I try to get a free port in DDP initialization of PyTorch. However, my code get stuck. The following snippet could repeat my description:
def get_open_port():
    with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
        s.bind(('', 0))
        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        return s.getsockname()[1]

def setup(rank, world_size):
    os.environ['MASTER_ADDR'] = 'localhost'
    port = get_open_port()
    os.environ['MASTER_PORT'] = str(port)   # '12345'

    # Initialize the process group.
    dist.init_process_group('NCCL', rank=rank, world_size=world_size)

class ToyModel(nn.Module):
    def __init__(self):
        super(ToyModel, self).__init__()
        self.net1 = nn.Linear(10, 5)

    def forward(self, x):
        print(f'x device={x.device}')
        return self.net1(x)


def demo_basic(rank, world_size):
    setup(rank, world_size)

    logger = logging.getLogger('train')
    logger.setLevel(logging.DEBUG)
    logger.info(f'Running DPP on rank={rank}.')

    # Create model and move it to GPU.
    model = ToyModel().to(rank)
    ddp_model = DDP(model, device_ids=[rank])

    loss_fn = nn.MSELoss()
    optimizer = optim.SGD(ddp_model.parameters(), lr=0.001)  # optimizer takes DDP model.

    optimizer.zero_grad()
    inputs = torch.randn(20, 10)  # .to(rank)

    print(f'inputs device={inputs.device}')
    outputs = ddp_model(inputs)
    print(f'output device={outputs.device}')

    labels = torch.randn(20, 5).to(rank)
    loss_fn(outputs, labels).backward()

    optimizer.step()

    cleanup()


def run_demo(demo_func, world_size):
    mp.spawn(
        demo_func,
        args=(world_size,),
        nprocs=world_size,
        join=True
    )

run_demo(demo_basic, 4)

The function get_open_port is supposed to free the port after invocation. My questions are: 1. How does it happen? 2. How to fix it?