docs: def _update_size(self, size, future):
          """
          Keep track of the file size during writing.
  
          If specified size value is greater than the current size, update the
          current size using specified value.
  
          Used as callback in default "_flush" implementation for files supporting
          random write access.
  
          Args:
              size (int): Size value.
              future (concurrent.futures._base.Future): future.
          """
          with self._size_lock:
              # Update value
              if size > self._size and future.done:
                  # Size can be lower if seek down on an 'a' mode open file.
                  self._size = size