Add custom object class support

Extension may have some database objects that need to exist/delete
consistently. We might use dependencies to record these relation-ship
and remove them consistently and clearly.

More generally, this patch allows extensions to create their own
auxiliary relations like system catalog in schema `pg_ext_aux`.
The auxiliary relations can't exist alone and have their own
dynamic oclass and callbacks to implement, including do_delete.

src/backend/catalog/dependency.c

@@ -1582,10 +1582,19 @@ doDeletion(const ObjectAddress *object, int flags)
 			elog(ERROR, "global objects cannot be deleted by doDeletion");
 			break;
 
+		default:
+		{
+			struct CustomObjectClass *coc;
+
+			coc = find_custom_object_class_by_classid(object->classId, false);
+			if (coc->do_delete)
+				coc->do_delete(coc, object, flags);
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
 			 */
+			break;
+		}
 	}
 }
 
@@ -2960,6 +2969,15 @@ getObjectClass(const ObjectAddress *object)
 
 		case TaskRelationId:
 			return OCLASS_TASK;
+
+		default:
+		{
+			struct CustomObjectClass *coc;
+			coc = find_custom_object_class_by_classid(object->classId, true);
+			if (coc)
+				return coc->oclass;
+			break;
+		}
 	}
 
 	/* shouldn't get here */
@@ -3097,3 +3115,67 @@ DeleteInitPrivs(const ObjectAddress *object)
 
 	table_close(relation, RowExclusiveLock);
 }
+
+/* Custom object class */
+#define FIRST_COSTOM_OBJECT_CLASS  512
+static List *custom_object_class_list = NIL;
+
+/*
+ * don't support unregister object class in a session.
+ * Return: the oclass allocated by the kernel
+ */
+int
+register_custom_object_class(struct CustomObjectClass *coc)
+{
+	MemoryContext saved_ctx;
+	/* check class_id and required callbacks */
+	Assert(OidIsValid(coc->class_id));
+	Assert(coc->do_delete);
+
+	if (find_custom_object_class_by_classid(coc->class_id, true))
+		elog(ERROR, "class_id %u exists already", coc->class_id);
+
+	coc->oclass = FIRST_COSTOM_OBJECT_CLASS + list_length(custom_object_class_list);
+	saved_ctx = MemoryContextSwitchTo(TopMemoryContext);
+	custom_object_class_list = lappend(custom_object_class_list, coc);
+	MemoryContextSwitchTo(saved_ctx);
+	return coc->oclass;
+}
+
+struct CustomObjectClass *
+find_custom_object_class(int oclass)
+{
+	int index = oclass - FIRST_COSTOM_OBJECT_CLASS;
+
+	if (oclass < FIRST_COSTOM_OBJECT_CLASS)
+		elog(ERROR, "unexpected oclass, found builtin one %u", oclass);
+	if (index >= list_length(custom_object_class_list))
+		elog(ERROR, "unexpected oclass beyond custom object class %u", oclass);
+
+	return (struct CustomObjectClass *) list_nth(custom_object_class_list, index);
+}
+
+struct CustomObjectClass *
+find_custom_object_class_by_classid(Oid class_id, bool missing_ok)
+{
+	ListCell *lc;
+
+#ifdef USE_ASSERT_CHECKING
+	int n = lengthof(object_classes);
+	Assert(n < FIRST_COSTOM_OBJECT_CLASS);
+
+	for (int i = 0; i < n; i++) {
+		if (object_classes[i] == class_id)
+			elog(ERROR, "find builtin object class by class id %u", class_id);
+	}
+#endif
+
+	foreach(lc, custom_object_class_list) {
+		struct CustomObjectClass *coc = (struct CustomObjectClass *)lfirst(lc);
+		if (coc->class_id == class_id)
+		  return coc;
+	}
+	if (!missing_ok)
+		ereport(ERROR, (errmsg("found no custom object class for class_id=%u", class_id)));
+	return NULL;
+}

src/backend/catalog/objectaddress.c

@@ -4079,6 +4079,15 @@ getObjectDescription(const ObjectAddress *object, bool missing_ok)
 					appendStringInfo(&buffer, _("history password for role %s"), username);
 				break;
 			}
+
+		default:
+			{
+				struct CustomObjectClass *coc;
+
+				coc = find_custom_object_class_by_classid(object->classId, missing_ok);
+				if (coc && coc->object_desc)
+					coc->object_desc(coc, object, missing_ok, &buffer);
+			}
 	}
 
 	/* an empty buffer is equivalent to no object found */
@@ -4650,10 +4659,19 @@ getObjectTypeDescription(const ObjectAddress *object, bool missing_ok)
 			appendStringInfoString(&buffer, "password_history");
 			break;
 
+		default:
+		{
+			struct CustomObjectClass *coc;
+
+			coc = find_custom_object_class_by_classid(object->classId, false);
+			if (coc->object_type_desc)
+				coc->object_type_desc(coc, object, missing_ok, &buffer);
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
 			 */
+			break;
+		}
 	}
 
 	/* the result can never be empty */
@@ -5988,10 +6006,19 @@ getObjectIdentityParts(const ObjectAddress *object,
 				break;
 			}
 
+		default:
+		{
+			struct CustomObjectClass *coc;
+
+			coc = find_custom_object_class_by_classid(object->classId, false);
+			if (coc->object_identity_parts)
+				coc->object_identity_parts(coc, object, objname, objargs, missing_ok, &buffer);
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
 			 */
+			break;
+		}
 	}
 
 	if (!missing_ok)

src/backend/commands/alter.c

@@ -736,10 +736,19 @@ AlterObjectNamespace_oid(Oid classId, Oid objid, Oid nspOid,
 			/* ignore object types that don't have schema-qualified names */
 			break;
 
+		default:
+		{
+			struct CustomObjectClass *coc;
+
+			coc = find_custom_object_class_by_classid(classId, false);
+			if (coc->alter_namespace)
+				coc->alter_namespace(coc, &dep, nspOid, objsMoved);
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
 			 */
+			break;
+		}
 	}
 
 	return oldNspOid;

src/backend/commands/event_trigger.c

@@ -1069,10 +1069,20 @@ EventTriggerSupportsObjectClass(ObjectClass objclass)
 		case OCLASS_TASK:
 			return false;
 
+		default:
+		{
+			struct CustomObjectClass *coc;
+
+			coc = find_custom_object_class(objclass);
+			Assert(coc);
+			if (coc->support_event_trigger)
+				return coc->support_event_trigger(coc);
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
 			 */
+			break;
+		}
 	}
 
 	return true;

src/backend/commands/tablecmds.c

@@ -14028,10 +14028,18 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 					 getObjectDescription(&foundObject, false));
 				break;
 
+			default:
+			{
+				struct CustomObjectClass *coc;
+				coc = find_custom_object_class_by_classid(foundObject.classId, false);
+				if (coc->alter_column_type)
+					coc->alter_column_type(coc);
 				/*
 				 * There's intentionally no default: case here; we want the
 				 * compiler to warn if a new OCLASS hasn't been handled above.
 				 */
+				break;
+			}
 		}
 	}
 

src/include/catalog/dependency.h

@@ -290,4 +290,39 @@ extern void recordProfileDependency(Oid roleId, Oid profileId);
 
 extern void changeProfileDependency(Oid roleId, Oid profileId);
 
+/* Custom object class */
+struct StringInfoData;
+struct CustomObjectClass {
+	Oid class_id;
+	int oclass;
+
+	void (*do_delete)(struct CustomObjectClass *self,
+					  const ObjectAddress *object, int flags);
+	void (*object_desc)(struct CustomObjectClass *self,
+						const ObjectAddress *object,
+						bool missing_ok,
+						struct StringInfoData *buffer);
+	void (*object_type_desc)(struct CustomObjectClass *self,
+							 const ObjectAddress *object,
+							 bool missing_ok,
+							 struct StringInfoData *buffer);
+	void (*object_identity_parts)(struct CustomObjectClass *self,
+								  const ObjectAddress *object,
+								  List **objname,
+								  List **objargs,
+								  bool missing_ok,
+								  struct StringInfoData *buffer);
+
+	Oid (*alter_namespace)(struct CustomObjectClass *self,
+						   const ObjectAddress *object,
+						   Oid nspOid,
+						   ObjectAddresses *objsMoved);
+	bool (*support_event_trigger)(struct CustomObjectClass *self);
+	// FIXME: unclear which arguments should pass
+	void (*alter_column_type)(struct CustomObjectClass *self);
+
+};
+extern int register_custom_object_class(struct CustomObjectClass *coc);
+extern struct CustomObjectClass *find_custom_object_class(int oclass);
+extern struct CustomObjectClass *find_custom_object_class_by_classid(Oid class_id, bool missing_ok);
 #endif							/* DEPENDENCY_H */