code: def construct_wal_name(sysinfo):
      
      log_hex, seg_hex = sysinfo["xlogpos"].split("/", 1)
      # seg_hex's topmost 8 bits are filename, low 24 bits are position in
      # file which we are not interested in
      return name_for_tli_log_seg(
          tli=int(sysinfo["timeline"]),
          log=int(log_hex, 16),
          seg=int(seg_hex, 16) >> 24)
docs: Get wal file name out of something like this:
      {'dbname': '', 'systemid': '6181331723016416192', 'timeline': '1', 'xlogpos': '0/90001B0'}