Extend `ResponseCache` to pass a context object into the callback (#10157)

This is the first of two PRs which seek to address #8518. This first PR lays the groundwork by extending ResponseCache; a second PR (#10158) will update the SyncHandler to actually use it, and fix the bug.

The idea here is that we allow the callback given to ResponseCache.wrap to decide whether its result should be cached or not. We do that by (optionally) passing a ResponseCacheContext into it, which it can modify.

1 files changed, 68 insertions, 7 deletions

diff --git a/tests/util/caches/test_responsecache.py b/tests/util/caches/test_response_cache.py

index f9a187b8de..1e83ef2f33 100644
--- a/tests/util/caches/test_responsecache.py
+++ b/tests/util/caches/test_response_cache.py
@@ -11,14 +11,17 @@
 # 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 parameterized import parameterized
 
-from synapse.util.caches.response_cache import ResponseCache
+from twisted.internet import defer
+
+from synapse.util.caches.response_cache import ResponseCache, ResponseCacheContext
 
 from tests.server import get_clock
 from tests.unittest import TestCase
 
 
-class DeferredCacheTestCase(TestCase):
+class ResponseCacheTestCase(TestCase):
     """
     A TestCase class for ResponseCache.
 
@@ -48,7 +51,9 @@ class DeferredCacheTestCase(TestCase):
 
         expected_result = "howdy"
 
-        wrap_d = cache.wrap(0, self.instant_return, expected_result)
+        wrap_d = defer.ensureDeferred(
+            cache.wrap(0, self.instant_return, expected_result)
+        )
 
         self.assertEqual(
             expected_result,
@@ -66,7 +71,9 @@ class DeferredCacheTestCase(TestCase):
 
         expected_result = "howdy"
 
-        wrap_d = cache.wrap(0, self.instant_return, expected_result)
+        wrap_d = defer.ensureDeferred(
+            cache.wrap(0, self.instant_return, expected_result)
+        )
 
         self.assertEqual(
             expected_result,
@@ -80,7 +87,9 @@ class DeferredCacheTestCase(TestCase):
 
         expected_result = "howdy"
 
-        wrap_d = cache.wrap(0, self.instant_return, expected_result)
+        wrap_d = defer.ensureDeferred(
+            cache.wrap(0, self.instant_return, expected_result)
+        )
 
         self.assertEqual(expected_result, self.successResultOf(wrap_d))
         self.assertEqual(
@@ -99,7 +108,10 @@ class DeferredCacheTestCase(TestCase):
 
         expected_result = "howdy"
 
-        wrap_d = cache.wrap(0, self.delayed_return, expected_result)
+        wrap_d = defer.ensureDeferred(
+            cache.wrap(0, self.delayed_return, expected_result)
+        )
+
         self.assertNoResult(wrap_d)
 
         # function wakes up, returns result
@@ -112,7 +124,9 @@ class DeferredCacheTestCase(TestCase):
 
         expected_result = "howdy"
 
-        wrap_d = cache.wrap(0, self.delayed_return, expected_result)
+        wrap_d = defer.ensureDeferred(
+            cache.wrap(0, self.delayed_return, expected_result)
+        )
         self.assertNoResult(wrap_d)
 
         # stop at 1 second to callback cache eviction callLater at that time, then another to set time at 2
@@ -129,3 +143,50 @@ class DeferredCacheTestCase(TestCase):
         self.reactor.pump((2,))
 
         self.assertIsNone(cache.get(0), "cache should not have the result now")
+
+    @parameterized.expand([(True,), (False,)])
+    def test_cache_context_nocache(self, should_cache: bool):
+        """If the callback clears the should_cache bit, the result should not be cached"""
+        cache = self.with_cache("medium_cache", ms=3000)
+
+        expected_result = "howdy"
+
+        call_count = 0
+
+        async def non_caching(o: str, cache_context: ResponseCacheContext[int]):
+            nonlocal call_count
+            call_count += 1
+            await self.clock.sleep(1)
+            cache_context.should_cache = should_cache
+            return o
+
+        wrap_d = defer.ensureDeferred(
+            cache.wrap(0, non_caching, expected_result, cache_context=True)
+        )
+        # there should be no result to start with
+        self.assertNoResult(wrap_d)
+
+        # a second call should also return a pending deferred
+        wrap2_d = defer.ensureDeferred(
+            cache.wrap(0, non_caching, expected_result, cache_context=True)
+        )
+        self.assertNoResult(wrap2_d)
+
+        # and there should have been exactly one call
+        self.assertEqual(call_count, 1)
+
+        # let the call complete
+        self.reactor.advance(1)
+
+        # both results should have completed
+        self.assertEqual(expected_result, self.successResultOf(wrap_d))
+        self.assertEqual(expected_result, self.successResultOf(wrap2_d))
+
+        if should_cache:
+            self.assertEqual(
+                expected_result,
+                self.successResultOf(cache.get(0)),
+                "cache should still have the result",
+            )
+        else:
+            self.assertIsNone(cache.get(0), "cache should not have the result")