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, 3 insertions, 6 deletions

diff --git a/tests/rest/client/v2_alpha/test_register.py b/tests/rest/client/v2_alpha/test_register.py

index 5637ce2090..7deaf5b24a 100644
--- a/tests/rest/client/v2_alpha/test_register.py
+++ b/tests/rest/client/v2_alpha/test_register.py
@@ -29,6 +29,7 @@ from synapse.rest.client.v1 import login, logout
 from synapse.rest.client.v2_alpha import account, account_validity, register, sync
 
 from tests import unittest
+from tests.unittest import override_config
 
 
 class RegisterRestServletTestCase(unittest.HomeserverTestCase):
@@ -146,10 +147,8 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEquals(channel.result["code"], b"403", channel.result)
         self.assertEquals(channel.json_body["error"], "Guest access is disabled")
 
+    @override_config({"rc_registration": {"per_second": 0.17, "burst_count": 5}})
     def test_POST_ratelimiting_guest(self):
-        self.hs.config.rc_registration.burst_count = 5
-        self.hs.config.rc_registration.per_second = 0.17
-
         for i in range(0, 6):
             url = self.url + b"?kind=guest"
             request, channel = self.make_request(b"POST", url, b"{}")
@@ -168,10 +167,8 @@ class RegisterRestServletTestCase(unittest.HomeserverTestCase):
 
         self.assertEquals(channel.result["code"], b"200", channel.result)
 
+    @override_config({"rc_registration": {"per_second": 0.17, "burst_count": 5}})
     def test_POST_ratelimiting(self):
-        self.hs.config.rc_registration.burst_count = 5
-        self.hs.config.rc_registration.per_second = 0.17
-
         for i in range(0, 6):
             params = {
                 "username": "kermit" + str(i),