Performance improvements and refactor of Ratelimiter (#7595)

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

1 files changed, 7 insertions, 10 deletions

diff --git a/synapse/server.py b/synapse/server.py
index ca2deb49bb..fe94836a2c 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -242,9 +242,12 @@ class HomeServer(object):
 
         self.clock = Clock(reactor)
         self.distributor = Distributor()
-        self.ratelimiter = Ratelimiter()
-        self.admin_redaction_ratelimiter = Ratelimiter()
-        self.registration_ratelimiter = Ratelimiter()
+
+        self.registration_ratelimiter = Ratelimiter(
+            clock=self.clock,
+            rate_hz=config.rc_registration.per_second,
+            burst_count=config.rc_registration.burst_count,
+        )
 
         self.datastores = None
 
@@ -314,15 +317,9 @@ class HomeServer(object):
     def get_distributor(self):
         return self.distributor
 
-    def get_ratelimiter(self):
-        return self.ratelimiter
-
-    def get_registration_ratelimiter(self):
+    def get_registration_ratelimiter(self) -> Ratelimiter:
         return self.registration_ratelimiter
 
-    def get_admin_redaction_ratelimiter(self):
-        return self.admin_redaction_ratelimiter
-
     def build_federation_client(self):
         return FederationClient(self)