Correctly ratelimit invites when creating a room (#9968)

* Correctly ratelimit invites when creating a room

Also allow ratelimiting for more than one action at a time.

1 files changed, 17 insertions, 5 deletions

diff --git a/synapse/api/ratelimiting.py b/synapse/api/ratelimiting.py

index 2244b8a340..b9a10283f4 100644
--- a/synapse/api/ratelimiting.py
+++ b/synapse/api/ratelimiting.py
@@ -57,6 +57,7 @@ class Ratelimiter:
         rate_hz: Optional[float] = None,
         burst_count: Optional[int] = None,
         update: bool = True,
+        n_actions: int = 1,
         _time_now_s: Optional[int] = None,
     ) -> Tuple[bool, float]:
         """Can the entity (e.g. user or IP address) perform the action?
@@ -76,6 +77,9 @@ class Ratelimiter:
             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
+            n_actions: The number of times the user wants to do this action. If the user
+                cannot do all of the actions, the user's action count is not incremented
+                at all.
             _time_now_s: The current time. Optional, defaults to the current time according
                 to self.clock. Only used by tests.
 
@@ -124,17 +128,20 @@ class Ratelimiter:
         time_delta = time_now_s - time_start
         performed_count = action_count - time_delta * rate_hz
         if performed_count < 0:
-            # Allow, reset back to count 1
-            allowed = True
+            performed_count = 0
             time_start = time_now_s
-            action_count = 1.0
-        elif performed_count > burst_count - 1.0:
+
+        # This check would be easier read as performed_count + n_actions > burst_count,
+        # but performed_count might be a very precise float (with lots of numbers
+        # following the point) in which case Python might round it up when adding it to
+        # n_actions. Writing it this way ensures it doesn't happen.
+        if performed_count > burst_count - n_actions:
             # Deny, we have exceeded our burst count
             allowed = False
         else:
             # We haven't reached our limit yet
             allowed = True
-            action_count += 1.0
+            action_count = performed_count + n_actions
 
         if update:
             self.actions[key] = (action_count, time_start, rate_hz)
@@ -182,6 +189,7 @@ class Ratelimiter:
         rate_hz: Optional[float] = None,
         burst_count: Optional[int] = None,
         update: bool = True,
+        n_actions: int = 1,
         _time_now_s: Optional[int] = None,
     ):
         """Checks if an action can be performed. If not, raises a LimitExceededError
@@ -201,6 +209,9 @@ class Ratelimiter:
             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
+            n_actions: The number of times the user wants to do this action. If the user
+                cannot do all of the actions, the user's action count is not incremented
+                at all.
             _time_now_s: The current time. Optional, defaults to the current time according
                 to self.clock. Only used by tests.
 
@@ -216,6 +227,7 @@ class Ratelimiter:
             rate_hz=rate_hz,
             burst_count=burst_count,
             update=update,
+            n_actions=n_actions,
             _time_now_s=time_now_s,
         )