Check whether to ratelimit sooner to avoid work

synapse/api/ratelimiting.py

@@ -23,7 +23,7 @@ class Ratelimiter(object):
     def __init__(self):
         self.message_counts = collections.OrderedDict()
 
-    def send_message(self, user_id, time_now_s, msg_rate_hz, burst_count):
+    def send_message(self, user_id, time_now_s, msg_rate_hz, burst_count, update=True):
         """Can the user send a message?
         Args:
             user_id: The user sending a message.
@@ -32,12 +32,15 @@ class Ratelimiter(object):
                 second.
             burst_count: How many messages the user can send before being
                 limited.
+            update (bool): Whether to update the message rates or not. This is
+                useful to check if a message would be allowed to be sent before
+                its ready to be actually sent.
         Returns:
             A pair of a bool indicating if they can send a message now and a
                 time in seconds of when they can next send a message.
         """
         self.prune_message_counts(time_now_s)
-        message_count, time_start, _ignored = self.message_counts.pop(
+        message_count, time_start, _ignored = self.message_counts.get(
             user_id, (0., time_now_s, None),
         )
         time_delta = time_now_s - time_start
@@ -52,6 +55,7 @@ class Ratelimiter(object):
             allowed = True
             message_count += 1
 
+        if update:
             self.message_counts[user_id] = (
                 message_count, time_start, msg_rate_hz
             )

synapse/handlers/message.py

@@ -16,7 +16,7 @@
 from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import AuthError, Codes, SynapseError
+from synapse.api.errors import AuthError, Codes, SynapseError, LimitExceededError
 from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.events.utils import serialize_event
 from synapse.events.validator import EventValidator
@@ -239,6 +239,18 @@ class MessageHandler(BaseHandler):
                 "Tried to send member event through non-member codepath"
             )
 
+        time_now = self.clock.time()
+        allowed, time_allowed = self.ratelimiter.send_message(
+            event.sender, time_now,
+            msg_rate_hz=self.hs.config.rc_messages_per_second,
+            burst_count=self.hs.config.rc_message_burst_count,
+            update=False,
+        )
+        if not allowed:
+            raise LimitExceededError(
+                retry_after_ms=int(1000 * (time_allowed - time_now)),
+            )
+
         user = UserID.from_string(event.sender)
 
         assert self.hs.is_mine(user), "User must be our own: %s" % (user,)