Error: This vat only supports a bootstrap interface, not the old Cap'n-Proto-0.4-style named exports

[LasseBlaauwbroek]

I'm trying to use capnp to connect an Ocaml process to a Python process. I have successfully followed the 'echo'-tutorial of capnp-rpc, getting an Ocaml client to talk to an Ocaml server. I created a similar setup in Python (having two python processes talk to each other). Now I wish to run the echo-server in a Python process and use an Ocaml client to connect to it. However, I am met with the following error:

atal error: exception (Failure
  "Error calling field(2) -> Failed: This vat only supports a bootstrap interface, not the old Cap'n-Proto-0.4-style named exports.(Echo.ping): Failed: This vat only supports a bootstrap interface, not the old Cap'n-Proto-0.4-style named exports.")

This error seems to originate from the C++ code that Python binds to:
https://github.com/capnproto/capnproto/blob/3f01eac812af3ca863d55981423fb90c08be0809/c%2B%2B/src/capnp/rpc.c%2B%2B#L2579-L2580

Am I doing something wrong while calling for the service, or is the implementation still using these 0.4-style named exports? Below is the code to reproduce:

@0xafda4797418def92;

interface Echo {
  ping @0 (msg :Text) -> (reply :Text);
}

open Lwt.Infix
open Capnp_rpc_lwt

let () =
  Logs.set_level (Some Logs.Warning);
  Logs.set_reporter (Logs_fmt.reporter ())

let run_client service =
  Echo_client.ping service "foo" >>= fun reply ->
  Fmt.pr "Got reply %S@." reply;
  Lwt.return_unit

let () =
  Lwt_main.run begin
    let client_vat = Capnp_rpc_unix.client_only_vat () in
    let sr = Capnp_rpc_unix.Vat.import_exn client_vat @@ Uri.of_string "capnp://insecure@localhost:5001" in
    Sturdy_ref.with_cap_exn sr run_client
  end

#!/usr/bin/env python3

import argparse
import capnp
import time

import echo_api_capnp

class EchoImpl(echo_api_capnp.Echo.Server):

    def ping(self, msg, _context):
        print(msg)
        print(_context)
        return "boe"


def parse_args():
    parser = argparse.ArgumentParser(usage='''Runs the server bound to the\
given address/port ADDRESS may be '*' to bind to all local addresses.\
:PORT may be omitted to choose a port automatically. ''')

    parser.add_argument("address", help="ADDRESS[:PORT]")

    return parser.parse_args()


def main():
    address = parse_args().address

    server = capnp.TwoPartyServer(address, bootstrap=EchoImpl())
    while True:
        server.poll_once()
        time.sleep(0.001)


if __name__ == '__main__':
    main()

[talex5]

Yes, it uses the original (simpler) bootstrapping scheme. I didn't know anyone was using the newer one (I thought it might have been abandoned).

I think you probably just need to use restorer instead of bootstrap in the Python server.

Possibly the OCaml code should skip setting the ID if it's the empty string so it can support either scheme.

capnp-rpc/capnp-rpc-net/serialise.ml

Line 73 in 8365bca

Schema.BuilderOps.write_string (Bootstrap.deprecated_object_id_get boot) object_id;

[LasseBlaauwbroek]

Thanks for the quick reply. Unfortunately, it looks like the workaround you suggested will not work, because the restorer named argument was removed from the python implementation in 2019 (even though the documentation still mentions it): capnproto/pycapnp@f6dd08d#diff-17d3a2bf99cbe0f9aeebce3362e3e7c5ce9a732f43c205cee6c00fee54a09790L2417

Do you have any ideas for other workarounds? Thanks.

[talex5]

Did the other change I suggested work? i.e. don't set the object_id if it's the empty string.

[LasseBlaauwbroek]

Ah sorry, I thought that was more of a meta-comment, not something for me to try. I tried it now, and I can indeed confirm it works. See PR #224.