code: def __unionfs_set_up(ro_dir, rw_dir, mount_dir):
      
      mount_dir.mkdir()
      rw_dir.mkdir()
      if not ro_dir.exists():
          LOG.error("Base dir does not exist: '%s'", ro_dir)
          raise ValueError("Base directory does not exist")
  
      from benchbuild.utils.cmd import unionfs as unionfs_cmd
      LOG.debug("Mounting UnionFS on %s with RO:%s RW:%s", mount_dir, ro_dir,
                rw_dir)
      return unionfs_cmd["-f", "-o", "auto_unmount,allow_other,cow", rw_dir +
                         "=RW:" + ro_dir + "=RO", mount_dir]
docs: Setup a unionfs via unionfs-fuse.
  
      Args:
          ro_base: base_directory of the project
          rw_image: virtual image of actual file system
          mountpoint: location where ro_base and rw_image merge