core: split _broker_shutdown() out into its own function.

Makes _broker_main() logic much clearer.

mitogen/core.py

@@ -2330,12 +2330,7 @@ class Broker(object):
         for (side, func) in self.poller.poll(timeout):
             self._call(side.stream, func)
 
-    def _broker_main(self):
-        try:
-            while self._alive:
-                self._loop_once()
-
-            fire(self, 'shutdown')
+    def _broker_shutdown(self):
         for _, (side, _) in self.poller.readers + self.poller.writers:
             self._call(side.stream, side.stream.on_shutdown)
 
@@ -2352,6 +2347,14 @@ class Broker(object):
         for _, (side, _) in self.poller.readers + self.poller.writers:
             LOG.error('_broker_main() force disconnecting %r', side)
             side.stream.on_disconnect(self)
+
+    def _broker_main(self):
+        try:
+            while self._alive:
+                self._loop_once()
+
+            fire(self, 'shutdown')
+            self._broker_shutdown()
         except Exception:
             LOG.exception('_broker_main() crashed')