Fix dependency handling of partitions and inheritance for ON COMMIT

This commit fixes a set of issues with ON COMMIT actions when used on
partitioned tables and tables with inheritance children:
- Applying ON COMMIT DROP on a partitioned table with partitions or on a
table with inheritance children caused a failure at commit time, with
complains about the children being already dropped as all relations are
dropped one at the same time.
- Applying ON COMMIT DELETE on a partition relying on a partitioned
table which uses ON COMMIT DROP would cause the partition truncation to
fail as the parent is removed first.

The solution to the first problem is to handle the removal of all the
dependencies in one go instead of dropping relations one-by-one, based
on a suggestion from Álvaro Herrera.  So instead all the relation OIDs
to remove are gathered and then processed in one round of multiple
deletions.

The solution to the second problem is to reorder the actions, with
truncation happening first and relation drop done after.  Even if it
means that a partition could be first truncated, then immediately
dropped if its partitioned table is dropped, this has the merit to keep
the code simple as there is no need to do existence checks on the
relations to drop.

Contrary to a manual TRUNCATE on a partitioned table, ON COMMIT DELETE
does not cascade to its partitions.  The ON COMMIT action defined on
each partition gets the priority.

Author: Michael Paquier
Reviewed-by: Amit Langote, Álvaro Herrera, Robert Haas
Discussion: https://postgr.es/m/68f17907-ec98-1192-f99f-8011400517f5@lab.ntt.co.jp
Backpatch-through: 10

src/backend/commands/tablecmds.c

@@ -13293,6 +13293,7 @@ PreCommit_on_commit_actions(void)
 {
 	ListCell   *l;
 	List	   *oids_to_truncate = NIL;
+	List	   *oids_to_drop = NIL;
 
 	foreach(l, on_commits)
 	{
@@ -13319,36 +13320,66 @@ PreCommit_on_commit_actions(void)
 					oids_to_truncate = lappend_oid(oids_to_truncate, oc->relid);
 				break;
 			case ONCOMMIT_DROP:
-				{
-					ObjectAddress object;
-
-					object.classId = RelationRelationId;
-					object.objectId = oc->relid;
-					object.objectSubId = 0;
-
-					/*
-					 * Since this is an automatic drop, rather than one
-					 * directly initiated by the user, we pass the
-					 * PERFORM_DELETION_INTERNAL flag.
-					 */
-					performDeletion(&object,
-									DROP_CASCADE, PERFORM_DELETION_INTERNAL);
-
-					/*
-					 * Note that table deletion will call
-					 * remove_on_commit_action, so the entry should get marked
-					 * as deleted.
-					 */
-					Assert(oc->deleting_subid != InvalidSubTransactionId);
-					break;
-				}
+				oids_to_drop = lappend_oid(oids_to_drop, oc->relid);
+				break;
 		}
 	}
+
+	/*
+	 * Truncate relations before dropping so that all dependencies between
+	 * relations are removed after they are worked on.  Doing it like this
+	 * might be a waste as it is possible that a relation being truncated will
+	 * be dropped anyway due to its parent being dropped, but this makes the
+	 * code more robust because of not having to re-check that the relation
+	 * exists at truncation time.
+	 */
 	if (oids_to_truncate != NIL)
 	{
 		heap_truncate(oids_to_truncate);
 		CommandCounterIncrement();	/* XXX needed? */
 	}
+	if (oids_to_drop != NIL)
+	{
+		ObjectAddresses *targetObjects = new_object_addresses();
+		ListCell   *l;
+
+		foreach(l, oids_to_drop)
+		{
+			ObjectAddress object;
+
+			object.classId = RelationRelationId;
+			object.objectId = lfirst_oid(l);
+			object.objectSubId = 0;
+
+			Assert(!object_address_present(&object, targetObjects));
+
+			add_exact_object_address(&object, targetObjects);
+		}
+
+		/*
+		 * Since this is an automatic drop, rather than one directly initiated
+		 * by the user, we pass the PERFORM_DELETION_INTERNAL flag.
+		 */
+		performMultipleDeletions(targetObjects, DROP_CASCADE,
+								 PERFORM_DELETION_INTERNAL | PERFORM_DELETION_QUIETLY);
+
+#ifdef USE_ASSERT_CHECKING
+
+		/*
+		 * Note that table deletion will call remove_on_commit_action, so the
+		 * entry should get marked as deleted.
+		 */
+		foreach(l, on_commits)
+		{
+			OnCommitItem *oc = (OnCommitItem *) lfirst(l);
+
+			if (oc->oncommit != ONCOMMIT_DROP)
+				continue;
+
+			Assert(oc->deleting_subid != InvalidSubTransactionId);
+		}
+#endif
+	}
 }
 
 /*