Method synchronized

3rdparty/libprocess/src/process.cpp:421–423

NOTE: even though `filter` is atomic we still need to synchronize updating it because once we return from this function the old filter might get deleted which could be bad if a thread is currently using the old filter in `ProcessManager::resume`.

    // a thread is currently using the old filter in
    // `ProcessManager::resume`.
    synchronized (filter_mutex) {
      filter.store(f);
    }
  }

  long workers() const