Issue #539: use a internal-only serializer function in json_object_new_double_s() to avoid potential conflicts with user code that uses the json_object_userdata_to_json_string serializer. Also, document the serializer-resetting behavior of json_object_set_double().

json_object.c

@@ -53,6 +53,7 @@ static json_object_to_json_string_fn json_object_double_to_json_string_default;
 static json_object_to_json_string_fn json_object_int_to_json_string;
 static json_object_to_json_string_fn json_object_string_to_json_string;
 static json_object_to_json_string_fn json_object_array_to_json_string;
+static json_object_to_json_string_fn _json_object_userdata_to_json_string;
 
 
 /* ref count debugging */
@@ -921,11 +922,22 @@ struct json_object* json_object_new_double_s(double d, const char *ds)
 		errno = ENOMEM;
 		return NULL;
 	}
-	json_object_set_serializer(jso, json_object_userdata_to_json_string,
+	json_object_set_serializer(jso, _json_object_userdata_to_json_string,
 	    new_ds, json_object_free_userdata);
 	return jso;
 }
 
+/*
+ * A wrapper around json_object_userdata_to_json_string() used only
+ * by json_object_new_double_s() just so json_object_set_double() can
+ * detect when it needs to reset the serializer to the default.
+ */
+static int _json_object_userdata_to_json_string(struct json_object *jso,
+	struct printbuf *pb, int level, int flags)
+{
+	return json_object_userdata_to_json_string(jso, pb, level, flags);
+}
+
 int json_object_userdata_to_json_string(struct json_object *jso,
 	struct printbuf *pb, int level, int flags)
 {
@@ -999,7 +1011,7 @@ int json_object_set_double(struct json_object *jso,double new_value){
 	if (!jso || jso->o_type!=json_type_double)
 		return 0;
 	jso->o.c_double=new_value;
-	if (jso->_to_json_string == &json_object_userdata_to_json_string)
+	if (jso->_to_json_string == &_json_object_userdata_to_json_string)
 	    json_object_set_serializer(jso, NULL, NULL, NULL);
 	return 1;
 }
@@ -1354,7 +1366,8 @@ static int json_object_copy_serializer_data(struct json_object *src, struct json
 	if (!src->_userdata && !src->_user_delete)
 		return 0;
 
-	if (dst->_to_json_string == json_object_userdata_to_json_string)
+	if (dst->_to_json_string == json_object_userdata_to_json_string ||
+	    dst->_to_json_string == _json_object_userdata_to_json_string)
 	{
 		dst->_userdata = strdup(src->_userdata);
 	}

json_object.h

@@ -801,7 +801,8 @@ JSON_EXPORT struct json_object* json_object_new_double(double d);
  * The userdata field is used to store the string representation, so it
  * can't be used for other data if this function is used.
  *
- * An equivalent sequence of calls is:
+ * A roughly equivalent sequence of calls, with the difference being that
+ *  the serialization function won't be reset by json_object_set_double(), is:
  * @code
  *   jso = json_object_new_double(d);
  *   json_object_set_serializer(jso, json_object_userdata_to_json_string,
@@ -885,6 +886,9 @@ JSON_EXPORT double json_object_get_double(const struct json_object *obj);
  * if it is not without any further actions. If type of obj is json_type_double
  * the object value is changed to new_value
  *
+ * If the object was created with json_object_new_double_s(), the serialization
+ * function is reset to the default and the cached serialized value is cleared.
+ *
  * @param obj the json_object instance
  * @param new_value the value to be set
  * @returns 1 if value is set correctly, 0 otherwise