From b28aaeb3a567ce1bbfd41796362b8bb0813ed0e3 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Wed, 21 Oct 2020 22:57:45 +0100
Subject: [PATCH] Optimise CacheDescriptor (#8594)

don't bother constricting a CacheContext unless we need one.
---
 changelog.d/8594.misc              |  1 +
 synapse/util/caches/descriptors.py | 12 +++++++-----
 2 files changed, 8 insertions(+), 5 deletions(-)
 create mode 100644 changelog.d/8594.misc

diff --git a/changelog.d/8594.misc b/changelog.d/8594.misc
new file mode 100644
index 000000000..d266ba19a
--- /dev/null
+++ b/changelog.d/8594.misc
@@ -0,0 +1 @@
+Minor optimisations in caching code.
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index a4172345e..5d7fffee6 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -201,14 +201,16 @@ class CacheDescriptor(_CacheDescriptorBase):
 
             cache_key = get_cache_key(args, kwargs)
 
-            # Add our own `cache_context` to argument list if the wrapped function
-            # has asked for one
-            if self.add_cache_context:
-                kwargs["cache_context"] = _CacheContext.get_instance(cache, cache_key)
-
             try:
                 ret = cache.get(cache_key, callback=invalidate_callback)
             except KeyError:
+                # Add our own `cache_context` to argument list if the wrapped function
+                # has asked for one
+                if self.add_cache_context:
+                    kwargs["cache_context"] = _CacheContext.get_instance(
+                        cache, cache_key
+                    )
+
                 ret = defer.maybeDeferred(preserve_fn(self.orig), obj, *args, **kwargs)
                 ret = cache.set(cache_key, ret, callback=invalidate_callback)