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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

16.09.2021 13:48, Markus Armbruster wrote:
> 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;
>      }

Looks simpler)

> 
> 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;
>      }
> 

Thanks for working that out, I'll go this way in v2

-- 
Best regards,
Vladimir