Does ros::Subscriber shutdown() block if there is an active callback?

edit

For the future reader: Yes, all of the above are confirmed.

struct B {
  int b = 1111;
  ~B() {
    b = 0; /* clear memory */
    std::cerr << "B is destroyed!\n";
  }
};
struct A {
  ros::NodeHandle n;
  ros::Publisher pub;
  ros::Subscriber sub;
  B b;
  A() {
    boost::function<void(const std_msgs::StringConstPtr &)> func = [this](const std_msgs::StringConstPtr &msg) {
      std::cerr << "B=" << b.b << " CALLBACK STARTED...waiting 3 seconds\n";
      std::this_thread::sleep_for(std::chrono::seconds(3));
      std::cerr << "B=" << b.b << " CALLBACK FINISHED!!!\n";
    };
    pub = n.advertise<std_msgs::String>("bah", 10, false);
    sub = n.subscribe("bah", 10, func);
  }
};
int main(int argc, char **argv) {
  std::cerr << "STARTED\n";
  ros::init(argc, argv, "a_blah");
  ros::Time::init();

  {
    A *a = new A;
    int start = 100000;
    int count = start;
    ros::AsyncSpinner spinner(0);
    spinner.start();
    while (ros::ok() && --count) {
      if (count == start - 1) {
        std::cerr << "Publishing message\n";
        std_msgs::String msg;
        a->pub.publish(msg);
      }
      if (count == start / 2) {
        std::cerr << "Deleting a...\n";
        delete a;
        a = nullptr;
        std::cerr << "Deleting a FINISHED####\n";
      }
    }
    spinner.stop();
  }

And this prints:

STARTED
Publishing message
B=1111 CALLBACK STARTED...waiting 3 seconds
Deleting a... 
B is destroyed!
                    <<<BLOCKS FOR ~3 SECONDS>>>
B=0 CALLBACK FINISHED!!!
Deleting a FINISHED####