Internal server error when looking up a invalid room alias

[DMRobertson]

https://sentry.tools.element.io/organizations/element/issues/923/?project=2&query=is%3Aunresolved

ValueError: At least one of the arguments destination and destination_is must be a nonempty bytestring.
  File "synapse/http/server.py", line 366, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "synapse/http/server.py", line 572, in _async_render
    callback_return = await raw_callback_return
  File "synapse/rest/client/directory.py", line 57, in on_GET
    res = await self.directory_handler.get_association(room_alias_obj)
  File "synapse/handlers/directory.py", line 262, in get_association
    fed_result: Optional[JsonDict] = await self.federation.make_query(
  File "synapse/federation/federation_client.py", line 183, in make_query
    return await self.transport_layer.make_query(
  File "synapse/federation/transport/client.py", line 260, in make_query
    return await self.client.get_json(
  File "synapse/http/matrixfederationclient.py", line 1069, in get_json
    response = await self._send_request_with_optional_trailing_slash(
  File "synapse/http/matrixfederationclient.py", line 405, in _send_request_with_optional_trailing_slash
    response = await self._send_request(request, **send_request_args)
  File "synapse/http/matrixfederationclient.py", line 555, in _send_request
    auth_headers = self.build_auth_headers(
  File "synapse/http/matrixfederationclient.py", line 735, in build_auth_headers
    raise ValueError(

This came from a GET request for url: /_matrix/client/r0/directory/room/%23Computer-security%3A, .

That's an invalid room alias because it doesn't contain a domain name. Very similar to #12690, then.

[DMRobertson]

To fix:

• There are three calls to RoomAlias.from_string in synapse.rest.client.directory.
• These should only be called if RoomAlias.is_valid confirms that the given alias is valid.
• If not, raise a SynapseError. Consult the CS spec to see what HTTP status code and what matrix error code is appropriate.

[santhoshivan23]

@DMRobertson I'll work on this!