sclass ManualTailFile extends TailFile {
  long lastCheck;
  Lock lock = lock();
  
  // onData: Runnable or voidfunc(S)
  *(final File *file, int *interval, fO *onData) {}
  *(final File *file, fO *onData) { interval = 100; }
  
  // updates in this thread
  void check() {
    long l2 = 0;
    {
      lock lock;
      long now = sysNow();
      if (now < lastCheck+interval) ret;
      
      lastCheck = now;
      l2 = l(file);
      if (l2 < l) l = 0; // file shrunk (log rotated!) - start over from beginning of file
      if (l2 == l) hasIdled = true;
      else pcall {
        if (debug) print("New data in " + f2s(file) + ": " + n2(l2-l, "byte"));
      }
    }
    
    // call onData outside of locked part
    for (S s : loadTextFilePart_iterator(file, l, l2))
      pcallF(onData, s);
      
    l = l2;
  }
}