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
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.