sclass TailFile implements AutoCloseable {
  File file;
  int interval;
  O onData;
  
  settable ISleeper sleeper = defaultSleeper();
  
  long l;
  gettable long currentFileSize;
  new Flag stopped;
  FlexibleRateTimer timer;
  ReliableSingleThread thread;
  volatile bool hasIdled, debug;
  
  *() {}
  *(final File *file, int *interval, O *onData) {
    thread = new ReliableSingleThread(r { update() });
  }
  
  void update {
    long l2 = currentFileSize = 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"));
      for (S s : loadTextFilePart_iterator(file, l, l2))
        pcallF(onData, s);
      l = l2;
    }
  }
  
  void start {
    timer = new FlexibleRateTimer(msIntervalToHertz(interval), -> {
      if (!stopped.isUp()) thread.trigger();
    });
    timer.sleeper(sleeper);
    timer.start();
  }
  
  void stop {
    timer.cancel();
    timer = null;
    if (stopped.raise())
      thread.triggerAndWait(); // One last time baby
  }
  
  public void close { stop(); }
  
  bool started() {
    ret timer != null;
  }
  
  void debugOn {
    if (debug) ret;
    debug = true;
    print("Watching file: " + f2s(file));
  }
  
  toString { ret "Tailing file " + file; }
}

Began life as a copy of #1009097