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.

1 files changed, 5 insertions, 11 deletions

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))