qom: Introduce object_property_try_add_child()

object_property_add() does not allow object_property_try_add()
to gracefully fail as &error_abort is passed as an error handle.

However such failure can easily be triggered from the QMP shell when,
for instance, one attempts to create an object with an id that already
exists. This is achieved from the following call path:

qmp_object_add -> user_creatable_add_dict -> user_creatable_add_type ->
object_property_add_child -> object_property_add

For instance, from the qmp-shell, call twice:
object-add qom-type=memory-backend-ram id=mem1 props.size=1073741824
and QEMU aborts.

This behavior is undesired as a user/management application mistake
in reusing a property ID shouldn't result in loss of the VM and live
data within.

This patch introduces a new function, object_property_try_add_child()
which takes an error handle and turn object_property_try_add() into
a non-static one.

Now the call path becomes:

user_creatable_add_type -> object_property_try_add_child ->
object_property_try_add

and the error is returned gracefully to the QMP client.

(QEMU) object-add qom-type=memory-backend-ram id=mem2  props.size=4294967296
{"return": {}}
(QEMU) object-add qom-type=memory-backend-ram id=mem2  props.size=4294967296
{"error": {"class": "GenericError", "desc": "attempt to add duplicate property
'mem2' to object (type 'container')"}}

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Fixes: d2623129a7 ("qom: Drop parameter @errp of object_property_add() & friends")
Reviewed-by: Markus Armbruster <armbru@redhat.com>

Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Message-Id: <20200629193424.30280-2-eric.auger@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qom/object.h

@@ -1047,7 +1047,7 @@ Object *object_ref(Object *obj);
 void object_unref(Object *obj);
 
 /**
- * object_property_add:
+ * object_property_try_add:
  * @obj: the object to add a property to
  * @name: the name of the property.  This can contain any character except for
  *  a forward slash.  In general, you should use hyphens '-' instead of
@@ -1064,10 +1064,23 @@ void object_unref(Object *obj);
  *   meant to allow a property to free its opaque upon object
  *   destruction.  This may be NULL.
  * @opaque: an opaque pointer to pass to the callbacks for the property
+ * @errp: pointer to error object
  *
  * Returns: The #ObjectProperty; this can be used to set the @resolve
  * callback for child and link properties.
  */
+ObjectProperty *object_property_try_add(Object *obj, const char *name,
+                                        const char *type,
+                                        ObjectPropertyAccessor *get,
+                                        ObjectPropertyAccessor *set,
+                                        ObjectPropertyRelease *release,
+                                        void *opaque, Error **errp);
+
+/**
+ * object_property_add:
+ * Same as object_property_try_add() with @errp hardcoded to
+ * &error_abort.
+ */
 ObjectProperty *object_property_add(Object *obj, const char *name,
                                     const char *type,
                                     ObjectPropertyAccessor *get,
@@ -1518,10 +1531,11 @@ Object *object_resolve_path_type(const char *path, const char *typename,
 Object *object_resolve_path_component(Object *parent, const char *part);
 
 /**
- * object_property_add_child:
+ * object_property_try_add_child:
  * @obj: the object to add a property to
  * @name: the name of the property
  * @child: the child object
+ * @errp: pointer to error object
  *
  * Child properties form the composition tree.  All objects need to be a child
  * of another object.  Objects can only be a child of one object.
@@ -1535,6 +1549,14 @@ Object *object_resolve_path_component(Object *parent, const char *part);
  *
  * Returns: The newly added property on success, or %NULL on failure.
  */
+ObjectProperty *object_property_try_add_child(Object *obj, const char *name,
+                                              Object *child, Error **errp);
+
+/**
+ * object_property_add_child:
+ * Same as object_property_try_add_child() with @errp hardcoded to
+ * &error_abort
+ */
 ObjectProperty *object_property_add_child(Object *obj, const char *name,
                                           Object *child);
 

qom/object.c

@@ -1146,7 +1146,7 @@ void object_unref(Object *obj)
     }
 }
 
-static ObjectProperty *
+ObjectProperty *
 object_property_try_add(Object *obj, const char *name, const char *type,
                         ObjectPropertyAccessor *get,
                         ObjectPropertyAccessor *set,
@@ -1675,8 +1675,8 @@ static void object_finalize_child_property(Object *obj, const char *name,
 }
 
 ObjectProperty *
-object_property_add_child(Object *obj, const char *name,
-                          Object *child)
+object_property_try_add_child(Object *obj, const char *name,
+                              Object *child, Error **errp)
 {
     g_autofree char *type = NULL;
     ObjectProperty *op;
@@ -1685,14 +1685,25 @@ object_property_add_child(Object *obj, const char *name,
 
     type = g_strdup_printf("child<%s>", object_get_typename(child));
 
-    op = object_property_add(obj, name, type, object_get_child_property, NULL,
-                             object_finalize_child_property, child);
+    op = object_property_try_add(obj, name, type, object_get_child_property,
+                                 NULL, object_finalize_child_property,
+                                 child, errp);
+    if (!op) {
+        return NULL;
+    }
     op->resolve = object_resolve_child_property;
     object_ref(child);
     child->parent = obj;
     return op;
 }
 
+ObjectProperty *
+object_property_add_child(Object *obj, const char *name,
+                          Object *child)
+{
+    return object_property_try_add_child(obj, name, child, &error_abort);
+}
+
 void object_property_allow_set_link(const Object *obj, const char *name,
                                     Object *val, Error **errp)
 {

qom/object_interfaces.c

@@ -83,8 +83,11 @@ Object *user_creatable_add_type(const char *type, const char *id,
     }
 
     if (id != NULL) {
-        object_property_add_child(object_get_objects_root(),
-                                  id, obj);
+        object_property_try_add_child(object_get_objects_root(),
+                                      id, obj, &local_err);
+        if (local_err) {
+            goto out;
+        }
     }
 
     if (!user_creatable_complete(USER_CREATABLE(obj), &local_err)) {