Re: [Qemu-devel] [PATCH 4/5] qom: allow creating an alias of an object

[Peter Crosthwaite]

On Wed, Jun 18, 2014 at 12:06 AM, Paolo Bonzini <address@hidden> wrote:
> Il 17/06/2014 15:55, Peter Crosthwaite ha scritto:
>
>> On Thu, Jun 12, 2014 at 2:49 AM, Paolo Bonzini <address@hidden>
>> wrote:
>>>
>>> Add a shorthand for creating an alias of a child<> property.  If you
>>> pass a NULL target_name to object_property_add_alias, the function
>>> will look up the child property that leads to target_obj, and create
>>> an alias for that property.
>>>
>>> This can be useful when an object wants to add a link to itself
>>> at a well-known location.  For example, a real-time clock device might
>>> add a link to itself at "/machine/rtc". Such well-known locations can
>>> then expose a standard set of properties that can be accessed via the
>>> "qom-get" and "qom-set" commands.
>>>
>>> Signed-off-by: Paolo Bonzini <address@hidden>
>>> ---
>>>  include/qom/object.h | 10 +++++++---
>>>  qom/object.c         | 16 +++++++++++++---
>>>  2 files changed, 20 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/qom/object.h b/include/qom/object.h
>>> index 9c4a5a4..9cd0ffa 100644
>>> --- a/include/qom/object.h
>>> +++ b/include/qom/object.h
>>> @@ -1256,11 +1256,15 @@ void object_property_add_uint64_ptr(Object *obj,
>>> const char *name,
>>>   * @obj: the object to add a property to
>>>   * @name: the name of the property
>>>   * @target_obj: the object to forward property access to
>>> - * @target_name: the name of the property on the forwarded object
>>> + * @target_name: the name of the property on the forwarded object, or
>>> + * #NULL to make an object alias.
>>>   * @errp: if an error occurs, a pointer to an area to store the error
>>>   *
>>> - * Add an alias for a property on an object.  This function will add a
>>> property
>>> - * of the same type as the forwarded property.
>>> + * Add an alias property on an object.  This function will add a
>>> property
>>> + * of the same type as the forwarded property or, if @target_name is
>>> #NULL,
>>> + * a link property that always resolves to @target_obj.  In fact, the
>>> case
>>> + * of a #NULL @target_obj actually creates an alias property that
>>> targets
>>> + * @target_obj's own child property.
>>>   *
>>>   * The caller must ensure that <code>@target_obj</code> stays alive as
>>> long as
>>>   * this property exists.  In the case of a child object or an alias on
>>> the same
>>> diff --git a/qom/object.c b/qom/object.c
>>> index ddf781e..1e8e6af 100644
>>> --- a/qom/object.c
>>> +++ b/qom/object.c
>>> @@ -1535,7 +1535,7 @@ void object_property_add_uint64_ptr(Object *obj,
>>> const char *name,
>>>  typedef struct
>>>  {
>>>      Object *target_obj;
>>> -    const char *target_name;
>>> +    char *target_name;
>>>  } AliasProperty;
>>>
>>>  static void property_get_alias(Object *obj, struct Visitor *v, void
>>> *opaque,
>>> @@ -1566,6 +1566,7 @@ static void property_release_alias(Object *obj,
>>> const char *name, void *opaque)
>>>  {
>>>      AliasProperty *prop = opaque;
>>>
>>> +    g_free(prop->target_name);
>>>      g_free(prop);
>>>  }
>>>
>>> @@ -1576,9 +1577,18 @@ void object_property_add_alias(Object *obj, const
>>> char *name,
>>>      AliasProperty *prop;
>>>      ObjectProperty *target_prop;
>>>      gchar *prop_type;
>>> +    gchar *the_target_name;
>>>
>>> -    target_prop = object_property_find(target_obj, target_name, errp);
>>> +    if (!target_name) {
>>> +        the_target_name =
>>> object_get_canonical_path_component(target_obj);
>>> +        target_obj = target_obj->parent;
>>
>>
>> This semantic seems a little tricky. It also get the target's
>> canon-parent entangled in the process whereas your original
>> object_property_add_alias is more self contained:
>>
>> http://lists.gnu.org/archive/html/qemu-devel/2014-06/msg01203.html
>>
>> For instance - could you unparent then reparent an object and have
>> it's aliases survive? I think the best implementation is to simply
>> place your original alias logic here under if (!target_name) and
>> return.
>
>
> Ok, so that would mean special casing target_prop == NULL.
>

target_name == NULL is the special case I think? It is already
something of a special case in this patch with a child to parent
traversal for who is getting a property aliased.

Regards,
Peter

> Paolo
>