Performance improvements and refactor of Ratelimiter (#7595)

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

10 files changed, 193 insertions, 162 deletions

diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py
index 7a049b3af7..ec6b3a69a2 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -1,4 +1,5 @@
 # Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2020 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.
@@ -16,75 +17,157 @@ from collections import OrderedDict
 from typing import Any, Optional, Tuple
 
 from synapse.api.errors import LimitExceededError
+from synapse.util import Clock
 
 
 class Ratelimiter(object):
     """
-    Ratelimit message sending by user.
+    Ratelimit actions marked by arbitrary keys.
+
+    Args:
+        clock: A homeserver clock, for retrieving the current time
+        rate_hz: The long term number of actions that can be performed in a second.
+        burst_count: How many actions that can be performed before being limited.
     """
 
-    def __init__(self):
-        self.message_counts = (
-            OrderedDict()
-        )  # type: OrderedDict[Any, Tuple[float, int, Optional[float]]]
+    def __init__(self, clock: Clock, rate_hz: float, burst_count: int):
+        self.clock = clock
+        self.rate_hz = rate_hz
+        self.burst_count = burst_count
+
+        # A ordered dictionary keeping track of actions, when they were last
+        # performed and how often. Each entry is a mapping from a key of arbitrary type
+        # to a tuple representing:
+        #   * How many times an action has occurred since a point in time
+        #   * The point in time
+        #   * The rate_hz of this particular entry. This can vary per request
+        self.actions = OrderedDict()  # type: OrderedDict[Any, Tuple[float, int, float]]
 
-    def can_do_action(self, key, time_now_s, rate_hz, burst_count, update=True):
+    def can_do_action(
+        self,
+        key: Any,
+        rate_hz: Optional[float] = None,
+        burst_count: Optional[int] = None,
+        update: bool = True,
+        _time_now_s: Optional[int] = None,
+    ) -> Tuple[bool, float]:
         """Can the entity (e.g. user or IP address) perform the action?
+
         Args:
             key: The key we should use when rate limiting. Can be a user ID
                 (when sending events), an IP address, etc.
-            time_now_s: The time now.
-            rate_hz: The long term number of messages a user can send in a
-                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.
+            rate_hz: The long term number of actions that can be performed in a second.
+                Overrides the value set during instantiation if set.
+            burst_count: How many actions that can be performed before being limited.
+                Overrides the value set during instantiation if set.
+            update: Whether to count this check as performing the action
+            _time_now_s: The current time. Optional, defaults to the current time according
+                to self.clock. Only used by tests.
+
         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.
+            A tuple containing:
+                * A bool indicating if they can perform the action now
+                * The reactor timestamp for when the action can be performed next.
+                  -1 if rate_hz is less than or equal to zero
         """
-        self.prune_message_counts(time_now_s)
-        message_count, time_start, _ignored = self.message_counts.get(
-            key, (0.0, time_now_s, None)
-        )
+        # Override default values if set
+        time_now_s = _time_now_s if _time_now_s is not None else self.clock.time()
+        rate_hz = rate_hz if rate_hz is not None else self.rate_hz
+        burst_count = burst_count if burst_count is not None else self.burst_count
+
+        # Remove any expired entries
+        self._prune_message_counts(time_now_s)
+
+        # Check if there is an existing count entry for this key
+        action_count, time_start, _ = self.actions.get(key, (0.0, time_now_s, 0.0))
+
+        # Check whether performing another action is allowed
         time_delta = time_now_s - time_start
-        sent_count = message_count - time_delta * rate_hz
-        if sent_count < 0:
+        performed_count = action_count - time_delta * rate_hz
+        if performed_count < 0:
+            # Allow, reset back to count 1
             allowed = True
             time_start = time_now_s
-            message_count = 1.0
-        elif sent_count > burst_count - 1.0:
+            action_count = 1.0
+        elif performed_count > burst_count - 1.0:
+            # Deny, we have exceeded our burst count
             allowed = False
         else:
+            # We haven't reached our limit yet
             allowed = True
-            message_count += 1
+            action_count += 1.0
 
         if update:
-            self.message_counts[key] = (message_count, time_start, rate_hz)
+            self.actions[key] = (action_count, time_start, rate_hz)
 
         if rate_hz > 0:
-            time_allowed = time_start + (message_count - burst_count + 1) / rate_hz
+            # Find out when the count of existing actions expires
+            time_allowed = time_start + (action_count - burst_count + 1) / rate_hz
+
+            # Don't give back a time in the past
             if time_allowed < time_now_s:
                 time_allowed = time_now_s
+
         else:
+            # XXX: Why is this -1? This seems to only be used in
+            # self.ratelimit. I guess so that clients get a time in the past and don't
+            # feel afraid to try again immediately
             time_allowed = -1
 
         return allowed, time_allowed
 
-    def prune_message_counts(self, time_now_s):
-        for key in list(self.message_counts.keys()):
-            message_count, time_start, rate_hz = self.message_counts[key]
+    def _prune_message_counts(self, time_now_s: int):
+        """Remove message count entries that have not exceeded their defined
+        rate_hz limit
+
+        Args:
+            time_now_s: The current time
+        """
+        # We create a copy of the key list here as the dictionary is modified during
+        # the loop
+        for key in list(self.actions.keys()):
+            action_count, time_start, rate_hz = self.actions[key]
+
+            # Rate limit = "seconds since we started limiting this action" * rate_hz
+            # If this limit has not been exceeded, wipe our record of this action
             time_delta = time_now_s - time_start
-            if message_count - time_delta * rate_hz > 0:
-                break
+            if action_count - time_delta * rate_hz > 0:
+                continue
             else:
-                del self.message_counts[key]
+                del self.actions[key]
+
+    def ratelimit(
+        self,
+        key: Any,
+        rate_hz: Optional[float] = None,
+        burst_count: Optional[int] = None,
+        update: bool = True,
+        _time_now_s: Optional[int] = None,
+    ):
+        """Checks if an action can be performed. If not, raises a LimitExceededError
+
+        Args:
+            key: An arbitrary key used to classify an action
+            rate_hz: The long term number of actions that can be performed in a second.
+                Overrides the value set during instantiation if set.
+            burst_count: How many actions that can be performed before being limited.
+                Overrides the value set during instantiation if set.
+            update: Whether to count this check as performing the action
+            _time_now_s: The current time. Optional, defaults to the current time according
+                to self.clock. Only used by tests.
+
+        Raises:
+            LimitExceededError: If an action could not be performed, along with the time in
+                milliseconds until the action can be performed again
+        """
+        time_now_s = _time_now_s if _time_now_s is not None else self.clock.time()
 
-    def ratelimit(self, key, time_now_s, rate_hz, burst_count, update=True):
         allowed, time_allowed = self.can_do_action(
-            key, time_now_s, rate_hz, burst_count, update
+            key,
+            rate_hz=rate_hz,
+            burst_count=burst_count,
+            update=update,
+            _time_now_s=time_now_s,
         )
 
         if not allowed:
diff --git a/synapse/config/ratelimiting.py b/synapse/config/ratelimiting.py
index 4a3bfc4354..2dd94bae2b 100644
--- a/synapse/config/ratelimiting.py
+++ b/synapse/config/ratelimiting.py
@@ -12,11 +12,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import Dict
+
 from ._base import Config
 
 
 class RateLimitConfig(object):
-    def __init__(self, config, defaults={"per_second": 0.17, "burst_count": 3.0}):
+    def __init__(
+        self,
+        config: Dict[str, float],
+        defaults={"per_second": 0.17, "burst_count": 3.0},
+    ):
         self.per_second = config.get("per_second", defaults["per_second"])
         self.burst_count = config.get("burst_count", defaults["burst_count"])
 
diff --git a/synapse/handlers/_base.py b/synapse/handlers/_base.py
index 3b781d9836..61dc4beafe 100644
--- a/synapse/handlers/_base.py
+++ b/synapse/handlers/_base.py
@@ -19,7 +19,7 @@ from twisted.internet import defer
 
 import synapse.types
 from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import LimitExceededError
+from synapse.api.ratelimiting import Ratelimiter
 from synapse.types import UserID
 
 logger = logging.getLogger(__name__)
@@ -44,11 +44,26 @@ class BaseHandler(object):
         self.notifier = hs.get_notifier()
         self.state_handler = hs.get_state_handler()
         self.distributor = hs.get_distributor()
-        self.ratelimiter = hs.get_ratelimiter()
-        self.admin_redaction_ratelimiter = hs.get_admin_redaction_ratelimiter()
         self.clock = hs.get_clock()
         self.hs = hs
 
+        # The rate_hz and burst_count are overridden on a per-user basis
+        self.request_ratelimiter = Ratelimiter(
+            clock=self.clock, rate_hz=0, burst_count=0
+        )
+        self._rc_message = self.hs.config.rc_message
+
+        # Check whether ratelimiting room admin message redaction is enabled
+        # by the presence of rate limits in the config
+        if self.hs.config.rc_admin_redaction:
+            self.admin_redaction_ratelimiter = Ratelimiter(
+                clock=self.clock,
+                rate_hz=self.hs.config.rc_admin_redaction.per_second,
+                burst_count=self.hs.config.rc_admin_redaction.burst_count,
+            )
+        else:
+            self.admin_redaction_ratelimiter = None
+
         self.server_name = hs.hostname
 
         self.event_builder_factory = hs.get_event_builder_factory()
@@ -70,7 +85,6 @@ class BaseHandler(object):
         Raises:
             LimitExceededError if the request should be ratelimited
         """
-        time_now = self.clock.time()
         user_id = requester.user.to_string()
 
         # The AS user itself is never rate limited.
@@ -83,48 +97,32 @@ class BaseHandler(object):
         if requester.app_service and not requester.app_service.is_rate_limited():
             return
 
+        messages_per_second = self._rc_message.per_second
+        burst_count = self._rc_message.burst_count
+
         # Check if there is a per user override in the DB.
         override = yield self.store.get_ratelimit_for_user(user_id)
         if override:
-            # If overriden with a null Hz then ratelimiting has been entirely
+            # If overridden with a null Hz then ratelimiting has been entirely
             # disabled for the user
             if not override.messages_per_second:
                 return
 
             messages_per_second = override.messages_per_second
             burst_count = override.burst_count
+
+        if is_admin_redaction and self.admin_redaction_ratelimiter:
+            # If we have separate config for admin redactions, use a separate
+            # ratelimiter as to not have user_ids clash
+            self.admin_redaction_ratelimiter.ratelimit(user_id, update=update)
         else:
-            # We default to different values if this is an admin redaction and
-            # the config is set
-            if is_admin_redaction and self.hs.config.rc_admin_redaction:
-                messages_per_second = self.hs.config.rc_admin_redaction.per_second
-                burst_count = self.hs.config.rc_admin_redaction.burst_count
-            else:
-                messages_per_second = self.hs.config.rc_message.per_second
-                burst_count = self.hs.config.rc_message.burst_count
-
-        if is_admin_redaction and self.hs.config.rc_admin_redaction:
-            # If we have separate config for admin redactions we use a separate
-            # ratelimiter
-            allowed, time_allowed = self.admin_redaction_ratelimiter.can_do_action(
-                user_id,
-                time_now,
-                rate_hz=messages_per_second,
-                burst_count=burst_count,
-                update=update,
-            )
-        else:
-            allowed, time_allowed = self.ratelimiter.can_do_action(
+            # Override rate and burst count per-user
+            self.request_ratelimiter.ratelimit(
                 user_id,
-                time_now,
                 rate_hz=messages_per_second,
                 burst_count=burst_count,
                 update=update,
             )
-        if not allowed:
-            raise LimitExceededError(
-                retry_after_ms=int(1000 * (time_allowed - time_now))
-            )
 
     async def maybe_kick_guest_users(self, event, context=None):
         # Technically this function invalidates current_state by changing it.
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 75b39e878c..119678e67b 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -108,7 +108,11 @@ class AuthHandler(BaseHandler):
 
         # Ratelimiter for failed auth during UIA. Uses same ratelimit config
         # as per `rc_login.failed_attempts`.
-        self._failed_uia_attempts_ratelimiter = Ratelimiter()
+        self._failed_uia_attempts_ratelimiter = Ratelimiter(
+            clock=self.clock,
+            rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
+            burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
+        )
 
         self._clock = self.hs.get_clock()
 
@@ -196,13 +200,7 @@ class AuthHandler(BaseHandler):
         user_id = requester.user.to_string()
 
         # Check if we should be ratelimited due to too many previous failed attempts
-        self._failed_uia_attempts_ratelimiter.ratelimit(
-            user_id,
-            time_now_s=self._clock.time(),
-            rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
-            burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
-            update=False,
-        )
+        self._failed_uia_attempts_ratelimiter.ratelimit(user_id, update=False)
 
         # build a list of supported flows
         flows = [[login_type] for login_type in self._supported_ui_auth_types]
@@ -212,14 +210,8 @@ class AuthHandler(BaseHandler):
                 flows, request, request_body, clientip, description
             )
         except LoginError:
-            # Update the ratelimite to say we failed (`can_do_action` doesn't raise).
-            self._failed_uia_attempts_ratelimiter.can_do_action(
-                user_id,
-                time_now_s=self._clock.time(),
-                rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
-                burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
-                update=True,
-            )
+            # Update the ratelimiter to say we failed (`can_do_action` doesn't raise).
+            self._failed_uia_attempts_ratelimiter.can_do_action(user_id)
             raise
 
         # find the completed login type
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 681f92cafd..649ca1f08a 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -362,7 +362,6 @@ class EventCreationHandler(object):
         self.profile_handler = hs.get_profile_handler()
         self.event_builder_factory = hs.get_event_builder_factory()
         self.server_name = hs.hostname
-        self.ratelimiter = hs.get_ratelimiter()
         self.notifier = hs.get_notifier()
         self.config = hs.config
         self.require_membership_for_aliases = hs.config.require_membership_for_aliases
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 55a03e53ea..cd746be7c8 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -425,14 +425,7 @@ class RegistrationHandler(BaseHandler):
         if not address:
             return
 
-        time_now = self.clock.time()
-
-        self.ratelimiter.ratelimit(
-            address,
-            time_now_s=time_now,
-            rate_hz=self.hs.config.rc_registration.per_second,
-            burst_count=self.hs.config.rc_registration.burst_count,
-        )
+        self.ratelimiter.ratelimit(address)
 
     def register_with_store(
         self,
diff --git a/synapse/rest/client/v1/login.py b/synapse/rest/client/v1/login.py
index 6ac7c5142b..dceb2792fa 100644
--- a/synapse/rest/client/v1/login.py
+++ b/synapse/rest/client/v1/login.py
@@ -87,11 +87,22 @@ class LoginRestServlet(RestServlet):
         self.auth_handler = self.hs.get_auth_handler()
         self.registration_handler = hs.get_registration_handler()
         self.handlers = hs.get_handlers()
-        self._clock = hs.get_clock()
         self._well_known_builder = WellKnownBuilder(hs)
-        self._address_ratelimiter = Ratelimiter()
-        self._account_ratelimiter = Ratelimiter()
-        self._failed_attempts_ratelimiter = Ratelimiter()
+        self._address_ratelimiter = Ratelimiter(
+            clock=hs.get_clock(),
+            rate_hz=self.hs.config.rc_login_address.per_second,
+            burst_count=self.hs.config.rc_login_address.burst_count,
+        )
+        self._account_ratelimiter = Ratelimiter(
+            clock=hs.get_clock(),
+            rate_hz=self.hs.config.rc_login_account.per_second,
+            burst_count=self.hs.config.rc_login_account.burst_count,
+        )
+        self._failed_attempts_ratelimiter = Ratelimiter(
+            clock=hs.get_clock(),
+            rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
+            burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
+        )
 
     def on_GET(self, request):
         flows = []
@@ -124,13 +135,7 @@ class LoginRestServlet(RestServlet):
         return 200, {}
 
     async def on_POST(self, request):
-        self._address_ratelimiter.ratelimit(
-            request.getClientIP(),
-            time_now_s=self.hs.clock.time(),
-            rate_hz=self.hs.config.rc_login_address.per_second,
-            burst_count=self.hs.config.rc_login_address.burst_count,
-            update=True,
-        )
+        self._address_ratelimiter.ratelimit(request.getClientIP())
 
         login_submission = parse_json_object_from_request(request)
         try:
@@ -198,13 +203,7 @@ class LoginRestServlet(RestServlet):
 
             # We also apply account rate limiting using the 3PID as a key, as
             # otherwise using 3PID bypasses the ratelimiting based on user ID.
-            self._failed_attempts_ratelimiter.ratelimit(
-                (medium, address),
-                time_now_s=self._clock.time(),
-                rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
-                burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
-                update=False,
-            )
+            self._failed_attempts_ratelimiter.ratelimit((medium, address), update=False)
 
             # Check for login providers that support 3pid login types
             (
@@ -238,13 +237,7 @@ class LoginRestServlet(RestServlet):
                 # If it returned None but the 3PID was bound then we won't hit
                 # this code path, which is fine as then the per-user ratelimit
                 # will kick in below.
-                self._failed_attempts_ratelimiter.can_do_action(
-                    (medium, address),
-                    time_now_s=self._clock.time(),
-                    rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
-                    burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
-                    update=True,
-                )
+                self._failed_attempts_ratelimiter.can_do_action((medium, address))
                 raise LoginError(403, "", errcode=Codes.FORBIDDEN)
 
             identifier = {"type": "m.id.user", "user": user_id}
@@ -263,11 +256,7 @@ class LoginRestServlet(RestServlet):
 
         # Check if we've hit the failed ratelimit (but don't update it)
         self._failed_attempts_ratelimiter.ratelimit(
-            qualified_user_id.lower(),
-            time_now_s=self._clock.time(),
-            rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
-            burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
-            update=False,
+            qualified_user_id.lower(), update=False
         )
 
         try:
@@ -279,13 +268,7 @@ class LoginRestServlet(RestServlet):
             # limiter. Using `can_do_action` avoids us raising a ratelimit
             # exception and masking the LoginError. The actual ratelimiting
             # should have happened above.
-            self._failed_attempts_ratelimiter.can_do_action(
-                qualified_user_id.lower(),
-                time_now_s=self._clock.time(),
-                rate_hz=self.hs.config.rc_login_failed_attempts.per_second,
-                burst_count=self.hs.config.rc_login_failed_attempts.burst_count,
-                update=True,
-            )
+            self._failed_attempts_ratelimiter.can_do_action(qualified_user_id.lower())
             raise
 
         result = await self._complete_login(
@@ -318,13 +301,7 @@ class LoginRestServlet(RestServlet):
         # Before we actually log them in we check if they've already logged in
         # too often. This happens here rather than before as we don't
         # necessarily know the user before now.
-        self._account_ratelimiter.ratelimit(
-            user_id.lower(),
-            time_now_s=self._clock.time(),
-            rate_hz=self.hs.config.rc_login_account.per_second,
-            burst_count=self.hs.config.rc_login_account.burst_count,
-            update=True,
-        )
+        self._account_ratelimiter.ratelimit(user_id.lower())
 
         if create_non_existent_users:
             canonical_uid = await self.auth_handler.check_user_exists(user_id)
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index addd4cae19..b9ffe86b2a 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -26,7 +26,6 @@ import synapse.types
 from synapse.api.constants import LoginType
 from synapse.api.errors import (
     Codes,
-    LimitExceededError,
     SynapseError,
     ThreepidValidationError,
     UnrecognizedRequestError,
@@ -396,20 +395,7 @@ class RegisterRestServlet(RestServlet):
 
         client_addr = request.getClientIP()
 
-        time_now = self.clock.time()
-
-        allowed, time_allowed = self.ratelimiter.can_do_action(
-            client_addr,
-            time_now_s=time_now,
-            rate_hz=self.hs.config.rc_registration.per_second,
-            burst_count=self.hs.config.rc_registration.burst_count,
-            update=False,
-        )
-
-        if not allowed:
-            raise LimitExceededError(
-                retry_after_ms=int(1000 * (time_allowed - time_now))
-            )
+        self.ratelimiter.ratelimit(client_addr, update=False)
 
         kind = b"user"
         if b"kind" in request.args:
diff --git a/synapse/server.py b/synapse/server.py
index ca2deb49bb..fe94836a2c 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -242,9 +242,12 @@ class HomeServer(object):
 
         self.clock = Clock(reactor)
         self.distributor = Distributor()
-        self.ratelimiter = Ratelimiter()
-        self.admin_redaction_ratelimiter = Ratelimiter()
-        self.registration_ratelimiter = Ratelimiter()
+
+        self.registration_ratelimiter = Ratelimiter(
+            clock=self.clock,
+            rate_hz=config.rc_registration.per_second,
+            burst_count=config.rc_registration.burst_count,
+        )
 
         self.datastores = None
 
@@ -314,15 +317,9 @@ class HomeServer(object):
     def get_distributor(self):
         return self.distributor
 
-    def get_ratelimiter(self):
-        return self.ratelimiter
-
-    def get_registration_ratelimiter(self):
+    def get_registration_ratelimiter(self) -> Ratelimiter:
         return self.registration_ratelimiter
 
-    def get_admin_redaction_ratelimiter(self):
-        return self.admin_redaction_ratelimiter
-
     def build_federation_client(self):
         return FederationClient(self)
 
diff --git a/synapse/util/ratelimitutils.py b/synapse/util/ratelimitutils.py
index 5ca4521ce3..e5efdfcd02 100644
--- a/synapse/util/ratelimitutils.py
+++ b/synapse/util/ratelimitutils.py
@@ -43,7 +43,7 @@ class FederationRateLimiter(object):
         self.ratelimiters = collections.defaultdict(new_limiter)
 
     def ratelimit(self, host):
-        """Used to ratelimit an incoming request from given host
+        """Used to ratelimit an incoming request from a given host
 
         Example usage: