Refactor ResponseCache usage

Adds a `.wrap` method to ResponseCache which wraps up the boilerplate of a
(get, set) pair, and then use it throughout the codebase.

This will be largely non-functional, but does include the following functional
changes:

* federation_server.on_context_state_request: drops use of _server_linearizer
  which looked redundant and could cause incorrect cache misses by yielding
  between the get and the set.
* RoomListHandler.get_remote_public_room_list(): fixes logcontext leaks
* the wrap function includes some logging. I'm hoping this won't be too noisy
  on production.

6 files changed, 87 insertions, 67 deletions

diff --git a/synapse/appservice/api.py b/synapse/appservice/api.py
index 11e9c37c63..00efff1464 100644
--- a/synapse/appservice/api.py
+++ b/synapse/appservice/api.py
@@ -18,7 +18,6 @@ from synapse.api.constants import ThirdPartyEntityKind
 from synapse.api.errors import CodeMessageException
 from synapse.http.client import SimpleHttpClient
 from synapse.events.utils import serialize_event
-from synapse.util.logcontext import preserve_fn, make_deferred_yieldable
 from synapse.util.caches.response_cache import ResponseCache
 from synapse.types import ThirdPartyInstanceID
 
@@ -194,12 +193,7 @@ class ApplicationServiceApi(SimpleHttpClient):
                 defer.returnValue(None)
 
         key = (service.id, protocol)
-        result = self.protocol_meta_cache.get(key)
-        if not result:
-            result = self.protocol_meta_cache.set(
-                key, preserve_fn(_get)()
-            )
-        return make_deferred_yieldable(result)
+        return self.protocol_meta_cache.wrap(key, _get)
 
     @defer.inlineCallbacks
     def push_bulk(self, service, events, txn_id=None):
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index e4ce037acf..d1611f39a9 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -30,7 +30,6 @@ import synapse.metrics
 from synapse.types import get_domain_from_id
 from synapse.util import async
 from synapse.util.caches.response_cache import ResponseCache
-from synapse.util.logcontext import make_deferred_yieldable, preserve_fn
 from synapse.util.logutils import log_function
 
 # when processing incoming transactions, we try to handle multiple rooms in
@@ -212,16 +211,11 @@ class FederationServer(FederationBase):
         if not in_room:
             raise AuthError(403, "Host not in room.")
 
-        result = self._state_resp_cache.get((room_id, event_id))
-        if not result:
-            with (yield self._server_linearizer.queue((origin, room_id))):
-                d = self._state_resp_cache.set(
-                    (room_id, event_id),
-                    preserve_fn(self._on_context_state_request_compute)(room_id, event_id)
-                )
-                resp = yield make_deferred_yieldable(d)
-        else:
-            resp = yield make_deferred_yieldable(result)
+        resp = yield self._state_resp_cache.wrap(
+            (room_id, event_id),
+            self._on_context_state_request_compute,
+            room_id, event_id,
+        )
 
         defer.returnValue((200, resp))
 
diff --git a/synapse/handlers/room_list.py b/synapse/handlers/room_list.py
index 8028d793c2..add3f9b009 100644
--- a/synapse/handlers/room_list.py
+++ b/synapse/handlers/room_list.py
@@ -20,7 +20,6 @@ from ._base import BaseHandler
 from synapse.api.constants import (
     EventTypes, JoinRules,
 )
-from synapse.util.logcontext import make_deferred_yieldable, preserve_fn
 from synapse.util.async import concurrently_execute
 from synapse.util.caches.descriptors import cachedInlineCallbacks
 from synapse.util.caches.response_cache import ResponseCache
@@ -78,18 +77,11 @@ class RoomListHandler(BaseHandler):
             )
 
         key = (limit, since_token, network_tuple)
-        result = self.response_cache.get(key)
-        if not result:
-            logger.info("No cached result, calculating one.")
-            result = self.response_cache.set(
-                key,
-                preserve_fn(self._get_public_room_list)(
-                    limit, since_token, network_tuple=network_tuple
-                )
-            )
-        else:
-            logger.info("Using cached deferred result.")
-        return make_deferred_yieldable(result)
+        return self.response_cache.wrap(
+            key,
+            self._get_public_room_list,
+            limit, since_token, network_tuple=network_tuple,
+        )
 
     @defer.inlineCallbacks
     def _get_public_room_list(self, limit=None, since_token=None,
@@ -423,18 +415,14 @@ class RoomListHandler(BaseHandler):
             server_name, limit, since_token, include_all_networks,
             third_party_instance_id,
         )
-        result = self.remote_response_cache.get(key)
-        if not result:
-            result = self.remote_response_cache.set(
-                key,
-                repl_layer.get_public_rooms(
-                    server_name, limit=limit, since_token=since_token,
-                    search_filter=search_filter,
-                    include_all_networks=include_all_networks,
-                    third_party_instance_id=third_party_instance_id,
-                )
-            )
-        return result
+        return self.remote_response_cache.wrap(
+            key,
+            repl_layer.get_public_rooms,
+            server_name, limit=limit, since_token=since_token,
+            search_filter=search_filter,
+            include_all_networks=include_all_networks,
+            third_party_instance_id=third_party_instance_id,
+        )
 
 
 class RoomListNextBatch(namedtuple("RoomListNextBatch", (
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 06d17ab20c..c6946831ab 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -15,7 +15,7 @@
 
 from synapse.api.constants import Membership, EventTypes
 from synapse.util.async import concurrently_execute
-from synapse.util.logcontext import LoggingContext, make_deferred_yieldable, preserve_fn
+from synapse.util.logcontext import LoggingContext
 from synapse.util.metrics import Measure, measure_func
 from synapse.util.caches.response_cache import ResponseCache
 from synapse.push.clientformat import format_push_rules_for_user
@@ -180,15 +180,11 @@ class SyncHandler(object):
         Returns:
             A Deferred SyncResult.
         """
-        result = self.response_cache.get(sync_config.request_key)
-        if not result:
-            result = self.response_cache.set(
-                sync_config.request_key,
-                preserve_fn(self._wait_for_sync_for_user)(
-                    sync_config, since_token, timeout, full_state
-                )
-            )
-        return make_deferred_yieldable(result)
+        return self.response_cache.wrap(
+            sync_config.request_key,
+            self._wait_for_sync_for_user,
+            sync_config, since_token, timeout, full_state,
+        )
 
     @defer.inlineCallbacks
     def _wait_for_sync_for_user(self, sync_config, since_token, timeout,
diff --git a/synapse/replication/http/send_event.py b/synapse/replication/http/send_event.py
index c6a6551d24..a9baa2c1c3 100644
--- a/synapse/replication/http/send_event.py
+++ b/synapse/replication/http/send_event.py
@@ -23,7 +23,6 @@ from synapse.events.snapshot import EventContext
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.util.async import sleep
 from synapse.util.caches.response_cache import ResponseCache
-from synapse.util.logcontext import make_deferred_yieldable, preserve_fn
 from synapse.util.metrics import Measure
 from synapse.types import Requester, UserID
 
@@ -118,17 +117,12 @@ class ReplicationSendEventRestServlet(RestServlet):
         self.response_cache = ResponseCache(hs, "send_event", timeout_ms=30 * 60 * 1000)
 
     def on_PUT(self, request, event_id):
-        result = self.response_cache.get(event_id)
-        if not result:
-            result = self.response_cache.set(
-                event_id,
-                self._handle_request(request)
-            )
-        else:
-            logger.warn("Returning cached response")
-        return make_deferred_yieldable(result)
-
-    @preserve_fn
+        return self.response_cache.wrap(
+            event_id,
+            self._handle_request,
+            request
+        )
+
     @defer.inlineCallbacks
     def _handle_request(self, request):
         with Measure(self.clock, "repl_send_event_parse"):
diff --git a/synapse/util/caches/response_cache.py b/synapse/util/caches/response_cache.py
index 066fa423fd..0c2c347953 100644
--- a/synapse/util/caches/response_cache.py
+++ b/synapse/util/caches/response_cache.py
@@ -12,9 +12,13 @@
 # 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 logging
 
 from synapse.util.async import ObservableDeferred
 from synapse.util.caches import metrics as cache_metrics
+from synapse.util.logcontext import make_deferred_yieldable, run_in_background
+
+logger = logging.getLogger(__name__)
 
 
 class ResponseCache(object):
@@ -31,6 +35,7 @@ class ResponseCache(object):
         self.clock = hs.get_clock()
         self.timeout_sec = timeout_ms / 1000.
 
+        self._name = name
         self._metrics = cache_metrics.register_cache(
             "response_cache",
             size_callback=lambda: self.size(),
@@ -47,7 +52,7 @@ class ResponseCache(object):
         so you'll probably want to make_deferred_yieldable it.
 
         Args:
-            key (str):
+            key (hashable):
 
         Returns:
             twisted.internet.defer.Deferred|None: None if there is no entry
@@ -76,7 +81,7 @@ class ResponseCache(object):
         to do it everywhere ResponseCache is used.)
 
         Args:
-            key (str):
+            key (hashable):
             deferred (twisted.internet.defer.Deferred):
 
         Returns:
@@ -97,3 +102,52 @@ class ResponseCache(object):
 
         result.addBoth(remove)
         return result.observe()
+
+    def wrap(self, key, callback, *args, **kwargs):
+        """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
+        follow the synapse logcontext rules and returns it.
+
+        Otherwise, makes a call to *callback(*args, **kwargs)*, which should
+        follow the synapse logcontext rules, and adds the result to the cache.
+
+        Example usage:
+
+            @defer.inlineCallbacks
+            def handle_request(request):
+                # etc
+                defer.returnValue(result)
+
+            result = yield response_cache.wrap(
+                key,
+                handle_request,
+                request,
+            )
+
+        Args:
+            key (hashable): key to get/set in the cache
+
+            callback (callable): function to call if the key is not found in
+                the cache
+
+            *args: positional parameters to pass to the callback, if it is used
+
+            **kwargs: named paramters to pass to the callback, if it is used
+
+        Returns:
+            twisted.internet.defer.Deferred: yieldable result
+        """
+        result = self.get(key)
+        if not result:
+            logger.info("[%s]: no cached result for [%s], calculating new one",
+                        self._name, key)
+            d = run_in_background(callback, *args, **kwargs)
+            result = self.set(key, d)
+        elif 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)