Fix error handling for rooms whose versions are unknown. (#5219)

If we remove support for a particular room version, we should behave more
gracefully. This should make client requests fail with a 400 rather than a 500,
and will ignore individiual PDUs in a federation transaction, rather than the
whole transaction.

changelog.d/5219.bugfix

@@ -0,0 +1 @@
+Fix error handling for rooms whose versions are unknown.

synapse/api/errors.py

@@ -328,9 +328,23 @@ class RoomKeysVersionError(SynapseError):
         self.current_version = current_version
 
 
+class UnsupportedRoomVersionError(SynapseError):
+    """The client's request to create a room used a room version that the server does
+    not support."""
+    def __init__(self):
+        super(UnsupportedRoomVersionError, self).__init__(
+            code=400,
+            msg="Homeserver does not support this room version",
+            errcode=Codes.UNSUPPORTED_ROOM_VERSION,
+        )
+
+
 class IncompatibleRoomVersionError(SynapseError):
-    """A server is trying to join a room whose version it does not support."""
+    """A server is trying to join a room whose version it does not support.
 
+    Unlike UnsupportedRoomVersionError, it is specific to the case of the make_join
+    failing.
+    """
     def __init__(self, room_version):
         super(IncompatibleRoomVersionError, self).__init__(
             code=400,

synapse/events/__init__.py

@@ -21,6 +21,7 @@ import six
 
 from unpaddedbase64 import encode_base64
 
+from synapse.api.errors import UnsupportedRoomVersionError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, EventFormatVersions
 from synapse.util.caches import intern_dict
 from synapse.util.frozenutils import freeze
@@ -369,12 +370,15 @@ def room_version_to_event_format(room_version):
 
     Returns:
         int
+
+    Raises:
+        UnsupportedRoomVersionError if the room version is unknown
     """
     v = KNOWN_ROOM_VERSIONS.get(room_version)
 
     if not v:
-        # We should have already checked version, so this should not happen
-        raise RuntimeError("Unrecognized room version %s" % (room_version,))
+        # this can happen if support is withdrawn for a room version
+        raise UnsupportedRoomVersionError()
 
     return v.event_format
 

synapse/events/builder.py

@@ -18,6 +18,7 @@ import attr
 from twisted.internet import defer
 
 from synapse.api.constants import MAX_DEPTH
+from synapse.api.errors import UnsupportedRoomVersionError
 from synapse.api.room_versions import (
     KNOWN_EVENT_FORMAT_VERSIONS,
     KNOWN_ROOM_VERSIONS,
@@ -178,9 +179,8 @@ class EventBuilderFactory(object):
         """
         v = KNOWN_ROOM_VERSIONS.get(room_version)
         if not v:
-            raise Exception(
-                "No event format defined for version %r" % (room_version,)
-            )
+            # this can happen if support is withdrawn for a room version
+            raise UnsupportedRoomVersionError()
         return self.for_room_version(v, key_values)
 
     def for_room_version(self, room_version, key_values):

synapse/federation/federation_server.py

@@ -33,6 +33,7 @@ from synapse.api.errors import (
     IncompatibleRoomVersionError,
     NotFoundError,
     SynapseError,
+    UnsupportedRoomVersionError,
 )
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.crypto.event_signing import compute_event_signature
@@ -198,11 +199,22 @@ class FederationServer(FederationBase):
 
             try:
                 room_version = yield self.store.get_room_version(room_id)
-                format_ver = room_version_to_event_format(room_version)
             except NotFoundError:
                 logger.info("Ignoring PDU for unknown room_id: %s", room_id)
                 continue
 
+            try:
+                format_ver = room_version_to_event_format(room_version)
+            except UnsupportedRoomVersionError:
+                # this can happen if support for a given room version is withdrawn,
+                # so that we still get events for said room.
+                logger.info(
+                    "Ignoring PDU for room %s with unknown version %s",
+                    room_id,
+                    room_version,
+                )
+                continue
+
             event = event_from_pdu_json(p, format_ver)
             pdus_by_room.setdefault(room_id, []).append(event)