qom: add object_property_add_alias()

Sometimes an object needs to present a property which is actually on
another object, or it needs to provide an alias name for an existing
property.

Examples:
  a.foo -> b.foo
  a.old_name -> a.new_name

The new object_property_add_alias() API allows objects to alias a
property on the same object or another object.  The source and target
names can be different.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>

include/qom/object.h

@@ -1230,6 +1230,26 @@ void object_property_add_uint32_ptr(Object *obj, const char *name,
 void object_property_add_uint64_ptr(Object *obj, const char *name,
                                     const uint64_t *v, Error **Errp);
 
+/**
+ * object_property_add_alias:
+ * @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
+ * @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.
+ *
+ * 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
+ * object this will be the case.  For aliases to other objects the caller is
+ * responsible for taking a reference.
+ */
+void object_property_add_alias(Object *obj, const char *name,
+                               Object *target_obj, const char *target_name,
+                               Error **errp);
+
 /**
  * object_child_foreach:
  * @obj: the object whose children will be navigated

qom/object.c

@@ -1550,6 +1550,57 @@ void object_property_add_uint64_ptr(Object *obj, const char *name,
                         NULL, NULL, (void *)v, errp);
 }
 
+typedef struct {
+    Object *target_obj;
+    const char *target_name;
+} AliasProperty;
+
+static void property_get_alias(Object *obj, struct Visitor *v, void *opaque,
+                               const char *name, Error **errp)
+{
+    AliasProperty *prop = opaque;
+
+    object_property_get(prop->target_obj, v, prop->target_name, errp);
+}
+
+static void property_set_alias(Object *obj, struct Visitor *v, void *opaque,
+                               const char *name, Error **errp)
+{
+    AliasProperty *prop = opaque;
+
+    object_property_set(prop->target_obj, v, prop->target_name, errp);
+}
+
+static void property_release_alias(Object *obj, const char *name, void *opaque)
+{
+    AliasProperty *prop = opaque;
+
+    g_free(prop);
+}
+
+void object_property_add_alias(Object *obj, const char *name,
+                               Object *target_obj, const char *target_name,
+                               Error **errp)
+{
+    AliasProperty *prop;
+    ObjectProperty *target_prop;
+
+    target_prop = object_property_find(target_obj, target_name, errp);
+    if (!target_prop) {
+        return;
+    }
+
+    prop = g_malloc(sizeof(*prop));
+    prop->target_obj = target_obj;
+    prop->target_name = target_name;
+
+    object_property_add(obj, name, target_prop->type,
+                        property_get_alias,
+                        property_set_alias,
+                        property_release_alias,
+                        prop, errp);
+}
+
 static void object_instance_init(Object *obj)
 {
     object_property_add_str(obj, "type", qdev_get_type, NULL, NULL);