispartition);
 
 	(void) SysCacheGetAttr(RELOID, tuple, Anum_pg_class_relpartbound,