Semaphore Files

      if (something()) {
        open F, $FILE or die ...;
        flock F, LOCK_SH;
        print while <F>;
      } else {
        open F, "> $FILE" or die ...;
        flock F, LOCK_EX;
        my $data = qx{some command};
        print F $data;
      }

Oops, it's still wrong
Maybe we can lock the file before we open it? ... Nope
Maybe the writer can open in update mode,
- lock the file,
- seek to the beginning,
- overwrite the contents,
- call truncate...

continued...