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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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