Fix for issues #461 and #471.

This handles None exception values that would otherwise fail when passed
to traceback.print_exception() and prevent the process from exiting. An associated test is also included.

gevent/hub.py

@@ -334,6 +334,11 @@ class Hub(greenlet):
                     cb.stop()
 
     def print_exception(self, context, type, value, tb):
+        # Python 3 does not gracefully handle None value or tb in
+        # traceback.print_exception() as previous versions did.
+        if value is None:
+            sys.stderr.write('%s\n' % type.__name__)
+        else:
             traceback.print_exception(type, value, tb)
         del tb
         if context is not None:

greentest/test__issues461_471.py

+'''Test for GitHub issues 461 and 471.
+
+When moving to Python 3, handling of KeyboardInterrupt exceptions caused
+by a Ctrl-C raise an exception while printing the traceback for a
+greenlet preventing the process from exiting. This test tests for proper
+handling of KeyboardInterrupt.
+'''
+
+import sys
+
+if sys.argv[1:] == ['subprocess']:
+    import gevent
+    def task():
+        sys.stdout.write('ready\n')
+        sys.stdout.flush()
+        gevent.sleep(30)
+    try:
+        gevent.spawn(task).get()
+    except KeyboardInterrupt:
+        pass
+else:
+    import signal
+    from subprocess import Popen, PIPE, TimeoutExpired
+    if sys.platform.startswith('win'):
+        from subprocess import CREATE_NEW_PROCESS_GROUP
+        kwargs = {'creationflags': CREATE_NEW_PROCESS_GROUP}
+    else:
+        kwargs = {}
+    p = Popen([sys.executable, __file__, 'subprocess'], stdout=PIPE, **kwargs)
+    p.stdout.readline()
+    p.send_signal(signal.SIGINT)
+    try:
+        p.wait(3)
+    except TimeoutExpired:
+        p.terminate()
+        sys.exit(1)
+    sys.exit(p.returncode)