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

[Paolo Bonzini]

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 <pbonz...@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;
+    } 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