From e9e6aacfbe016922ed6a25071c0e9ce77334b4e8 Mon Sep 17 00:00:00 2001
From: reivilibre <oliverw@matrix.org>
Date: Thu, 4 Aug 2022 15:27:04 +0000
Subject: [PATCH] Faster Room Joins: prevent Synapse from answering federated
 join requests for a room which it has not fully joined yet. (#13416)

---
 changelog.d/13416.misc                  |  1 +
 synapse/federation/federation_server.py | 17 +++++++++++++++++
 synapse/handlers/federation.py          | 17 +++++++++++++++++
 3 files changed, 35 insertions(+)
 create mode 100644 changelog.d/13416.misc

diff --git a/changelog.d/13416.misc b/changelog.d/13416.misc
new file mode 100644
index 000000000..2904e7337
--- /dev/null
+++ b/changelog.d/13416.misc
@@ -0,0 +1 @@
+Faster Room Joins: prevent Synapse from answering federated join requests for a room which it has not fully joined yet.
\ No newline at end of file
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 1d6013741..db4b83a50 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -843,8 +843,25 @@ class FederationServer(FederationBase):
                 Codes.BAD_JSON,
             )
 
+        # Note that get_room_version throws if the room does not exist here.
         room_version = await self.store.get_room_version(room_id)
 
+        if await self.store.is_partial_state_room(room_id):
+            # If our server is still only partially joined, we can't give a complete
+            # response to /send_join, /send_knock or /send_leave.
+            # This is because we will not be able to provide the server list (for partial
+            # joins) or the full state (for full joins).
+            # Return a 404 as we would if we weren't in the room at all.
+            logger.info(
+                f"Rejecting /send_{membership_type} to %s because it's a partial state room",
+                room_id,
+            )
+            raise SynapseError(
+                404,
+                f"Unable to handle /send_{membership_type} right now; this server is not fully joined.",
+                errcode=Codes.NOT_FOUND,
+            )
+
         if membership_type == Membership.KNOCK and not room_version.msc2403_knocking:
             raise SynapseError(
                 403,
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 30f1585a8..504223674 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -754,6 +754,23 @@ class FederationHandler:
         # (and return a 404 otherwise)
         room_version = await self.store.get_room_version(room_id)
 
+        if await self.store.is_partial_state_room(room_id):
+            # If our server is still only partially joined, we can't give a complete
+            # response to /make_join, so return a 404 as we would if we weren't in the
+            # room at all.
+            # The main reason we can't respond properly is that we need to know about
+            # the auth events for the join event that we would return.
+            # We also should not bother entertaining the /make_join since we cannot
+            # handle the /send_join.
+            logger.info(
+                "Rejecting /make_join to %s because it's a partial state room", room_id
+            )
+            raise SynapseError(
+                404,
+                "Unable to handle /make_join right now; this server is not fully joined.",
+                errcode=Codes.NOT_FOUND,
+            )
+
         # now check that we are *still* in the room
         is_in_room = await self._event_auth_handler.check_host_in_room(
             room_id, self.server_name