Fix regression in WSGI server SIGHUP behavior

The SIGHUP receipt used to pop us out of an os.wait() where now, it's in
a "green" wait() and Timeout() combo, some part of which eats the signal
receipt.  This causes the while loop condition to never get checked and
SIGHUP no longer works as a server reload command.

The fix is to loop at least every 0.5 seconds, as a trade-off between
not busy-waiting and checking the "keep running" condition often enough
to feel responsive.

Change-Id: I95283b8b7cfc2998ab5813e0ad3ca1fa231696c8
Closes-Bug: #1479972

swift/common/wsgi.py

@@ -461,10 +461,14 @@ class WorkersStrategy(object):
 
     def loop_timeout(self):
         """
-        :returns: None; to block in :py:func:`green.os.wait`
+        We want to keep from busy-waiting, but we also need a non-None value so
+        the main loop gets a chance to tell whether it should keep running or
+        not (e.g. SIGHUP received).
+
+        So we return 0.5.
         """
 
-        return None
+        return 0.5
 
     def bind_ports(self):
         """

test/unit/common/test_wsgi.py

@@ -1090,9 +1090,10 @@ class TestWorkersStrategy(unittest.TestCase):
         self.addCleanup(patcher.stop)
 
     def test_loop_timeout(self):
-        # This strategy should block in the green.os.wait() until a worker
-        # process exits.
-        self.assertEqual(None, self.strategy.loop_timeout())
+        # This strategy should sit in the green.os.wait() for a bit (to avoid
+        # busy-waiting) but not forever (so the keep-running flag actually
+        # gets checked).
+        self.assertEqual(0.5, self.strategy.loop_timeout())
 
     def test_binding(self):
         self.assertEqual(None, self.strategy.bind_ports())