Fixes to the federation rate limiter (#5621)

- Put the default window_size back to 1000ms (broken by #5181)
- Make the `rc_federation` config actually do something
- fix an off-by-one error in the 'concurrent' limit
- Avoid creating an unused `_PerHostRatelimiter` object for every single
  incoming request

tests/util/test_ratelimitutils.py

@@ -0,0 +1,97 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from synapse.config.homeserver import HomeServerConfig
+from synapse.util.ratelimitutils import FederationRateLimiter
+
+from tests.server import get_clock
+from tests.unittest import TestCase
+from tests.utils import default_config
+
+
+class FederationRateLimiterTestCase(TestCase):
+    def test_ratelimit(self):
+        """A simple test with the default values"""
+        reactor, clock = get_clock()
+        rc_config = build_rc_config()
+        ratelimiter = FederationRateLimiter(clock, rc_config)
+
+        with ratelimiter.ratelimit("testhost") as d1:
+            # shouldn't block
+            self.successResultOf(d1)
+
+    def test_concurrent_limit(self):
+        """Test what happens when we hit the concurrent limit"""
+        reactor, clock = get_clock()
+        rc_config = build_rc_config({"rc_federation": {"concurrent": 2}})
+        ratelimiter = FederationRateLimiter(clock, rc_config)
+
+        with ratelimiter.ratelimit("testhost") as d1:
+            # shouldn't block
+            self.successResultOf(d1)
+
+            cm2 = ratelimiter.ratelimit("testhost")
+            d2 = cm2.__enter__()
+            # also shouldn't block
+            self.successResultOf(d2)
+
+            cm3 = ratelimiter.ratelimit("testhost")
+            d3 = cm3.__enter__()
+            # this one should block, though ...
+            self.assertNoResult(d3)
+
+            # ... until we complete an earlier request
+            cm2.__exit__(None, None, None)
+            self.successResultOf(d3)
+
+    def test_sleep_limit(self):
+        """Test what happens when we hit the sleep limit"""
+        reactor, clock = get_clock()
+        rc_config = build_rc_config(
+            {"rc_federation": {"sleep_limit": 2, "sleep_delay": 500}}
+        )
+        ratelimiter = FederationRateLimiter(clock, rc_config)
+
+        with ratelimiter.ratelimit("testhost") as d1:
+            # shouldn't block
+            self.successResultOf(d1)
+
+        with ratelimiter.ratelimit("testhost") as d2:
+            # nor this
+            self.successResultOf(d2)
+
+        with ratelimiter.ratelimit("testhost") as d3:
+            # this one should block, though ...
+            self.assertNoResult(d3)
+            sleep_time = _await_resolution(reactor, d3)
+            self.assertAlmostEqual(sleep_time, 500, places=3)
+
+
+def _await_resolution(reactor, d):
+    """advance the clock until the deferred completes.
+
+    Returns the number of milliseconds it took to complete.
+    """
+    start_time = reactor.seconds()
+    while not d.called:
+        reactor.advance(0.01)
+    return (reactor.seconds() - start_time) * 1000
+
+
+def build_rc_config(settings={}):
+    config_dict = default_config("test")
+    config_dict.update(settings)
+    config = HomeServerConfig()
+    config.parse_config_dict(config_dict, "", "")
+    return config.rc_federation