object_new() has a failure scenario where it will assert() if given
an abstract type. Callers which are creating objects based on user
input, or unknown/untrusted type names, must manually check the
result of object_class_is_abstract() before calling object_new()
to propagate an Error, instead of asserting.

Introduce a object_new_dynamic() method which is a counterpart to
object_new() that directly returns an Error, instead of asserting.
This new method is to be used where the typename is specified
dynamically by code separate from the immediate caller.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 include/qom/object.h | 27 +++++++++++++++++++++++++++
 qom/object.c         |  9 +++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/qom/object.h b/include/qom/object.h
index 11ee472719..4fc01336c4 100644
--- a/include/qom/object.h
+++ b/include/qom/object.h
@@ -624,14 +624,41 @@ Object *object_new_with_class(ObjectClass *klass, Error **errp);
  * object_new:
  * @typename: The name of the type of the object to instantiate.
  *
+ * This method should be used where @typename is statically specified
+ * from a const string at build time, where the caller does not expect
+ * failure to be possible.
+ *
  * This function will initialize a new object using heap allocated memory.
  * The returned object has a reference count of 1, and will be freed when
  * the last reference is dropped.
  *
+ * If an instance of @typename is not permitted to be instantiated, an
+ * assert will be raised. This can happen if @typename is abstract.
+ *
  * Returns: The newly allocated and instantiated object.
  */
 Object *object_new(const char *typename);
 
+/**
+ * object_new_dynamic:
+ * @typename: The name of the type of the object to instantiate.
+ * @errp: pointer to be filled with error details on failure
+ *
+ * This method should be used where @typename is dynamically chosen
+ * at runtime, which has the possibility of unexpected choices leading
+ * to failures.
+ *
+ * This function will initialize a new object using heap allocated memory.
+ * The returned object has a reference count of 1, and will be freed when
+ * the last reference is dropped.
+ *
+ * If an instance of @typename is not permitted to be instantiated, an
+ * error will be raised. This can happen if @typename is abstract.
+ *
+ * Returns: The newly allocated and instantiated object.
+ */
+Object *object_new_dynamic(const char *typename, Error **errp);
+
 /**
  * object_new_with_props:
  * @typename:  The name of the type of the object to instantiate.
diff --git a/qom/object.c b/qom/object.c
index ad5b3b9582..42ef40a1fd 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -806,6 +806,15 @@ Object *object_new(const char *typename)
     return object_new_with_type(ti, &error_abort);
 }
 
+Object *object_new_dynamic(const char *typename, Error **errp)
+{
+    TypeImpl *ti = type_get_or_load_by_name(typename, errp);
+    if (!ti) {
+        return NULL;
+    }
+
+    return object_new_with_type(ti, errp);
+}
 
 Object *object_new_with_props(const char *typename,
                               Object *parent,
-- 
2.46.0