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.

tests/api/test_ratelimiting.py

@@ -1,39 +1,97 @@
-from synapse.api.ratelimiting import Ratelimiter
+from synapse.api.ratelimiting import LimitExceededError, Ratelimiter
 
 from tests import unittest
 
 
 class TestRatelimiter(unittest.TestCase):
-    def test_allowed(self):
-        limiter = Ratelimiter()
-        allowed, time_allowed = limiter.can_do_action(
-            key="test_id", time_now_s=0, rate_hz=0.1, burst_count=1
-        )
+    def test_allowed_via_can_do_action(self):
+        limiter = Ratelimiter(clock=None, rate_hz=0.1, burst_count=1)
+        allowed, time_allowed = limiter.can_do_action(key="test_id", _time_now_s=0)
         self.assertTrue(allowed)
         self.assertEquals(10.0, time_allowed)
 
-        allowed, time_allowed = limiter.can_do_action(
-            key="test_id", time_now_s=5, rate_hz=0.1, burst_count=1
-        )
+        allowed, time_allowed = limiter.can_do_action(key="test_id", _time_now_s=5)
         self.assertFalse(allowed)
         self.assertEquals(10.0, time_allowed)
 
-        allowed, time_allowed = limiter.can_do_action(
-            key="test_id", time_now_s=10, rate_hz=0.1, burst_count=1
-        )
+        allowed, time_allowed = limiter.can_do_action(key="test_id", _time_now_s=10)
         self.assertTrue(allowed)
         self.assertEquals(20.0, time_allowed)
 
+    def test_allowed_via_ratelimit(self):
+        limiter = Ratelimiter(clock=None, rate_hz=0.1, burst_count=1)
+
+        # Shouldn't raise
+        limiter.ratelimit(key="test_id", _time_now_s=0)
+
+        # Should raise
+        with self.assertRaises(LimitExceededError) as context:
+            limiter.ratelimit(key="test_id", _time_now_s=5)
+        self.assertEqual(context.exception.retry_after_ms, 5000)
+
+        # Shouldn't raise
+        limiter.ratelimit(key="test_id", _time_now_s=10)
+
+    def test_allowed_via_can_do_action_and_overriding_parameters(self):
+        """Test that we can override options of can_do_action that would otherwise fail
+        an action
+        """
+        # Create a Ratelimiter with a very low allowed rate_hz and burst_count
+        limiter = Ratelimiter(clock=None, rate_hz=0.1, burst_count=1)
+
+        # First attempt should be allowed
+        allowed, time_allowed = limiter.can_do_action(("test_id",), _time_now_s=0,)
+        self.assertTrue(allowed)
+        self.assertEqual(10.0, time_allowed)
+
+        # Second attempt, 1s later, will fail
+        allowed, time_allowed = limiter.can_do_action(("test_id",), _time_now_s=1,)
+        self.assertFalse(allowed)
+        self.assertEqual(10.0, time_allowed)
+
+        # But, if we allow 10 actions/sec for this request, we should be allowed
+        # to continue.
+        allowed, time_allowed = limiter.can_do_action(
+            ("test_id",), _time_now_s=1, rate_hz=10.0
+        )
+        self.assertTrue(allowed)
+        self.assertEqual(1.1, time_allowed)
+
+        # Similarly if we allow a burst of 10 actions
+        allowed, time_allowed = limiter.can_do_action(
+            ("test_id",), _time_now_s=1, burst_count=10
+        )
+        self.assertTrue(allowed)
+        self.assertEqual(1.0, time_allowed)
+
+    def test_allowed_via_ratelimit_and_overriding_parameters(self):
+        """Test that we can override options of the ratelimit method that would otherwise
+        fail an action
+        """
+        # Create a Ratelimiter with a very low allowed rate_hz and burst_count
+        limiter = Ratelimiter(clock=None, rate_hz=0.1, burst_count=1)
+
+        # First attempt should be allowed
+        limiter.ratelimit(key=("test_id",), _time_now_s=0)
+
+        # Second attempt, 1s later, will fail
+        with self.assertRaises(LimitExceededError) as context:
+            limiter.ratelimit(key=("test_id",), _time_now_s=1)
+        self.assertEqual(context.exception.retry_after_ms, 9000)
+
+        # But, if we allow 10 actions/sec for this request, we should be allowed
+        # to continue.
+        limiter.ratelimit(key=("test_id",), _time_now_s=1, rate_hz=10.0)
+
+        # Similarly if we allow a burst of 10 actions
+        limiter.ratelimit(key=("test_id",), _time_now_s=1, burst_count=10)
+
     def test_pruning(self):
-        limiter = Ratelimiter()
-        allowed, time_allowed = limiter.can_do_action(
-            key="test_id_1", time_now_s=0, rate_hz=0.1, burst_count=1
-        )
+        limiter = Ratelimiter(clock=None, rate_hz=0.1, burst_count=1)
+        limiter.can_do_action(key="test_id_1", _time_now_s=0)
 
-        self.assertIn("test_id_1", limiter.message_counts)
+        self.assertIn("test_id_1", limiter.actions)
 
-        allowed, time_allowed = limiter.can_do_action(
-            key="test_id_2", time_now_s=10, rate_hz=0.1, burst_count=1
-        )
+        limiter.can_do_action(key="test_id_2", _time_now_s=10)
 
-        self.assertNotIn("test_id_1", limiter.message_counts)
+        self.assertNotIn("test_id_1", limiter.actions)