Re: [PATCH 5/8] qdev: improve find_device_state() to distinguish simple not found case

Vladimir Sementsov-Ogievskiy <vsementsov(a)virtuozzo.com&gt; writes: Awkward. Before, find_device_state() either finds something (and returns it) or doesn't (and sets @errp). Afterward, it can fail to find in two ways, and only one of it sets @errp. The existing callers laboriously fuse the two back together. The next commit adds a caller that doesn't. Failure modes that need to be handled differently are often the result of a function doing too much. Let's have a closer look at this one before the patch: static DeviceState *find_device_state(const char *id, Error **errp) { Object *obj; if (id[0] == '/') { obj = object_resolve_path(id, NULL); This interprets @id as a QOM path, and tries to resolve it. On failure, @obj becomes NULL. On success, it points to an object of arbitrary type. } else { char *root_path = object_get_canonical_path(qdev_get_peripheral()); char *path = g_strdup_printf("%s/%s", root_path, id); g_free(root_path); obj = object_resolve_path_type(path, TYPE_DEVICE, NULL); g_free(path); This interprets @id as qdev ID, maps it to a QOM path, and tries to resolve it to a TYPE_DEVICE. Fails when the path doesn't resolve, and when it resolves to something that isn't a TYPE_DEVICE. The latter can't happen as long as we put only devices under /machine/peripheral/. On failure, @obj becomes NULL. On success, it points to a TYPE_DEVICE object. } if (!obj) { error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND, "Device '%s' not found", id); return NULL; } if (!object_dynamic_cast(obj, TYPE_DEVICE)) { error_setg(errp, "%s is not a hotpluggable device", id); return NULL; } Unclean. If we somehow ended up with a non-device /machine/peripheral/foo, then find_device_state("foo", errp) would fail the first way, but find_device_state("/machine/peripheral/foo", errp) would fail the second way. They should fail the exact same way instead. return DEVICE(obj); } Better: static DeviceState *find_device_state(const char *id, Error **errp) { Object *obj; DeviceState *dev; if (id[0] == '/') { obj = object_resolve_path(id, NULL); } else { obj = object_resolve_path_component(qdev_get_peripheral(), id); } if (!obj) { error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND, "Device '%s' not found", id); return NULL; } dev = (DeviceState *)object_dynamic_cast(obj, TYPE_DEVICE); if (!dev) { error_setg(errp, "%s is not a hotpluggable device", id); return NULL; } return dev; } I'll post this as a cleanup patch. Note that this function does two things, one after the other, namely 1. resolve a "qdev ID or qom path" string, and 2. convert to TYPE_DEVICE, with error checking. Factor out the core of 1. into its own helper resolve_id_or_qom_path(), and the next commit can do something like obj = resolve_id_or_qom_path(parent_id); if (!obj) { return 0; } dev = object_dynamic_cast(obj, TYPE_DEVICE); if (!dev) { error_setg(errp, ...); return -EINVAL; }