No need to cheat with the SingleAck test.

dulwich/tests/test_server.py

@@ -599,7 +599,6 @@ class SingleAckGraphWalkerImplTestCase(AckGraphWalkerImplTestCase):
         self.assertNoAck()
 
         self.assertNextEquals(ONE)
-        self._walker.done = True
         self._impl.ack(ONE)
         self.assertAck(ONE)
 
@@ -618,7 +617,6 @@ class SingleAckGraphWalkerImplTestCase(AckGraphWalkerImplTestCase):
         self.assertNoAck()
 
         self.assertNextEquals(ONE)
-        self._walker.done = True
         self._impl.ack(ONE)
         self.assertAck(ONE)