Restarting a kernel hangs

[davidbrochart]

Description

Restarting a kernel fails with e.g. ipykernel.

Reproduce

1. In JupyterLab, open a notebook and click "Restart kernel".
2. See that it hangs.

Jupyverse uses a shutdown_request with restart: true in the content. It looks like ipykernel doesn't support this, while it works fine with akernel.

Expected behavior

Restarting a kernel should work. Maybe first shutdown the kernel then start it again, instead of relying on the kernel to do it in one step.

Context

See:

• Kernel shutdown and restart ipython/ipykernel#840
• Clarify "shutdown and restart" specification jupyter/jupyter_client#736