Enable reuse_port option to HTTPServer.bind.

This makes solutions that restart the process (e.g., envconsul) work
properly.  Without port reuse, you will get a very unpleasant "port
already in use" when the service restarts.

This requires Tornado >= 4.4

docs/history.rst

@@ -3,6 +3,10 @@
 Release History
 ===============
 
+`Next Release`_
+---------------
+- Enable port reuse for Tornado versions newer than 4.3.
+
 `1.4.2`_ (25 Jan 2018)
 ---------------------
 - Allow max_body_size and max_buffer_size to be specified on the http server.

sprockets/http/runner.py

@@ -12,6 +12,7 @@ import signal
 import sys
 
 from tornado import httpserver, ioloop
+import tornado
 
 import sprockets.http.app
 
@@ -90,7 +91,12 @@ class Runner(object):
             self.server.listen(port_number)
         else:
             self.logger.info('starting processes on port %d', port_number)
-            self.server.bind(port_number)
+            if tornado.version_info >= (4, 4):
+                self.server.bind(port_number, reuse_port=True)
+            else:
+                self.logger.warning('port reuse disabled, please upgrade to'
+                                    'at least Tornado 4.4')
+                self.server.bind(port_number)
             self.server.start(number_of_procs)
 
     def stop_server(self):

tests.py

@@ -14,7 +14,8 @@ except ImportError:
     import mock
     open_name = '__builtin__.open'
 
-from tornado import concurrent, httputil, ioloop, testing, web
+from tornado import concurrent, httpserver, httputil, ioloop, testing, web
+import tornado
 
 import sprockets.http.mixins
 import sprockets.http.runner
@@ -383,7 +384,7 @@ class RunnerTests(MockHelper, unittest.TestCase):
         ioloop_module = self.start_mock('sprockets.http.runner.ioloop')
         ioloop_module.IOLoop.instance.return_value = self.io_loop
 
-        self.http_server = mock.Mock()
+        self.http_server = mock.Mock(spec=httpserver.HTTPServer)
         self.httpserver_module = \
             self.start_mock('sprockets.http.runner.httpserver')
         self.httpserver_module.HTTPServer.return_value = self.http_server
@@ -402,9 +403,24 @@ class RunnerTests(MockHelper, unittest.TestCase):
     def test_that_production_run_starts_in_multiprocess_mode(self):
         runner = sprockets.http.runner.Runner(self.application)
         runner.run(8000)
-        self.http_server.bind.assert_called_once_with(8000)
+
+        self.assertTrue(self.http_server.bind.called)
+        args, kwargs = self.http_server.bind.call_args_list[0]
+        self.assertEqual(args, (8000, ))
+
         self.http_server.start.assert_called_once_with(0)
 
+    @unittest.skipUnless(tornado.version_info >= (4, 4),
+                         'port reuse requries newer tornado')
+    def test_that_production_enables_reuse_port(self):
+        runner = sprockets.http.runner.Runner(self.application)
+        runner.run(8000)
+
+        self.assertTrue(self.http_server.bind.called)
+        args, kwargs = self.http_server.bind.call_args_list[0]
+        self.assertEqual(args, (8000, ))
+        self.assertEqual(kwargs['reuse_port'], True)
+
     def test_that_debug_run_starts_in_singleprocess_mode(self):
         self.application.settings['debug'] = True
         runner = sprockets.http.runner.Runner(self.application)