Fix ratelimiting for federation /send requests. (#8342)

c.f. #8295 for rationale

synapse/federation/transport/server.py

@@ -45,7 +45,6 @@ from synapse.logging.opentracing import (
 )
 from synapse.server import HomeServer
 from synapse.types import ThirdPartyInstanceID, get_domain_from_id
-from synapse.util.ratelimitutils import FederationRateLimiter
 from synapse.util.versionstring import get_version_string
 
 logger = logging.getLogger(__name__)
@@ -72,9 +71,7 @@ class TransportLayerServer(JsonResource):
         super(TransportLayerServer, self).__init__(hs, canonical_json=False)
 
         self.authenticator = Authenticator(hs)
-        self.ratelimiter = FederationRateLimiter(
-            self.clock, config=hs.config.rc_federation
-        )
+        self.ratelimiter = hs.get_federation_ratelimiter()
 
         self.register_servlets()
 
@@ -272,6 +269,8 @@ class BaseFederationServlet:
 
     PREFIX = FEDERATION_V1_PREFIX  # Allows specifying the API version
 
+    RATELIMIT = True  # Whether to rate limit requests or not
+
     def __init__(self, handler, authenticator, ratelimiter, server_name):
         self.handler = handler
         self.authenticator = authenticator
@@ -335,7 +334,7 @@ class BaseFederationServlet:
                 )
 
             with scope:
-                if origin:
+                if origin and self.RATELIMIT:
                     with ratelimiter.ratelimit(origin) as d:
                         await d
                         if request._disconnected:
@@ -372,6 +371,10 @@ class BaseFederationServlet:
 class FederationSendServlet(BaseFederationServlet):
     PATH = "/send/(?P<transaction_id>[^/]*)/?"
 
+    # We ratelimit manually in the handler as we queue up the requests and we
+    # don't want to fill up the ratelimiter with blocked requests.
+    RATELIMIT = False
+
     def __init__(self, handler, server_name, **kwargs):
         super(FederationSendServlet, self).__init__(
             handler, server_name=server_name, **kwargs