2 files changed, 10 insertions, 3 deletions
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 675bfd5feb..998de70d29 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -478,6 +478,11 @@ class CacheListDescriptor(object):
 
 
 class _CacheContext(namedtuple("_CacheContext", ("cache", "key"))):
+    # We rely on _CacheContext implementing __eq__ and __hash__ sensibly,
+    # which namedtuple does for us (i.e. two _CacheContext are the same if
+    # their caches and keys match). This is important in particular to
+    # dedupe when we add callbacks to lru cache nodes, otherwise the number
+    # of callbacks would grow.
     def invalidate(self):
         self.cache.invalidate(self.key)
 
diff --git a/synapse/util/retryutils.py b/synapse/util/retryutils.py
index b94ae369cf..153ef001ad 100644
--- a/synapse/util/retryutils.py
+++ b/synapse/util/retryutils.py
@@ -129,11 +129,13 @@ class RetryDestinationLimiter(object):
             # APIs may expect to never received e.g. a 404. It's important to
             # handle 404 as some remote servers will return a 404 when the HS
             # has been decommissioned.
+            # If we get a 401, then we should probably back off since they
+            # won't accept our requests for at least a while.
+            # 429 is us being aggresively rate limited, so lets rate limit
+            # ourselves.
             if exc_val.code == 404 and self.backoff_on_404:
                 valid_err_code = False
-            elif exc_val.code == 429:
-                # 429 is us being aggresively rate limited, so lets rate limit
-                # ourselves.
+            elif exc_val.code in (401, 429):
                 valid_err_code = False
             elif exc_val.code < 500:
                 valid_err_code = True