From 2b3af01791120de5d0b829395109b42870d9e465 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Fri, 16 Oct 2020 13:16:02 +0100
Subject: [PATCH] optimise DeferredCache.set

---
 changelog.d/8593.misc                 |  1 +
 synapse/util/caches/deferred_cache.py | 15 ++++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)
 create mode 100644 changelog.d/8593.misc

diff --git a/changelog.d/8593.misc b/changelog.d/8593.misc
new file mode 100644
index 0000000000..d266ba19a4
--- /dev/null
+++ b/changelog.d/8593.misc
@@ -0,0 +1 @@
+Minor optimisations in caching code.
diff --git a/synapse/util/caches/deferred_cache.py b/synapse/util/caches/deferred_cache.py
index 6c162e9f34..fc01026285 100644
--- a/synapse/util/caches/deferred_cache.py
+++ b/synapse/util/caches/deferred_cache.py
@@ -214,9 +214,6 @@ 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 +221,18 @@ 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:
+            self.cache.set(key, value.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():