Fix overzealous cache invalidation

Fixes an issue where a cache invalidation would invalidate *all* pending
entries, rather than just the entry that we intended to invalidate.

tests/util/caches/test_descriptors.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -12,6 +13,7 @@
 # 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.
+from functools import partial
 import logging
 
 import mock
@@ -25,6 +27,50 @@ from tests import unittest
 logger = logging.getLogger(__name__)
 
 
+class CacheTestCase(unittest.TestCase):
+    def test_invalidate_all(self):
+        cache = descriptors.Cache("testcache")
+
+        callback_record = [False, False]
+
+        def record_callback(idx):
+            callback_record[idx] = True
+
+        # add a couple of pending entries
+        d1 = defer.Deferred()
+        cache.set("key1", d1, partial(record_callback, 0))
+
+        d2 = defer.Deferred()
+        cache.set("key2", d2, partial(record_callback, 1))
+
+        # lookup should return the deferreds
+        self.assertIs(cache.get("key1"), d1)
+        self.assertIs(cache.get("key2"), d2)
+
+        # let one of the lookups complete
+        d2.callback("result2")
+        self.assertEqual(cache.get("key2"), "result2")
+
+        # now do the invalidation
+        cache.invalidate_all()
+
+        # lookup should return none
+        self.assertIsNone(cache.get("key1", None))
+        self.assertIsNone(cache.get("key2", None))
+
+        # both callbacks should have been callbacked
+        self.assertTrue(
+            callback_record[0], "Invalidation callback for key1 not called",
+        )
+        self.assertTrue(
+            callback_record[1], "Invalidation callback for key2 not called",
+        )
+
+        # letting the other lookup complete should do nothing
+        d1.callback("result1")
+        self.assertIsNone(cache.get("key1", None))
+
+
 class DescriptorTestCase(unittest.TestCase):
     @defer.inlineCallbacks
     def test_cache(self):