Parent header missing in response

[anutosh491]

Check jupyter-server/jupyter_server#1497 where this has been explained in detail.

And check comment jupyter-server/jupyter_server#1497 (comment)

We may want to debug the reply message creation and find why you don't have a parent header in the response !!

[kr-2003]

Experiments & Observations

1. When running jupyter notebook and then selecting xcpp20 as kernel, the xcpp is invoked locally.
2. I ran xcpp locally and tried to trace the message format in which the xeus kernel sends message to xeus-cpp.
3. Stack trace when the kernel is invoked.

Thread 1 "xcpp" received signal SIGINT, Interrupt.
0x00007ffff7b2fbbf in __GI___poll (fds=0x7fffffffb670, nfds=2, timeout=-1)
    at ../sysdeps/unix/sysv/linux/poll.c:29
29	../sysdeps/unix/sysv/linux/poll.c: No such file or directory.
(gdb) bt
#0  0x00007ffff7b2fbbf in __GI___poll (fds=0x7fffffffb670, nfds=2, timeout=-1)
    at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007fffebc1fd66 in zmq_poll () from /home/abhinav/micromamba/envs/xeus-cpp/lib/./libzmq.so.5
#2  0x00007ffff7f141d6 in xeus::xserver_zmq_impl::poll_channels(long) ()
   from /home/abhinav/micromamba/envs/xeus-cpp/lib/libxeus-zmq.so.6
#3  0x00007ffff7f18f05 in xeus::xserver_zmq::poll_channels(long) ()
   from /home/abhinav/micromamba/envs/xeus-cpp/lib/libxeus-zmq.so.6
#4  0x00007ffff7f0f534 in xeus::xserver_zmq_default::start_impl(xeus::xpub_message) ()
   from /home/abhinav/micromamba/envs/xeus-cpp/lib/libxeus-zmq.so.6
#5  0x00007ffff7e985a6 in xeus::xserver::start(xeus::xpub_message) ()
   from /home/abhinav/micromamba/envs/xeus-cpp/lib/libxeus.so.11
#6  0x00007ffff7e71c24 in xeus::xkernel::start() ()
   from /home/abhinav/micromamba/envs/xeus-cpp/lib/libxeus.so.11
#7  0x0000555555557a39 in main ()

4. After main(), the execution enters into xeus::xkernel::start().

void xkernel::start()
{
    xpub_message start_msg = p_core->build_start_msg();
    p_server->start(std::move(start_msg));
}

5. Here is the build_start_msg() function.

xpub_message xkernel_core::build_start_msg() const
{
    std::string topic = "kernel_core." + m_kernel_id + ".status";
    nl::json content;
    content["execution_state"] = "starting";

    xpub_message msg(topic,
                     make_header("status", m_user_name, m_session_id),
                     nl::json::object(),
                     nl::json::object(),
                     std::move(content),
                     buffer_sequence());
    return msg;
}

6. The third argument in the xpub_message msg(...) is the parent_header that is being passed down.

Constructor for xpub_message

xpub_message(const std::string& topic,
             nl::json header,
             nl::json parent_header,
             nl::json metadata,
             nl::json content,
             buffer_sequence buffers);

[kr-2003]

@anutosh491
I think this bug is more related to xeus. Since, xeus xeus provides two implementations of the server component, one built upon ZeroMQ and one built for in-memory communication in the browser. So, the messaging thing is handled by xeus and implementation of interpreter part in xeus is handled by xeus-cpp.

[anutosh491]

Yes. Shall look into it on the xeus side. Thanks for the debug. Great work .

[anutosh491]

Closing this as has been addressed through jupyter server.

Read discussion following jupyter-server/jupyter_server#1497 (comment) and below.