title: A confusing BUG on continiousg linking to nonblocking TCP socket when bind to port 80 (python3)?
body: <p>I'm writing a multi-thread web server. I tried to bind a non-blocking tcp socket to port 80. When I tested it in two terminals, I found the server cannot recv message until the client socket was closed. (But when binding to other port, the server recv message from the client immediently).</p>
  <p>Blow are the minimal code of this problem.</p>
  <pre class="lang-py prettyprint-override"><code># server.py
  
  from socket import *
  import time
  
  port = 80
  
  so = socket()   # TCP for default
  so.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
  so.setblocking(False)
  so.bind(('', port))
  so.listen()
  print('Ready to server...')
  while True:
      try:
          client_so, client_addr = so.accept()
      except:
          time.sleep(0.001)
      else:
          # now we have a connection, but ...
          # Press Ctrl-C in client to see what will happend.
          # And change the &quot;port&quot; in both file, then do the same procedure.
          print(f'link to {client_addr}')
          while True:
              try:
                  msg = client_so.recv(1024).decode()
              except:
                  time.sleep(0.001)
              else:
                  if(msg):
                      print(msg)
  
  </code></pre>
  <pre class="lang-py prettyprint-override"><code>from socket import *
  import time
  
  host = 'localhost'
  port = 80
  
  so = socket()
  so.connect((host, port))
  so.send('hi'.encode())
  
  while True:
      time.sleep(0.01)
  </code></pre>
  <ol>
  <li>run <code>python server.py</code> in one terminal.</li>
  <li>run <code>python client.py</code> in another terminal.</li>
  </ol>
  <p>When port is 80, the server cannot recive 'hi' until client.py was stoped. But other ports do work.</p>
  <p>I tried this both on Windows 10 and Ubuntu 20.04 (python version is 3.9.7) and get the same result.</p>
  <p>So what happened when bind a tcp socket to port 80? Will it make some difference to behavior of the socket?</p>