Fixed #26011 -- Prevented random LiveServerTestCase test failures on Windows.

Prevented LiveServerTestCase from stealing ports used by concurrent
processes on Windows.

django/core/servers/basehttp.py

@@ -70,6 +70,7 @@ class WSGIServer(simple_server.WSGIServer, object):
     def __init__(self, *args, **kwargs):
         if kwargs.pop('ipv6', False):
             self.address_family = socket.AF_INET6
+        self.allow_reuse_address = kwargs.pop('allow_reuse_address', True)
         super(WSGIServer, self).__init__(*args, **kwargs)
 
     def server_bind(self):

django/test/testcases.py

@@ -1252,7 +1252,7 @@ class LiveServerThread(threading.Thread):
             self.is_ready.set()
 
     def _create_server(self, port):
-        return WSGIServer((self.host, port), QuietWSGIRequestHandler)
+        return WSGIServer((self.host, port), QuietWSGIRequestHandler, allow_reuse_address=False)
 
     def terminate(self):
         if hasattr(self, 'httpd'):

tests/servers/tests.py

@@ -5,6 +5,7 @@ Tests for django.core.servers.
 from __future__ import unicode_literals
 
 import contextlib
+import errno
 import os
 import socket
 
@@ -174,3 +175,31 @@ class LiveServerDatabase(LiveServerBase):
             ['jane', 'robert', 'emily'],
             lambda b: b.name
         )
+
+
+class LiveServerPort(LiveServerBase):
+
+    def test_port_bind(self):
+        """
+        Each LiveServerTestCase binds to a unique port or fails to start a
+        server thread when run concurrently (#26011).
+        """
+        TestCase = type(str("TestCase"), (LiveServerBase,), {})
+        try:
+            TestCase.setUpClass()
+        except socket.error as e:
+            if e.ernrno == errno.EADDRINUSE:
+                # We're out of ports, LiveServerTestCase correctly fails with
+                # a socket error.
+                return
+            # Unexpected error.
+            raise
+        try:
+            # We've acquired a port, ensure our server threads acquired
+            # different addresses.
+            self.assertNotEqual(
+                self.live_server_url, TestCase.live_server_url,
+                "Acquired duplicate server addresses for server threads: %s" % self.live_server_url
+            )
+        finally:
+            TestCase.tearDownClass()