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

changelog.d/5621.bugfix

@@ -0,0 +1 @@
+Various minor fixes to the federation request rate limiter.

synapse/config/ratelimiting.py

@@ -23,7 +23,7 @@ class RateLimitConfig(object):
 
 class FederationRateLimitConfig(object):
     _items_and_default = {
-        "window_size": 10000,
+        "window_size": 1000,
         "sleep_limit": 10,
         "sleep_delay": 500,
         "reject_limit": 50,
@@ -54,7 +54,7 @@ class RatelimitConfig(Config):
 
         # Load the new-style federation config, if it exists. Otherwise, fall
         # back to the old method.
-        if "federation_rc" in config:
+        if "rc_federation" in config:
             self.rc_federation = FederationRateLimitConfig(**config["rc_federation"])
         else:
             self.rc_federation = FederationRateLimitConfig(

synapse/util/ratelimitutils.py

@@ -36,9 +36,11 @@ class FederationRateLimiter(object):
             clock (Clock)
             config (FederationRateLimitConfig)
         """
-        self.clock = clock
+
-        self._config = config
+        def new_limiter():
-        self.ratelimiters = {}
+            return _PerHostRatelimiter(clock=clock, config=config)
+
+        self.ratelimiters = collections.defaultdict(new_limiter)
 
     def ratelimit(self, host):
         """Used to ratelimit an incoming request from given host
@@ -53,11 +55,9 @@ class FederationRateLimiter(object):
             host (str): Origin of incoming request.
 
         Returns:
-            _PerHostRatelimiter
+            context manager which returns a deferred.
         """
-        return self.ratelimiters.setdefault(
+        return self.ratelimiters[host].ratelimit()
-            host, _PerHostRatelimiter(clock=self.clock, config=self._config)
-        ).ratelimit()
 
 
 class _PerHostRatelimiter(object):
@@ -122,7 +122,7 @@ class _PerHostRatelimiter(object):
         self.request_times.append(time_now)
 
         def queue_request():
-            if len(self.current_processing) > self.concurrent_requests:
+            if len(self.current_processing) >= self.concurrent_requests:
                 queue_defer = defer.Deferred()
                 self.ready_request_queue[request_id] = queue_defer
                 logger.info(

tests/config/test_ratelimiting.py

@@ -0,0 +1,40 @@
+# -*- 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 tests.unittest import TestCase
+from tests.utils import default_config
+
+
+class RatelimitConfigTestCase(TestCase):
+    def test_parse_rc_federation(self):
+        config_dict = default_config("test")
+        config_dict["rc_federation"] = {
+            "window_size": 20000,
+            "sleep_limit": 693,
+            "sleep_delay": 252,
+            "reject_limit": 198,
+            "concurrent": 7,
+        }
+
+        config = HomeServerConfig()
+        config.parse_config_dict(config_dict, "", "")
+        config_obj = config.rc_federation
+
+        self.assertEqual(config_obj.window_size, 20000)
+        self.assertEqual(config_obj.sleep_limit, 693)
+        self.assertEqual(config_obj.sleep_delay, 252)
+        self.assertEqual(config_obj.reject_limit, 198)
+        self.assertEqual(config_obj.concurrent, 7)

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

tests/utils.py

@@ -152,12 +152,6 @@ def default_config(name, parse=False):
         "mau_stats_only": False,
         "mau_limits_reserved_threepids": [],
         "admin_contact": None,
-        "rc_federation": {
-            "reject_limit": 10,
-            "sleep_limit": 10,
-            "sleep_delay": 10,
-            "concurrent": 10,
-        },
         "rc_message": {"per_second": 10000, "burst_count": 10000},
         "rc_registration": {"per_second": 10000, "burst_count": 10000},
         "rc_login": {