Potential deadlock in statestore error path

 {
    lock_guard<mutex> l(subscribers_lock_);
    // ... snip ...
    if (state == FailureDetector::FAILED) {
      if (is_heartbeat) {
        // TODO: Consider if a metric to track the number of failures would be useful.
        LOG(INFO) << "Subscriber '" << subscriber->id() << "' has failed, disconnected "
                  << "or re-registered (last known registration ID: " << update.second
                  << ")";
        UnregisterSubscriber(subscriber.get());
      }
    } else {
      // Schedule the next message.
      VLOG(3) << "Next " << (is_heartbeat ? "heartbeat" : "update") << " deadline for: "
              << subscriber->id() << " is in " << deadline_ms << "ms";
      // vvvvvvvvvvvvvvvvvv oops vvvvvvvvvvvvvvvvvv
      OfferUpdate(make_pair(deadline_ms, subscriber->id()), is_heartbeat ?
          &subscriber_heartbeat_threadpool_ : &subscriber_topic_update_threadpool_);
    }
  }