Allow @cached-wrapped functions to have a prefill method for setting entries

1 files changed, 15 insertions, 8 deletions

diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 78ba5f25ea..4b1ec687c9 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -48,24 +48,30 @@ def cached(max_entries=1000):
 
     The wrapped function has an additional member, a callable called
     "invalidate". This can be used to remove individual entries from the cache.
+
+    The wrapped function has another additional callable, called "prefill",
+    which can be used to insert values into the cache specifically, without
+    calling the calculation function.
     """
     def wrap(orig):
         cache = {}
 
-        @defer.inlineCallbacks
-        def wrapped(self, key):
-            if key in cache:
-                defer.returnValue(cache[key])
-
-            ret = yield orig(self, key)
-
+        def prefill(key, value):
             while len(cache) > max_entries:
                 # TODO(paul): This feels too biased. However, a random index
                 #   would be a bit inefficient, walking the list of keys just
                 #   to ignore most of them?
                 del cache[cache.keys()[0]]
 
-            cache[key] = ret;
+            cache[key] = value
+
+        @defer.inlineCallbacks
+        def wrapped(self, key):
+            if key in cache:
+                defer.returnValue(cache[key])
+
+            ret = yield orig(self, key)
+            prefill(key, ret)
             defer.returnValue(ret)
 
         def invalidate(key):
@@ -73,6 +79,7 @@ def cached(max_entries=1000):
                 del cache[key]
 
         wrapped.invalidate = invalidate
+        wrapped.prefill    = prefill
         return wrapped
 
     return wrap