add locked_file.flush()

youtube_dl/utils.py

@@ -3309,6 +3309,9 @@ class locked_file(object):
     def read(self, *args):
         return self.f.read(*args)
 
+    def flush(self, *args):
+        self.f.flush()
+
     def close(self, *args):
         self.__exit__(self, *args, value=False, traceback=False)