test_build: exit rather than hanging on event handler exception

Event handlers decorated with @celery_app_task don't raise an exception -
they just log the exception, leaving the Moksha hub running. This meant
that any failures in test_build would result in the test suite hanging
rather than failing usefully.

We solve this by adding a new class EventTrap which acts as a context
manager. Any exceptions that occur in event handlers are set on the
current EventTrap, and the test_build tests re-raise the exception.

tests/test_build/test_build.py

@@ -83,6 +83,16 @@ def make_simple_stop_condition():
     return stop_condition
 
 
+def make_trapped_stop_condition(stop_condition):
+    def trapped_stop_condition(message):
+        if events.EventTrap.get_exception():
+            return True
+
+        return stop_condition(message)
+
+    return trapped_stop_condition
+
+
 def main(initial_messages, stop_condition):
     """ Run the consumer until some condition is met.
 
@@ -108,18 +118,25 @@ def main(initial_messages, stop_condition):
 
     consumers = [module_build_service.scheduler.consumer.MBSConsumer]
 
-    # Note that the hub we kick off here cannot send any message.  You
-    # should use fedmsg.publish(...) still for that.
-    moksha.hub.main(
-        # Pass in our config dict
-        options=config,
-        # Only run the specified consumers if any are so specified.
-        consumers=consumers,
-        # Do not run default producers.
-        producers=[],
-        # Tell moksha to quiet its logging.
-        framework=False,
-    )
+    # The events.EventTrap context handler allows us to detect exceptions
+    # in event handlers and re-raise them here so that tests fail usefully
+    # rather than just hang.
+    with events.EventTrap() as trap:
+        # Note that the hub we kick off here cannot send any message.  You
+        # should use fedmsg.publish(...) still for that.
+        moksha.hub.main(
+            # Pass in our config dict
+            options=config,
+            # Only run the specified consumers if any are so specified.
+            consumers=consumers,
+            # Do not run default producers.
+            producers=[],
+            # Tell moksha to quiet its logging.
+            framework=False,
+        )
+
+        if trap.exception:
+            raise trap.exception
 
 
 class FakeSCM(object):
@@ -388,9 +405,10 @@ def cleanup_moksha():
 class BaseTestBuild:
 
     def run_scheduler(self, msgs=None, stop_condition=None):
+        stop_condition = stop_condition or make_simple_stop_condition()
         main(
             msgs or [],
-            stop_condition or make_simple_stop_condition()
+            make_trapped_stop_condition(stop_condition)
         )