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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/06/2014 15:55, Peter Crosthwaite ha scritto:
> On Thu, Jun 12, 2014 at 2:49 AM, Paolo Bonzini <pbonzini@redhat.com> 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 <pbonzini@redhat.com>
>> ---
>>  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.

Paolo