static <A, B> B syncMapGetOrPut(Map<A, B> map, A key, B value) {
  if (map == null || key == null) ret value;
  synchronized(collectionMutex(map)) {
    B existingValue = map.get(key);
    if (existingValue != null)
      ret existingValue;
    else {
      map.put(key, value);
      ret value;
    }
  }
}

Began life as a copy of #1031558