Allow plugins to specify signal event reason

Currently, we have logic in the Resource.signal base-class implementation
which works out the reason string based on knowledge of some of the
signal-handling resources.  This is invariably going to be fragile, and it
won't work at all with any out of tree or contrib plugins.

So instead, move towards a model where we accept an optional return value from
handle_signal, and use that as the reason string instead.  Since no resources
yet return a reason, maintain the old logic as a fallback path, until this
gets migrated into the appropriate resources.

Change-Id: I45fdf112e6cd050bb1f4111977f653ba6440df02
Partial-Bug: #1340894

heat/engine/resource.py

@@ -897,8 +897,12 @@ class Resource(object):
                        str(self))
                 raise Exception(msg)
 
-            self._add_event('signal', self.status, get_string_details())
-            self.handle_signal(details)
+            signal_result = self.handle_signal(details)
+            if signal_result:
+                reason_string = "Signal: %s" % signal_result
+            else:
+                reason_string = get_string_details()
+            self._add_event('signal', self.status, reason_string)
         except Exception as ex:
             LOG.exception(_('signal %(name)s : %(msg)s') % {'name': str(self),
                                                             'msg': ex})

heat/tests/test_signal.py

@@ -297,6 +297,30 @@ class SignalTest(HeatTestCase):
 
         self.m.VerifyAll()
 
+    def test_signal_plugin_reason(self):
+        # Ensure if handle_signal returns data, we use it as the reason
+        self.stack = self.create_stack()
+        self.stack.create()
+
+        rsrc = self.stack['signal_handler']
+        self.assertEqual((rsrc.CREATE, rsrc.COMPLETE), rsrc.state)
+
+        self.m.StubOutWithMock(generic_resource.SignalResource,
+                               'handle_signal')
+        signal_details = {'status': 'COMPLETE'}
+        ret_expected = "Received COMPLETE signal"
+        generic_resource.SignalResource.handle_signal(
+            signal_details).AndReturn(ret_expected)
+
+        self.m.StubOutWithMock(generic_resource.SignalResource,
+                               '_add_event')
+        generic_resource.SignalResource._add_event(
+            'signal', 'COMPLETE', 'Signal: %s' % ret_expected).AndReturn(None)
+        self.m.ReplayAll()
+
+        rsrc.signal(details=signal_details)
+        self.m.VerifyAll()
+
     def test_signal_wrong_resource(self):
         # assert that we get the correct exception when calling a
         # resource.signal() that does not have a handle_signal()