qom: add a generic mechanism to resolve paths

It may be desirable to have custom link<> properties that do more
than just store an object.  Even the addition of a "check"
function is not enough if setting the link has side effects
or if a non-standard reference counting is preferrable.

Avoid the assumption that the opaque field of a link<> is a
LinkProperty struct, by adding a generic "resolve" callback
to ObjectProperty.  This fixes aliases of link properties.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qom/object.h

@@ -303,6 +303,25 @@ typedef void (ObjectPropertyAccessor)(Object *obj,
                                       const char *name,
                                       Error **errp);
 
+/**
+ * ObjectPropertyResolve:
+ * @obj: the object that owns the property
+ * @opaque: the opaque registered with the property
+ * @part: the name of the property
+ *
+ * Resolves the #Object corresponding to property @part.
+ *
+ * The returned object can also be used as a starting point
+ * to resolve a relative path starting with "@part".
+ *
+ * Returns: If @path is the path that led to @obj, the function
+ * returns the #Object corresponding to "@path/@part".
+ * If "@path/@part" is not a valid object path, it returns #NULL.
+ */
+typedef Object *(ObjectPropertyResolve)(Object *obj,
+                                        void *opaque,
+                                        const char *part);
+
 /**
  * ObjectPropertyRelease:
  * @obj: the object that owns the property
@@ -321,6 +340,7 @@ typedef struct ObjectProperty
     gchar *type;
     ObjectPropertyAccessor *get;
     ObjectPropertyAccessor *set;
+    ObjectPropertyResolve *resolve;
     ObjectPropertyRelease *release;
     void *opaque;
 
@@ -787,12 +807,16 @@ void object_unref(Object *obj);
  *   destruction.  This may be NULL.
  * @opaque: an opaque pointer to pass to the callbacks for the property
  * @errp: returns an error if this function fails
+ *
+ * Returns: The #ObjectProperty; this can be used to set the @resolve
+ * callback for child and link properties.
  */
-void object_property_add(Object *obj, const char *name, const char *type,
+ObjectProperty *object_property_add(Object *obj, const char *name,
-                         ObjectPropertyAccessor *get,
+                                    const char *type,
-                         ObjectPropertyAccessor *set,
+                                    ObjectPropertyAccessor *get,
-                         ObjectPropertyRelease *release,
+                                    ObjectPropertyAccessor *set,
-                         void *opaque, Error **errp);
+                                    ObjectPropertyRelease *release,
+                                    void *opaque, Error **errp);
 
 void object_property_del(Object *obj, const char *name, Error **errp);
 

qom/object.c

@@ -356,11 +356,6 @@ static inline bool object_property_is_child(ObjectProperty *prop)
     return strstart(prop->type, "child<", NULL);
 }
 
-static inline bool object_property_is_link(ObjectProperty *prop)
-{
-    return strstart(prop->type, "link<", NULL);
-}
-
 static void object_property_del_all(Object *obj)
 {
     while (!QTAILQ_EMPTY(&obj->properties)) {
@@ -728,11 +723,12 @@ void object_unref(Object *obj)
     }
 }
 
-void object_property_add(Object *obj, const char *name, const char *type,
+ObjectProperty *
-                         ObjectPropertyAccessor *get,
+object_property_add(Object *obj, const char *name, const char *type,
-                         ObjectPropertyAccessor *set,
+                    ObjectPropertyAccessor *get,
-                         ObjectPropertyRelease *release,
+                    ObjectPropertyAccessor *set,
-                         void *opaque, Error **errp)
+                    ObjectPropertyRelease *release,
+                    void *opaque, Error **errp)
 {
     ObjectProperty *prop;
 
@@ -741,7 +737,7 @@ void object_property_add(Object *obj, const char *name, const char *type,
             error_setg(errp, "attempt to add duplicate property '%s'"
                        " to object (type '%s')", name,
                        object_get_typename(obj));
-            return;
+            return NULL;
         }
     }
 
@@ -756,6 +752,7 @@ void object_property_add(Object *obj, const char *name, const char *type,
     prop->opaque = opaque;
 
     QTAILQ_INSERT_TAIL(&obj->properties, prop, node);
+    return prop;
 }
 
 ObjectProperty *object_property_find(Object *obj, const char *name,
@@ -1028,6 +1025,11 @@ static void object_get_child_property(Object *obj, Visitor *v, void *opaque,
     g_free(path);
 }
 
+static Object *object_resolve_child_property(Object *parent, void *opaque, const gchar *part)
+{
+    return opaque;
+}
+
 static void object_finalize_child_property(Object *obj, const char *name,
                                            void *opaque)
 {
@@ -1041,15 +1043,18 @@ void object_property_add_child(Object *obj, const char *name,
 {
     Error *local_err = NULL;
     gchar *type;
+    ObjectProperty *op;
 
     type = g_strdup_printf("child<%s>", object_get_typename(OBJECT(child)));
 
-    object_property_add(obj, name, type, object_get_child_property, NULL,
+    op = object_property_add(obj, name, type, object_get_child_property, NULL,
-                        object_finalize_child_property, child, &local_err);
+                             object_finalize_child_property, child, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         goto out;
     }
+
+    op->resolve = object_resolve_child_property;
     object_ref(child);
     g_assert(child->parent == NULL);
     child->parent = obj;
@@ -1163,6 +1168,13 @@ static void object_set_link_property(Object *obj, Visitor *v, void *opaque,
     }
 }
 
+static Object *object_resolve_link_property(Object *parent, void *opaque, const gchar *part)
+{
+    LinkProperty *lprop = opaque;
+
+    return *lprop->child;
+}
+
 static void object_release_link_property(Object *obj, const char *name,
                                          void *opaque)
 {
@@ -1184,6 +1196,7 @@ void object_property_add_link(Object *obj, const char *name,
     Error *local_err = NULL;
     LinkProperty *prop = g_malloc(sizeof(*prop));
     gchar *full_type;
+    ObjectProperty *op;
 
     prop->child = child;
     prop->check = check;
@@ -1191,17 +1204,21 @@ void object_property_add_link(Object *obj, const char *name,
 
     full_type = g_strdup_printf("link<%s>", type);
 
-    object_property_add(obj, name, full_type,
+    op = object_property_add(obj, name, full_type,
-                        object_get_link_property,
+                             object_get_link_property,
-                        check ? object_set_link_property : NULL,
+                             check ? object_set_link_property : NULL,
-                        object_release_link_property,
+                             object_release_link_property,
-                        prop,
+                             prop,
-                        &local_err);
+                             &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         g_free(prop);
+        goto out;
     }
 
+    op->resolve = object_resolve_link_property;
+
+out:
     g_free(full_type);
 }
 
@@ -1260,11 +1277,8 @@ Object *object_resolve_path_component(Object *parent, const gchar *part)
         return NULL;
     }
 
-    if (object_property_is_link(prop)) {
+    if (prop->resolve) {
-        LinkProperty *lprop = prop->opaque;
+        return prop->resolve(parent, prop->opaque, part);
-        return *lprop->child;
-    } else if (object_property_is_child(prop)) {
-        return prop->opaque;
     } else {
         return NULL;
     }
@@ -1571,6 +1585,14 @@ static void property_set_alias(Object *obj, struct Visitor *v, void *opaque,
     object_property_set(prop->target_obj, v, prop->target_name, errp);
 }
 
+static Object *property_resolve_alias(Object *obj, void *opaque,
+                                      const gchar *part)
+{
+    AliasProperty *prop = opaque;
+
+    return object_resolve_path_component(prop->target_obj, prop->target_name);
+}
+
 static void property_release_alias(Object *obj, const char *name, void *opaque)
 {
     AliasProperty *prop = opaque;
@@ -1583,6 +1605,7 @@ void object_property_add_alias(Object *obj, const char *name,
                                Error **errp)
 {
     AliasProperty *prop;
+    ObjectProperty *op;
     ObjectProperty *target_prop;
 
     target_prop = object_property_find(target_obj, target_name, errp);
@@ -1594,11 +1617,12 @@ void object_property_add_alias(Object *obj, const char *name,
     prop->target_obj = target_obj;
     prop->target_name = target_name;
 
-    object_property_add(obj, name, target_prop->type,
+    op = object_property_add(obj, name, target_prop->type,
-                        property_get_alias,
+                             property_get_alias,
-                        property_set_alias,
+                             property_set_alias,
-                        property_release_alias,
+                             property_release_alias,
-                        prop, errp);
+                             prop, errp);
+    op->resolve = property_resolve_alias;
 }
 
 static void object_instance_init(Object *obj)