// the lockFile muste be a file separate from any data files.
// It is created & deleted by this class, and will always have
// size 0.
sclass FileBasedLock implements AutoCloseable {
  File lockFile;
  double timeout = 60.0; // in seconds. refresh happens twice as often
  bool verbose;
  bool haveLock;
  java.util.Timer touchTimer;
  
  *() {}
  *(File *lockFile) {}
  *(File *lockFile, double *timeout) {}
  
  // returns true iff lock was acquired (or kept)
  synchronized bool tryToLock() {
    if (haveLock) true;
    
    if (fileExists(lockFile)) {
      double age = fileAgeInSeconds(lockFile);
      if (verbose) print("Lock file age: " + lockFile + " = " + iround(age) + " s");
      if (age >= timeout) {
        print("Deleting old lock file (program crashed?): " + lockFile + " (age: " + iround(age) + " seconds)");
        deleteFile(lockFile);
      }
    }
    
    try {
      mkdirsForFile(lockFile);
      java.nio.file.Files.createFile(toPath(lockFile));
      set haveLock;
      startTouchTimer();
      true;
    } catch e {
      printExceptionShort(e);
      false;
    }
  }
  
  synchronized void startTouchTimer {
    if (touchTimer != null) ret;
    double interval = timeout/2;
    touchTimer = doEvery(interval, r doTouch);
    if (verbose) print("Touch timer started for " + lockFile +  " (" + interval + "s)");
  }

  synchronized void doTouch {
    if (haveLock) {
      if (verbose) print("Touching lock file: " + lockFile);
      touchExistingFile(lockFile);
    }
  }
     
  public synchronized void close() pcall {
    dispose touchTimer;
    if (haveLock) {
      haveLock = false;
      if (verbose) print("Deleting lock file: " + lockFile);
      deleteFile(lockFile);
    }
  }
  
  synchronized void _simulateCrash {
    dispose touchTimer;
  }
}