Fix background_update tests

A bit of a cleanup for background_updates, and make sure that the real
background updates have run before we start the unit tests, so that they don't
interfere with the tests.

synapse/storage/background_updates.py

@@ -88,10 +88,12 @@ class BackgroundUpdateStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def start_doing_background_updates(self):
-        while True:
-            if self._background_update_timer is not None:
-                return
+        assert(self._background_update_timer is not None,
+               "background updates already running")
+
+        logger.info("Starting background schema updates")
 
+        while True:
             sleep = defer.Deferred()
             self._background_update_timer = self._clock.call_later(
                 self.BACKGROUND_UPDATE_INTERVAL_MS / 1000., sleep.callback, None
@@ -102,7 +104,7 @@ class BackgroundUpdateStore(SQLBaseStore):
                 self._background_update_timer = None
 
             try:
-                result = yield self.do_background_update(
+                result = yield self.do_next_background_update(
                     self.BACKGROUND_UPDATE_DURATION_MS
                 )
             except:
@@ -113,11 +115,12 @@ class BackgroundUpdateStore(SQLBaseStore):
                         "No more background updates to do."
                         " Unscheduling background update task."
                     )
-                    return
+                    defer.returnValue()
 
     @defer.inlineCallbacks
-    def do_background_update(self, desired_duration_ms):
-        """Does some amount of work on a background update
+    def do_next_background_update(self, desired_duration_ms):
+        """Does some amount of work on the next queued background update
+
         Args:
             desired_duration_ms(float): How long we want to spend
                 updating.
@@ -136,11 +139,21 @@ class BackgroundUpdateStore(SQLBaseStore):
                 self._background_update_queue.append(update['update_name'])
 
         if not self._background_update_queue:
+            # no work left to do
             defer.returnValue(None)
 
+        # pop from the front, and add back to the back
         update_name = self._background_update_queue.pop(0)
         self._background_update_queue.append(update_name)
 
+        res = yield self._do_background_update(update_name, desired_duration_ms)
+        defer.returnValue(res)
+
+    @defer.inlineCallbacks
+    def _do_background_update(self, update_name, desired_duration_ms):
+        logger.info("Starting update batch on background update '%s'",
+                    update_name)
+
         update_handler = self._background_update_handlers[update_name]
 
         performance = self._background_update_performance.get(update_name)

tests/storage/test_background_update.py

@@ -10,7 +10,7 @@ class BackgroundUpdateTestCase(unittest.TestCase):
 
     @defer.inlineCallbacks
     def setUp(self):
-        hs = yield setup_test_homeserver()
+        hs = yield setup_test_homeserver()  # type: synapse.server.HomeServer
         self.store = hs.get_datastore()
         self.clock = hs.get_clock()
 
@@ -20,11 +20,20 @@ class BackgroundUpdateTestCase(unittest.TestCase):
             "test_update", self.update_handler
         )
 
+        # run the real background updates, to get them out the way
+        # (perhaps we should run them as part of the test HS setup, since we
+        # run all of the other schema setup stuff there?)
+        while True:
+            res = yield self.store.do_next_background_update(1000)
+            if res is None:
+                break
+
     @defer.inlineCallbacks
     def test_do_background_update(self):
         desired_count = 1000
         duration_ms = 42
 
+        # first step: make a bit of progress
         @defer.inlineCallbacks
         def update(progress, count):
             self.clock.advance_time_msec(count * duration_ms)
@@ -42,7 +51,7 @@ class BackgroundUpdateTestCase(unittest.TestCase):
         yield self.store.start_background_update("test_update", {"my_key": 1})
 
         self.update_handler.reset_mock()
-        result = yield self.store.do_background_update(
+        result = yield self.store.do_next_background_update(
             duration_ms * desired_count
         )
         self.assertIsNotNone(result)
@@ -50,24 +59,25 @@ class BackgroundUpdateTestCase(unittest.TestCase):
             {"my_key": 1}, self.store.DEFAULT_BACKGROUND_BATCH_SIZE
         )
 
+        # second step: complete the update
         @defer.inlineCallbacks
         def update(progress, count):
             yield self.store._end_background_update("test_update")
             defer.returnValue(count)
 
         self.update_handler.side_effect = update
-
         self.update_handler.reset_mock()
-        result = yield self.store.do_background_update(
-            duration_ms * desired_count
+        result = yield self.store.do_next_background_update(
+           duration_ms * desired_count
         )
         self.assertIsNotNone(result)
         self.update_handler.assert_called_once_with(
             {"my_key": 2}, desired_count
         )
 
+        # third step: we don't expect to be called any more
         self.update_handler.reset_mock()
-        result = yield self.store.do_background_update(
+        result = yield self.store.do_next_background_update(
             duration_ms * desired_count
         )
         self.assertIsNone(result)