text: from mock import Mock
  
  from twisted.internet import defer
  
  from synapse.storage.background_updates import BackgroundUpdater
  
  from tests import unittest
  
  
  class BackgroundUpdateTestCase(unittest.HomeserverTestCase):
      def prepare(self, reactor, clock, homeserver):
          self.updates = (
              self.hs.get_datastore().db_pool.updates
          )  # type: BackgroundUpdater
          # the base test class should have run the real bg updates for us
          self.assertTrue(
              self.get_success(self.updates.has_completed_background_updates())
          )
  
          self.update_handler = Mock()
          self.updates.register_background_update_handler(
              "test_update", self.update_handler
          )
  
      def test_do_background_update(self):
          # the time we claim each update takes
          duration_ms = 42
  
          # the target runtime for each bg update
          target_background_update_duration_ms = 50000
  
          store = self.hs.get_datastore()
          self.get_success(
              store.db_pool.simple_insert(
                  "background_updates",
                  values={"update_name": "test_update", "progress_json": '{"my_key": 1}'},
              )
          )
  
          # first step: make a bit of progress
          @defer.inlineCallbacks
          def update(progress, count):
              yield self.clock.sleep((count * duration_ms) / 1000)
              progress = {"my_key": progress["my_key"] + 1}
              yield store.db_pool.runInteraction(
                  "update_progress",
                  self.updates._background_update_progress_txn,
                  "test_update",
                  progress,
              )
              return count
  
          self.update_handler.side_effect = update
          self.update_handler.reset_mock()
          res = self.get_success(
              self.updates.do_next_background_update(
                  target_background_update_duration_ms
              ),
              by=0.1,
          )
          self.assertFalse(res)
  
          # on the first call, we should get run with the default background update size
          self.update_handler.assert_called_once_with(
              {"my_key": 1}, self.updates.DEFAULT_BACKGROUND_BATCH_SIZE
          )
  
          # second step: complete the update
          # we should now get run with a much bigger number of items to update
          @defer.inlineCallbacks
          def update(progress, count):
              self.assertEqual(progress, {"my_key": 2})
              self.assertAlmostEqual(
                  count, target_background_update_duration_ms / duration_ms, places=0,
              )
              yield self.updates._end_background_update("test_update")
              return count
  
          self.update_handler.side_effect = update
          self.update_handler.reset_mock()
          result = self.get_success(
              self.updates.do_next_background_update(target_background_update_duration_ms)
          )
          self.assertFalse(result)
          self.update_handler.assert_called_once()
  
          # third step: we don't expect to be called any more
          self.update_handler.reset_mock()
          result = self.get_success(
              self.updates.do_next_background_update(target_background_update_duration_ms)
          )
          self.assertTrue(result)
          self.assertFalse(self.update_handler.called)
  