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

[Markus Armbruster]

Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:

> We'll need this for realizing qdev_find_child() in the next commit.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  softmmu/qdev-monitor.c | 48 +++++++++++++++++++++++++++++-------------
>  1 file changed, 33 insertions(+), 15 deletions(-)
>
> diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
> index 721dec2d82..0117989009 100644
> --- a/softmmu/qdev-monitor.c
> +++ b/softmmu/qdev-monitor.c
> @@ -819,7 +819,12 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, 
> Error **errp)
>      object_unref(OBJECT(dev));
>  }
>  
> -static DeviceState *find_device_state(const char *id, Error **errp)
> +/*
> + * Returns: 1 when found, @dev set
> + *          0 not found, @dev and @errp untouched
> + *         <0 error, or id is ambiguous, @errp set
> + */
> +static int find_device_state(const char *id, DeviceState **dev, Error **errp)
>  {
>      Object *obj;
>  
> @@ -835,17 +840,16 @@ static DeviceState *find_device_state(const char *id, 
> Error **errp)
>      }
>  
>      if (!obj) {
> -        error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
> -                  "Device '%s' not found", id);
> -        return NULL;
> +        return 0;
>      }
>  
>      if (!object_dynamic_cast(obj, TYPE_DEVICE)) {
>          error_setg(errp, "%s is not a hotpluggable device", id);
> -        return NULL;
> +        return -EINVAL;
>      }
>  
> -    return DEVICE(obj);
> +    *dev = DEVICE(obj);
> +    return 1;
>  }
>  
>  void qdev_unplug(DeviceState *dev, Error **errp)
> @@ -894,16 +898,25 @@ void qdev_unplug(DeviceState *dev, Error **errp)
>  
>  void qmp_device_del(const char *id, Error **errp)
>  {
> -    DeviceState *dev = find_device_state(id, errp);
> -    if (dev != NULL) {
> -        if (dev->pending_deleted_event) {
> -            error_setg(errp, "Device %s is already in the "
> -                             "process of unplug", id);
> -            return;
> +    int ret;
> +    DeviceState *dev;
> +
> +    ret = find_device_state(id, &dev, errp);
> +    if (ret <= 0) {
> +        if (ret == 0) {
> +            error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
> +                      "Device '%s' not found", id);
>          }
> +        return;
> +    }
>  
> -        qdev_unplug(dev, errp);
> +    if (dev->pending_deleted_event) {
> +        error_setg(errp, "Device %s is already in the "
> +                         "process of unplug", id);
> +        return;
>      }
> +
> +    qdev_unplug(dev, errp);
>  }
>  
>  void hmp_device_add(Monitor *mon, const QDict *qdict)
> @@ -925,11 +938,16 @@ void hmp_device_del(Monitor *mon, const QDict *qdict)
>  
>  BlockBackend *blk_by_qdev_id(const char *id, Error **errp)
>  {
> +    int ret;
>      DeviceState *dev;
>      BlockBackend *blk;
>  
> -    dev = find_device_state(id, errp);
> -    if (dev == NULL) {
> +    ret = find_device_state(id, &dev, errp);
> +    if (ret <= 0) {
> +        if (ret == 0) {
> +            error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
> +                      "Device '%s' not found", id);
> +        }
>          return NULL;
>      }

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;
    }