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/storage/test_keys.py

@@ -0,0 +1,53 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 Vector Creations 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 signedjson.key
+from twisted.internet import defer
+
+import tests.unittest
+import tests.utils
+
+
+class KeyStoreTestCase(tests.unittest.TestCase):
+    def __init__(self, *args, **kwargs):
+        super(KeyStoreTestCase, self).__init__(*args, **kwargs)
+        self.store = None  # type: synapse.storage.keys.KeyStore
+
+    @defer.inlineCallbacks
+    def setUp(self):
+        hs = yield tests.utils.setup_test_homeserver()
+        self.store = hs.get_datastore()
+
+    @defer.inlineCallbacks
+    def test_get_server_verify_keys(self):
+        key1 = signedjson.key.decode_verify_key_base64(
+            "ed25519", "key1", "fP5l4JzpZPq/zdbBg5xx6lQGAAOM9/3w94cqiJ5jPrw"
+        )
+        key2 = signedjson.key.decode_verify_key_base64(
+            "ed25519", "key2", "Noi6WqcDj0QmPxCNQqgezwTlBKrfqehY1u2FyWP9uYw"
+        )
+        yield self.store.store_server_verify_key(
+            "server1", "from_server", 0, key1
+        )
+        yield self.store.store_server_verify_key(
+            "server1", "from_server", 0, key2
+        )
+
+        res = yield self.store.get_server_verify_keys(
+            "server1", ["ed25519:key1", "ed25519:key2", "ed25519:key3"])
+
+        self.assertEqual(len(res.keys()), 2)
+        self.assertEqual(res["ed25519:key1"].version, "key1")
+        self.assertEqual(res["ed25519:key2"].version, "key2")