Add a ratelimiter for 3pid invite

changelog.d/11892.feature

@@ -0,0 +1 @@
+Use a dedicated configurable rate limiter for 3PID invites.

docs/sample_config.yaml

@@ -857,6 +857,9 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #   - one for ratelimiting how often a user or IP can attempt to validate a 3PID.
 #   - two for ratelimiting how often invites can be sent in a room or to a
 #     specific user.
+#   - one for ratelimiting 3PID invites (i.e. invites sent to a third-party ID
+#     such as an email address or a phone number) based on the account that's
+#     sending the invite.
 #
 # The defaults are as shown below.
 #
@@ -906,6 +909,10 @@ log_config: "CONFDIR/SERVERNAME.log.config"
 #  per_user:
 #    per_second: 0.003
 #    burst_count: 5
+#
+#rc_third_party_invite:
+#  per_second: 0.2
+#  burst_count: 10
 
 # Ratelimiting settings for incoming federation
 #

synapse/config/ratelimiting.py

@@ -134,6 +134,11 @@ def read_config(self, config, **kwargs):
             defaults={"per_second": 0.003, "burst_count": 5},
         )
 
+        self.rc_third_party_invite = RateLimitConfig(
+            config.get("rc_third_party_invite", {}),
+            defaults={"per_second": 0.2, "burst_count": 10},
+        )
+
     def generate_config_section(self, **kwargs):
         return """\
         ## Ratelimiting ##
@@ -168,6 +173,9 @@ def generate_config_section(self, **kwargs):
         #   - one for ratelimiting how often a user or IP can attempt to validate a 3PID.
         #   - two for ratelimiting how often invites can be sent in a room or to a
         #     specific user.
+        #   - one for ratelimiting 3PID invites (i.e. invites sent to a third-party ID
+        #     such as an email address or a phone number) based on the account that's
+        #     sending the invite.
         #
         # The defaults are as shown below.
         #
@@ -217,6 +225,10 @@ def generate_config_section(self, **kwargs):
         #  per_user:
         #    per_second: 0.003
         #    burst_count: 5
+        #
+        #rc_third_party_invite:
+        #  per_second: 0.2
+        #  burst_count: 10
 
         # Ratelimiting settings for incoming federation
         #

synapse/handlers/room_member.py

@@ -116,6 +116,13 @@ def __init__(self, hs: "HomeServer"):
             burst_count=hs.config.ratelimiting.rc_invites_per_user.burst_count,
         )
 
+        self._third_party_invite_limiter = Ratelimiter(
+            store=self.store,
+            clock=self.clock,
+            rate_hz=hs.config.ratelimiting.rc_third_party_invite.per_second,
+            burst_count=hs.config.ratelimiting.rc_third_party_invite.burst_count,
+        )
+
         self.request_ratelimiter = hs.get_request_ratelimiter()
 
     @abc.abstractmethod
@@ -1295,7 +1302,10 @@ async def do_3pid_invite(
 
         # We need to rate limit *before* we send out any 3PID invites, so we
         # can't just rely on the standard ratelimiting of events.
-        await self.request_ratelimiter.ratelimit(requester)
+        await self._third_party_invite_limiter.ratelimit(
+            requester=requester,
+            key=requester.user.to_string(),
+        )
 
         can_invite = await self.third_party_event_rules.check_threepid_can_be_invited(
             medium, address, room_id