From 42c45e7119ce4efe9f5f9cd28e31bc5b449f9869 Mon Sep 17 00:00:00 2001
From: David Robertson <davidr@element.io>
Date: Tue, 12 Jul 2022 12:03:34 +0100
Subject: [PATCH 1/2] Notifier: accept callbacks to fire on room joins

---
 synapse/notifier.py | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/synapse/notifier.py b/synapse/notifier.py
index 54b0ec4b97b4..c42bb8266add 100644
--- a/synapse/notifier.py
+++ b/synapse/notifier.py
@@ -228,6 +228,7 @@ def __init__(self, hs: "HomeServer"):
 
         # Called when there are new things to stream over replication
         self.replication_callbacks: List[Callable[[], None]] = []
+        self._new_join_in_room_callbacks: List[Callable[[str, str], None]] = []
 
         self._federation_client = hs.get_federation_http_client()
 
@@ -280,6 +281,19 @@ def add_replication_callback(self, cb: Callable[[], None]) -> None:
         """
         self.replication_callbacks.append(cb)
 
+    def add_new_join_in_room_callback(self, cb: Callable[[str, str], None]) -> None:
+        """Add a callback that will be called when a user joins a room.
+
+        This only fires on genuine membership changes, e.g. "invite" -> "join".
+        Membership transitions like "join" -> "join" (for e.g. displayname changes) do
+        not trigger the callback.
+
+        When called, the callback receives two arguments: the event ID and the room ID.
+        It should *not* return a Deferred - if it needs to do any asynchronous work, a
+        background thread should be started and wrapped with run_as_background_process.
+        """
+        self._new_join_in_room_callbacks.append(cb)
+
     async def on_new_room_event(
         self,
         event: EventBase,
@@ -723,6 +737,10 @@ def notify_replication(self) -> None:
         for cb in self.replication_callbacks:
             cb()
 
+    def notify_user_joined_room(self, event_id: str, room_id: str) -> None:
+        for cb in self._new_join_in_room_callbacks:
+            cb(event_id, room_id)
+
     def notify_remote_server_up(self, server: str) -> None:
         """Notify any replication that a remote server has come back up"""
         # We call federation_sender directly rather than registering as a

From ddde86a2069adc56a8ef2dece5ce2dd22fa4badd Mon Sep 17 00:00:00 2001
From: David Robertson <davidr@element.io>
Date: Tue, 12 Jul 2022 12:11:52 +0100
Subject: [PATCH 2/2] Changelog

---
 changelog.d/13254.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/13254.misc

diff --git a/changelog.d/13254.misc b/changelog.d/13254.misc
new file mode 100644
index 000000000000..cba6b9ee0ff0
--- /dev/null
+++ b/changelog.d/13254.misc
@@ -0,0 +1 @@
+Preparatory work for a per-room rate limiter on joins.