From 550308c7a1e8e6f7bf6ee27baf2d67796591657a Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 19 Oct 2016 10:45:24 +0100
Subject: [PATCH 1/2] Check whether to ratelimit sooner to avoid work

---
 synapse/api/ratelimiting.py | 14 +++++++++-----
 synapse/handlers/message.py | 14 +++++++++++++-
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index 660dfb56e..06cc8d90b 100644
--- a/synapse/api/ratelimiting.py
+++ b/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
@@ -52,9 +55,10 @@ class Ratelimiter(object):
             allowed = True
             message_count += 1
 
-        self.message_counts[user_id] = (
-            message_count, time_start, msg_rate_hz
-        )
+        if update:
+            self.message_counts[user_id] = (
+                message_count, time_start, msg_rate_hz
+            )
 
         if msg_rate_hz > 0:
             time_allowed = (
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 30ea9630f..a94a51433 100644
--- a/synapse/handlers/message.py
+++ b/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,)

From f2f74ffce6e77234dad571b61b70e59a7534a681 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 19 Oct 2016 14:21:28 +0100
Subject: [PATCH 2/2] Comment

---
 synapse/handlers/message.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index a94a51433..59eb26bea 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -239,6 +239,9 @@ class MessageHandler(BaseHandler):
                 "Tried to send member event through non-member codepath"
             )
 
+        # We check here if we are currently being rate limited, so that we
+        # don't do unnecessary work. We check again just before we actually
+        # send the event.
         time_now = self.clock.time()
         allowed, time_allowed = self.ratelimiter.send_message(
             event.sender, time_now,