Fix caching of remote servers' signature keys

The `@cached` decorator on `KeyStore._get_server_verify_key` was missing
its `num_args` parameter, which meant that it was returning the wrong key for
any server which had more than one recorded key.

By way of a fix, change the default for `num_args` to be *all* arguments. To
implement that, factor out a common base class for `CacheDescriptor` and `CacheListDescriptor`.

tests/util/caches/test_descriptors.py

@@ -0,0 +1,86 @@
+# -*- coding: utf-8 -*-
+# Copyright 2016 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import mock
+from twisted.internet import defer
+from synapse.util.caches import descriptors
+from tests import unittest
+
+
+class DescriptorTestCase(unittest.TestCase):
+    @defer.inlineCallbacks
+    def test_cache(self):
+        class Cls(object):
+            def __init__(self):
+                self.mock = mock.Mock()
+
+            @descriptors.cached()
+            def fn(self, arg1, arg2):
+                return self.mock(arg1, arg2)
+
+        obj = Cls()
+
+        obj.mock.return_value = 'fish'
+        r = yield obj.fn(1, 2)
+        self.assertEqual(r, 'fish')
+        obj.mock.assert_called_once_with(1, 2)
+        obj.mock.reset_mock()
+
+        # a call with different params should call the mock again
+        obj.mock.return_value = 'chips'
+        r = yield obj.fn(1, 3)
+        self.assertEqual(r, 'chips')
+        obj.mock.assert_called_once_with(1, 3)
+        obj.mock.reset_mock()
+
+        # the two values should now be cached
+        r = yield obj.fn(1, 2)
+        self.assertEqual(r, 'fish')
+        r = yield obj.fn(1, 3)
+        self.assertEqual(r, 'chips')
+        obj.mock.assert_not_called()
+
+    @defer.inlineCallbacks
+    def test_cache_num_args(self):
+        """Only the first num_args arguments should matter to the cache"""
+
+        class Cls(object):
+            def __init__(self):
+                self.mock = mock.Mock()
+
+            @descriptors.cached(num_args=1)
+            def fn(self, arg1, arg2):
+                return self.mock(arg1, arg2)
+
+        obj = Cls()
+        obj.mock.return_value = 'fish'
+        r = yield obj.fn(1, 2)
+        self.assertEqual(r, 'fish')
+        obj.mock.assert_called_once_with(1, 2)
+        obj.mock.reset_mock()
+
+        # a call with different params should call the mock again
+        obj.mock.return_value = 'chips'
+        r = yield obj.fn(2, 3)
+        self.assertEqual(r, 'chips')
+        obj.mock.assert_called_once_with(2, 3)
+        obj.mock.reset_mock()
+
+        # the two values should now be cached; we should be able to vary
+        # the second argument and still get the cached result.
+        r = yield obj.fn(1, 4)
+        self.assertEqual(r, 'fish')
+        r = yield obj.fn(2, 5)
+        self.assertEqual(r, 'chips')
+        obj.mock.assert_not_called()