Return the file path referenced but rel_path, or None if
        it can't be found. If an upstream is declared, it will try to get the file
        from the upstream before declaring failure.

def get(self, rel_path, cb=None):
        '''Return the file path referenced but rel_path, or None if
        it can't be found. If an upstream is declared, it will try to get the file
        from the upstream before declaring failure.
        '''
        import shutil

        global_logger.debug("FC {} get looking for {}".format(self.repo_id,rel_path))

        path = os.path.join(self.cache_dir, rel_path)

        # If is already exists in the repo, just return it.
        if os.path.exists(path):

            if not os.path.isfile(path):
                raise ValueError("Path does not point to a file")

            global_logger.debug( "FC {} get {} found ".format(self.repo_id,path))
            return path

        if not self.upstream:
            # If we don't have an upstream, then we are done.
            return None

        stream = self.upstream.get_stream(rel_path, cb=cb)

        if not stream:
            global_logger.debug(
                "FC {} get not found in upstream ()".format(
                    self.repo_id,
                    rel_path))
            return None

        # Got a stream from upstream, so put the file in this cache.
        dirname = os.path.dirname(path)
        if not os.path.isdir(dirname):
            os.makedirs(dirname)

        try:
            with open(path, 'w') as f:
                #shutil.copyfileobj(stream, f)
                copy_file_or_flo(stream, f, cb=cb)
        except:
            os.remove(path)
            raise

        try:
            stream.close()
        except AttributeError:
            pass  # HTTPResponse objects don't have close()

        if not os.path.exists(path):
            raise Exception("Failed to copy upstream data to {} ".format(path))

        global_logger.debug(
            "FC {} got return from upstream {}".format(
                self.repo_id,
                rel_path))
        return path