1 files changed, 15 insertions, 3 deletions
diff --git a/synapse/util/caches/deferred_cache.py b/synapse/util/caches/deferred_cache.py
index 6c162e9f34..601305487c 100644
--- a/synapse/util/caches/deferred_cache.py
+++ b/synapse/util/caches/deferred_cache.py
@@ -31,6 +31,7 @@ from typing import (
 from prometheus_client import Gauge
 
 from twisted.internet import defer
+from twisted.python import failure
 
 from synapse.util.async_helpers import ObservableDeferred
 from synapse.util.caches.lrucache import LruCache
@@ -214,9 +215,6 @@ class DeferredCache(Generic[KT, VT]):
 
         callbacks = [callback] if callback else []
         self.check_thread()
-        observable = ObservableDeferred(value, consumeErrors=True)
-        observer = observable.observe()
-        entry = CacheEntry(deferred=observable, callbacks=callbacks)
 
         existing_entry = self._pending_deferred_cache.pop(key, None)
         if existing_entry:
@@ -224,6 +222,20 @@ class DeferredCache(Generic[KT, VT]):
 
         # XXX: why don't we invalidate the entry in `self.cache` yet?
 
+        # we can save a whole load of effort if the deferred is ready.
+        if value.called:
+            result = value.result
+            if not isinstance(result, failure.Failure):
+                self.cache.set(key, result, callbacks)
+            return value
+
+        # otherwise, we'll add an entry to the _pending_deferred_cache for now,
+        # and add callbacks to add it to the cache properly later.
+
+        observable = ObservableDeferred(value, consumeErrors=True)
+        observer = observable.observe()
+        entry = CacheEntry(deferred=observable, callbacks=callbacks)
+
         self._pending_deferred_cache[key] = entry
 
         def compare_and_pop():