Refactor and comment ratelimiting. Set limits in constructor

synapse/api/ratelimiting.py

@@ -13,78 +13,130 @@
 # limitations under the License.
 
 from collections import OrderedDict
-from typing import Any, Optional, Tuple
+from typing import Any, Tuple
 
 from synapse.api.errors import LimitExceededError
 
 
 class Ratelimiter(object):
     """
-    Ratelimit message sending by user.
+    Ratelimit actions marked by arbitrary keys.
+
+    Args:
+        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, rate_hz: float, burst_count: int):
+        # 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
+        #   * That point in time
+        self.actions = OrderedDict()  # type: OrderedDict[Any, Tuple[float, int]]
+        self.rate_hz = rate_hz
+        self.burst_count = burst_count
 
-    def can_do_action(self, key, time_now_s, rate_hz, burst_count, update=True):
+    def can_do_action(
+        self,
+        key: Any,
+        time_now_s: int,
+        update: bool = True,
+    ) -> 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.
+            time_now_s: The time now
+            update: Whether to count this check as performing the action
         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 time in seconds of when it can next be performed.
+                  -1 if a rate_hz has not been defined for this Ratelimiter
         """
-        self.prune_message_counts(time_now_s)
-        message_count, time_start, _ignored = self.message_counts.get(
-            key, (0.0, time_now_s, None)
+        # 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)
         )
+
+        # 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 * self.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 > self.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)
 
-        if rate_hz > 0:
-            time_allowed = time_start + (message_count - burst_count + 1) / rate_hz
+        # Figure out the time when an action can be performed again
+        if self.rate_hz > 0:
+            time_allowed = (
+                time_start + (action_count - self.burst_count + 1) / self.rate_hz
+            )
+
+            # Don't give back a time in the past
             if time_allowed < time_now_s:
                 time_allowed = time_now_s
         else:
+            # This does not apply
             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 are older than
+
+        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 = self.actions[key]
+
             time_delta = time_now_s - time_start
-            if message_count - time_delta * rate_hz > 0:
+            if action_count - time_delta * self.rate_hz > 0:
+                # XXX: Should this be a continue?
                 break
             else:
-                del self.message_counts[key]
+                del self.actions[key]
 
-    def ratelimit(self, key, time_now_s, rate_hz, burst_count, update=True):
+    def ratelimit(
+        self,
+        key: Any,
+        time_now_s: int,
+        update: bool = True,
+    ):
+        """Checks if an action can be performed. If not, raises a LimitExceededError
+
+        Args:
+            key: An arbitrary key used to classify an action
+            time_now_s: The current time
+            update: Whether to count this check as performing the action
+
+        Raises:
+            LimitExceededError: If an action could not be performed, along with the time in
+                milliseconds until the action can be performed again
+        """
         allowed, time_allowed = self.can_do_action(
-            key, time_now_s, rate_hz, burst_count, update
+            key, time_now_s, update
         )
 
         if not allowed: