def host_impl(self, run, tool):
    """Returns available hosts for the run and tool in the log directory.

    In the plugin log directory, each directory contains profile data for a
    single run (identified by the directory name), and files in the run
    directory contains data for different tools and hosts. The file that
    contains profile for a specific tool "x" will have a prefix name TOOLS["x"].

    Example:
      log/
        run1/
          plugins/
            profile/
              host1.trace
              host2.trace
        run2/
          plugins/
            profile/
              host1.trace
              host2.trace

    Returns:
      A list of host names e.g.
        {"host1", "host2", "host3"} for the example.
    """
    hosts = {}
    run_dir = self._run_dir(run)
    if not run_dir:
      logger.warn("Cannot find asset directory for: %s", run)
      return hosts
    tool_pattern = '*' + TOOLS[tool]
    try:
      files = tf.io.gfile.glob(os.path.join(run_dir, tool_pattern))
      hosts = [os.path.basename(f).replace(TOOLS[tool], '') for f in files]
    except tf.errors.OpError as e:
      logger.warn("Cannot read asset directory: %s, OpError %s",
                      run_dir, e)
    return hosts