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.

Author: richvdh

changelog.d/10157.misc

@@ -0,0 +1 @@
+Extend `ResponseCache` to pass a context object into the callback.

synapse/replication/http/_base.py

@@ -285,7 +285,7 @@ def register(self, http_server):
             self.__class__.__name__,
         )
 
-    def _check_auth_and_handle(self, request, **kwargs):
+    async def _check_auth_and_handle(self, request, **kwargs):
         """Called on new incoming requests when caching is enabled. Checks
         if there is a cached response for the request and returns that,
         otherwise calls `_handle_request` and caches its response.
@@ -300,8 +300,8 @@ def _check_auth_and_handle(self, request, **kwargs):
         if self.CACHE:
             txn_id = kwargs.pop("txn_id")
 
-            return self.response_cache.wrap(
+            return await self.response_cache.wrap(
                 txn_id, self._handle_request, request, **kwargs
             )
 
-        return self._handle_request(request, **kwargs)
+        return await self._handle_request(request, **kwargs)

synapse/replication/http/membership.py

@@ -345,7 +345,7 @@ async def _serialize_payload(  # type: ignore
 
         return {}
 
-    def _handle_request(  # type: ignore
+    async def _handle_request(  # type: ignore
         self, request: Request, room_id: str, user_id: str, change: str
     ) -> Tuple[int, JsonDict]:
         logger.info("user membership change: %s in %s", user_id, room_id)

synapse/util/caches/response_cache.py

@@ -12,7 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Any, Callable, Dict, Generic, Optional, TypeVar
+from typing import Any, Awaitable, Callable, Dict, Generic, Optional, TypeVar
+
+import attr
 
 from twisted.internet import defer
 
@@ -23,10 +25,36 @@
 
 logger = logging.getLogger(__name__)
 
-T = TypeVar("T")
+# the type of the key in the cache
+KV = TypeVar("KV")
+
+# the type of the result from the operation
+RV = TypeVar("RV")
+
 
+@attr.s(auto_attribs=True)
+class ResponseCacheContext(Generic[KV]):
+    """Information about a missed ResponseCache hit
 
-class ResponseCache(Generic[T]):
+    This object can be passed into the callback for additional feedback
+    """
+
+    cache_key: KV
+    """The cache key that caused the cache miss
+
+    This should be considered read-only.
+
+    TODO: in attrs 20.1, make it frozen with an on_setattr.
+    """
+
+    should_cache: bool = True
+    """Whether the result should be cached once the request completes.
+
+    This can be modified by the callback if it decides its result should not be cached.
+    """
+
+
+class ResponseCache(Generic[KV]):
     """
     This caches a deferred response. Until the deferred completes it will be
     returned from the cache. This means that if the client retries the request
@@ -35,8 +63,10 @@ class ResponseCache(Generic[T]):
     """
 
     def __init__(self, clock: Clock, name: str, timeout_ms: float = 0):
-        # Requests that haven't finished yet.
-        self.pending_result_cache = {}  # type: Dict[T, ObservableDeferred]
+        # This is poorly-named: it includes both complete and incomplete results.
+        # We keep complete results rather than switching to absolute values because
+        # that makes it easier to cache Failure results.
+        self.pending_result_cache = {}  # type: Dict[KV, ObservableDeferred]
 
         self.clock = clock
         self.timeout_sec = timeout_ms / 1000.0
@@ -50,16 +80,13 @@ def size(self) -> int:
     def __len__(self) -> int:
         return self.size()
 
-    def get(self, key: T) -> Optional[defer.Deferred]:
+    def get(self, key: KV) -> Optional[defer.Deferred]:
         """Look up the given key.
 
-        Can return either a new Deferred (which also doesn't follow the synapse
-        logcontext rules), or, if the request has completed, the actual
-        result. You will probably want to make_deferred_yieldable the result.
+        Returns a new Deferred (which also doesn't follow the synapse
+        logcontext rules). You will probably want to make_deferred_yieldable the result.
 
-        If there is no entry for the key, returns None. It is worth noting that
-        this means there is no way to distinguish a completed result of None
-        from an absent cache entry.
+        If there is no entry for the key, returns None.
 
         Args:
             key: key to get/set in the cache
@@ -76,42 +103,56 @@ def get(self, key: T) -> Optional[defer.Deferred]:
             self._metrics.inc_misses()
             return None
 
-    def set(self, key: T, deferred: defer.Deferred) -> defer.Deferred:
+    def _set(
+        self, context: ResponseCacheContext[KV], deferred: defer.Deferred
+    ) -> defer.Deferred:
         """Set the entry for the given key to the given deferred.
 
         *deferred* should run its callbacks in the sentinel logcontext (ie,
         you should wrap normal synapse deferreds with
         synapse.logging.context.run_in_background).
 
-        Can return either a new Deferred (which also doesn't follow the synapse
-        logcontext rules), or, if *deferred* was already complete, the actual
-        result. You will probably want to make_deferred_yieldable the result.
+        Returns a new Deferred (which also doesn't follow the synapse logcontext rules).
+        You will probably want to make_deferred_yieldable the result.
 
         Args:
-            key: key to get/set in the cache
+            context: Information about the cache miss
             deferred: The deferred which resolves to the result.
 
         Returns:
             A new deferred which resolves to the actual result.
         """
         result = ObservableDeferred(deferred, consumeErrors=True)
+        key = context.cache_key
         self.pending_result_cache[key] = result
 
-        def remove(r):
-            if self.timeout_sec:
+        def on_complete(r):
+            # if this cache has a non-zero timeout, and the callback has not cleared
+            # the should_cache bit, we leave it in the cache for now and schedule
+            # its removal later.
+            if self.timeout_sec and context.should_cache:
                 self.clock.call_later(
                     self.timeout_sec, self.pending_result_cache.pop, key, None
                 )
             else:
+                # otherwise, remove the result immediately.
                 self.pending_result_cache.pop(key, None)
             return r
 
-        result.addBoth(remove)
+        # make sure we do this *after* adding the entry to pending_result_cache,
+        # in case the result is already complete (in which case flipping the order would
+        # leave us with a stuck entry in the cache).
+        result.addBoth(on_complete)
         return result.observe()
 
-    def wrap(
-        self, key: T, callback: Callable[..., Any], *args: Any, **kwargs: Any
-    ) -> defer.Deferred:
+    async def wrap(
+        self,
+        key: KV,
+        callback: Callable[..., Awaitable[RV]],
+        *args: Any,
+        cache_context: bool = False,
+        **kwargs: Any,
+    ) -> RV:
         """Wrap together a *get* and *set* call, taking care of logcontexts
 
         First looks up the key in the cache, and if it is present makes it
@@ -140,22 +181,28 @@ async def handle_request(request):
 
             *args: positional parameters to pass to the callback, if it is used
 
+            cache_context: if set, the callback will be given a `cache_context` kw arg,
+                which will be a ResponseCacheContext object.
+
             **kwargs: named parameters to pass to the callback, if it is used
 
         Returns:
-            Deferred which resolves to the result
+            The result of the callback (from the cache, or otherwise)
         """
         result = self.get(key)
         if not result:
             logger.debug(
                 "[%s]: no cached result for [%s], calculating new one", self._name, key
             )
+            context = ResponseCacheContext(cache_key=key)
+            if cache_context:
+                kwargs["cache_context"] = context
             d = run_in_background(callback, *args, **kwargs)
-            result = self.set(key, d)
+            result = self._set(context, d)
         elif not isinstance(result, defer.Deferred) or result.called:
             logger.info("[%s]: using completed cached result for [%s]", self._name, key)
         else:
             logger.info(
                 "[%s]: using incomplete cached result for [%s]", self._name, key
             )
-        return make_deferred_yieldable(result)
+        return await make_deferred_yieldable(result)

tests/util/caches/test_responsecache.py renamed to 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 @@ def test_cache_hit(self):
 
         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 @@ def test_cache_miss(self):
 
         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 @@ def test_cache_expire(self):
 
         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 @@ def test_cache_wait_hit(self):
 
         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 @@ def test_cache_wait_expire(self):
 
         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 @@ def test_cache_wait_expire(self):
         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")