A unittest for the metaclass.

src/allmydata/protocol_switch.py

@@ -31,13 +31,15 @@ from .storage.server import StorageServer
 
 class _PretendToBeNegotiation(type):
     """
-    Metaclass that allows ``_FoolscapOrHttps`` to pretend to be a ``Negotiation``
-    instance, since Foolscap has some ``assert isinstance(protocol,
-    Negotiation`` checks.
+    Metaclass that allows ``_FoolscapOrHttps`` to pretend to be a
+    ``Negotiation`` instance, since Foolscap does some checks like
+    ``assert isinstance(protocol, tub.negotiationClass)`` in its internals,
+    and sometimes that ``protocol`` is a ``_FoolscapOrHttps`` instance, but
+    sometimes it's a ``Negotiation`` instance.
     """
 
     def __instancecheck__(self, instance):
-        return (instance.__class__ == self) or isinstance(instance, Negotiation)
+        return issubclass(instance.__class__, self) or isinstance(instance, Negotiation)
 
 
 class _FoolscapOrHttps(Protocol, metaclass=_PretendToBeNegotiation):

src/allmydata/test/test_protocol_switch.py

@@ -0,0 +1,43 @@
+"""
+Unit tests for ``allmydata.protocol_switch``.
+
+By its nature, most of the testing needs to be end-to-end; essentially any test
+that uses real Foolscap (``test_system.py``, integration tests) ensures
+Foolscap still works.  ``test_istorageserver.py`` tests the HTTP support.
+"""
+
+from foolscap.negotiate import Negotiation
+
+from .common import TestCase
+from ..protocol_switch import _PretendToBeNegotiation
+
+
+class UtilityTests(TestCase):
+    """Tests for utilities in the protocol switch code."""
+
+    def test_metaclass(self):
+        """
+        A class that has the ``_PretendToBeNegotiation`` metaclass will support
+        ``isinstance()``'s normal semantics on its own instances, but will also
+        indicate that ``Negotiation`` instances are its instances.
+        """
+
+        class Parent(metaclass=_PretendToBeNegotiation):
+            pass
+
+        class Child(Parent):
+            pass
+
+        class Other:
+            pass
+
+        p = Parent()
+        self.assertIsInstance(p, Parent)
+        self.assertIsInstance(Negotiation(), Parent)
+        self.assertNotIsInstance(Other(), Parent)
+
+        c = Child()
+        self.assertIsInstance(c, Child)
+        self.assertIsInstance(c, Parent)
+        self.assertIsInstance(Negotiation(), Child)
+        self.assertNotIsInstance(Other(), Child)