From 59e011220942e642968c453a36fff4862c753ce3 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Sat, 23 Feb 2019 14:31:08 +0000
Subject: [PATCH 1/2] MSC 1866 - Use M_UNSUPPORTED_ROOM_VERSION for invite API

---
 synapse/federation/federation_client.py | 18 +++++++++++++++++-
 synapse/federation/federation_server.py | 10 +++++++++-
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 4e4f58b418..58e04d81ab 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -33,6 +33,7 @@ from synapse.api.constants import (
 )
 from synapse.api.errors import (
     CodeMessageException,
+    Codes,
     FederationDeniedError,
     HttpResponseException,
     SynapseError,
@@ -792,10 +793,25 @@ class FederationClient(FederationBase):
             defer.returnValue(content)
         except HttpResponseException as e:
             if e.code in [400, 404]:
+                err = e.to_synapse_error()
+
+                # If we receive an error response that isn't a generic error, we
+                # assume that the remote understands the v2 invite API and this
+                # is a legitimate error.
+                if err.errcode != Codes.UNKNOWN:
+                    raise err
+
+                # Otherwise, we assume that the remote server doesn't understand
+                # the v2 invite API.
+
                 if room_version in (RoomVersions.V1, RoomVersions.V2):
                     pass  # We'll fall through
                 else:
-                    raise Exception("Remote server is too old")
+                    raise SynapseError(
+                        400,
+                        "User's homeserver does not support this room version",
+                        Codes.UNSUPPORTED_ROOM_VERSION,
+                    )
             elif e.code == 403:
                 raise e.to_synapse_error()
             else:
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 3da86d4ba6..e44342bc85 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -25,9 +25,10 @@ from twisted.internet import defer
 from twisted.internet.abstract import isIPAddress
 from twisted.python import failure
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import KNOWN_ROOM_VERSIONS, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
+    Codes,
     FederationError,
     IncompatibleRoomVersionError,
     NotFoundError,
@@ -386,6 +387,13 @@ class FederationServer(FederationBase):
 
     @defer.inlineCallbacks
     def on_invite_request(self, origin, content, room_version):
+        if room_version not in KNOWN_ROOM_VERSIONS:
+            raise SynapseError(
+                400,
+                "Homeserver does not support this room version",
+                Codes.UNSUPPORTED_ROOM_VERSION,
+            )
+
         format_ver = room_version_to_event_format(room_version)
 
         pdu = event_from_pdu_json(content, format_ver)

From 71304bfc8d6e0ff66833a04f5df72117decf59d7 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Sat, 23 Feb 2019 14:53:15 +0000
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/4721.feature | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/4721.feature

diff --git a/changelog.d/4721.feature b/changelog.d/4721.feature
new file mode 100644
index 0000000000..f932843ce7
--- /dev/null
+++ b/changelog.d/4721.feature
@@ -0,0 +1 @@
+Return correct error code when inviting a remote user to a room whose homeserver does not support the room version.