Fix a long-standing bug which meant that rate limiting was not restrictive enough in some cases. (#13018)

synapse/api/ratelimiting.py

@@ -128,6 +128,9 @@ class Ratelimiter:
         performed_count = action_count - time_delta * rate_hz
         if performed_count < 0:
             performed_count = 0
+
+            # Reset the start time and forgive all actions
+            action_count = 0
             time_start = time_now_s
 
         # This check would be easier read as performed_count + n_actions > burst_count,
@@ -140,7 +143,7 @@ class Ratelimiter:
         else:
             # We haven't reached our limit yet
             allowed = True
-            action_count = performed_count + n_actions
+            action_count = action_count + n_actions
 
         if update:
             self.actions[key] = (action_count, time_start, rate_hz)