Monkeypatch startService and stopService to assert on running

This adds a new monkeypatch module which will hopefully be useful in
adding more monkey patches later.  It should make tracking down
service-management bugs in tests *much* easier.

buildbot/broken_test/runs/test_web.py

@@ -6,7 +6,7 @@
 from twisted.python import components
 
 from twisted.trial import unittest
-from buildbot.broken_test.runutils import RunMixin
+from buildbot.broken_test.runutils import RunMixin, monkeypatch
 
 from twisted.internet import reactor, defer, protocol
 from twisted.internet.interfaces import IReactorUNIX
@@ -22,6 +22,8 @@
 from buildbot.process.buildstep import BuildStep
 from buildbot.broken_test.runutils import setupBuildStepStatus
 
+monkeypatch()
+
 class SimpleMaster(master.BuildMaster):
     """This BuildMaster variant uses a local sqlite DB, and gives more
     control over the loading of the config file."""

buildbot/broken_test/runutils.py

@@ -614,3 +614,5 @@ def _get_buildstatus(build_set_status):
     d.addCallback(_get_buildstatus)
     return d
 
+# convenient access to *new* test utilities
+from buildbot.test.util.monkeypatches import monkeypatch

buildbot/test/unit/test_util_loop.py

@@ -41,9 +41,12 @@
 from twisted.python import log
 
 from buildbot.test.fake.state import State
+from buildbot.test.util.monkeypatches import monkeypatch
 
 from buildbot.util import loop, eventual
 
+monkeypatch()
+
 class TestLoopMixin(object):
     def setUpTestLoop(self, skipConstructor=False):
         if not skipConstructor:

buildbot/test/util/monkeypatches.py

@@ -0,0 +1,18 @@
+# Import this module and call the monkeypatch() method to add a bunch of useful
+# Twisted monkeypatches that help to catch stupid errors.
+
+def monkeypatch_startService():
+    from twisted.application.service import Service
+    old_startService = Service.startService
+    old_stopService = Service.stopService
+    def startService(self):
+        assert not self.running
+        return old_startService(self)
+    def stopService(self):
+        assert self.running
+        return old_stopService(self)
+    Service.startService = startService
+    Service.stopService = stopService
+
+def monkeypatch():
+    monkeypatch_startService()

docs/developer.texinfo

@@ -417,6 +417,7 @@ stopped.
 
 @menu
 * Keeping State in Tests::
+* Better Debugging through Monkeypatching::
 @end menu
 
 @node Keeping State in Tests
@@ -457,6 +458,20 @@ def test_localVariable(self):
 
 This is almost as readable as the first example, but it actually works. 
 
+@node Better Debugging through Monkeypatching
+@subsection Better Debugging through Monkeypatching
+@dvindex buildbot.test.util.monkeypatches
+
+The module @code{buildbot.test.util.monkeypatches} contains a few
+monkey-patches to Twisted that detect errors a bit better.  These patches
+shouldn't affect correct behavior, so it's worthwhile including something like
+this in the header of every test file:
+
+@example
+from buildbot.test.util.monkeypatches import monkeypatch
+monkeypatch()
+@end example
+
 @node Developer Class Index
 @section Developer Class Index
 @printindex dv