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

[Peter Crosthwaite]

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.

Another possibility, is to add a self-reffing link (perhaps lazily)
from the object to itself ("." would be a good name in spirit with
canon paths looking like file paths) and then alias that.

Note that even though patch 3 prepares for this and my suggestion
would obsolete that, I think P3 still stands in it's own right.

Regards,
Peter

> +    } else {
> +        the_target_name = g_strdup(target_name);
> +    }
> +
> +    target_prop = object_property_find(target_obj, the_target_name, errp);
>      if (!target_prop) {
> +        g_free(the_target_name);
>          return;
>      }
>
> @@ -1590,7 +1600,7 @@ void object_property_add_alias(Object *obj, const char 
> *name,
>
>      prop = g_malloc(sizeof(*prop));
>      prop->target_obj = target_obj;
> -    prop->target_name = target_name;
> +    prop->target_name = the_target_name;
>
>      object_property_add_full(obj, name, prop_type,
>                               property_get_alias,
> --
> 1.8.3.1
>
>
>