please include function appendToFile.

static void rotateLogFile(File f) {
  lock appendToFile_lock;
  if (!f.exists()) ret;
  int n = 1;
  File p;
  while ((p = newFile(f.getParentFile(), f.getName() + ".part" + n + ".gz")).exists())
    ++n;
  gzipFile(f, p);
  assertTrue(f.delete());
}