[arbiter] close sockets on shutdown

Close all the listeners when the arbiter shuts down. By doing so,
workers can close the socket at the beginning of a graceful shut
down thereby informing the operating system that the socket can
be cleaned up. With this change, graceful exits with such workers
will refuse new connections while draining, allowing load balancers
to respond more quickly and avoiding leaving connections dangling
in the listen backlog, unaccepted.

Ref #922

gunicorn/arbiter.py

@@ -334,6 +334,8 @@ class Arbiter(object):
         :attr graceful: boolean, If True (the default) workers will be
         killed gracefully  (ie. trying to wait for the current connection)
         """
+        for l in self.LISTENERS:
+            l.close()
         self.LISTENERS = []
         sig = signal.SIGTERM
         if not graceful:

tests/test_arbiter.py

@@ -5,15 +5,18 @@
 
 import os
 
+try:
+    import unittest.mock as mock
+except ImportError:
+    import mock
+
 import gunicorn.app.base
 import gunicorn.arbiter
 
 
-class PreloadedAppWithEnvSettings(gunicorn.app.base.BaseApplication):
+class DummyApplication(gunicorn.app.base.BaseApplication):
     """
-    Simple application that makes use of the 'preload' feature to
-    start the application before spawning worker processes and sets
-    environmental variable configuration settings.
+    Dummy application that has an default configuration.
     """
 
     def init(self, parser, opts, args):
@@ -22,6 +25,27 @@ class PreloadedAppWithEnvSettings(gunicorn.app.base.BaseApplication):
     def load(self):
         """No-op"""
 
+    def load_config(self):
+        """No-op"""
+
+
+def test_arbiter_shutdown_closes_listeners():
+    arbiter = gunicorn.arbiter.Arbiter(DummyApplication())
+    listener1 = mock.Mock()
+    listener2 = mock.Mock()
+    arbiter.LISTENERS = [listener1, listener2]
+    arbiter.stop()
+    listener1.close.assert_called_with()
+    listener2.close.assert_called_with()
+
+
+class PreloadedAppWithEnvSettings(DummyApplication):
+    """
+    Simple application that makes use of the 'preload' feature to
+    start the application before spawning worker processes and sets
+    environmental variable configuration settings.
+    """
+
     def load_config(self):
         """Set the 'preload_app' and 'raw_env' settings in order to verify their
         interaction below.