Allow input on comm messages

[jasongrout]

The execute request message has an allow_stdin parameter to instruct the kernel if input could be handled. However, the comm messages do not have a similar parameter, which means, for example, you can't set a breakpoint in a comm message handler and have interactive debugging.

If we had such a parameter in comm messages, they would no longer be symmetric, but this is still an important usecase.

See jupyter-widgets/ipywidgets#1603, CC @TimShawver.

For an implementation, it looks like essentially, we'd need to do something like https://github.com/ipython/ipykernel/blob/8acaee80c83c9a64a22afde522f9df2d0c3638e4/ipykernel/ipkernel.py#L200 and https://github.com/ipython/ipykernel/blob/8acaee80c83c9a64a22afde522f9df2d0c3638e4/ipykernel/ipkernel.py#L206 when processing comm messages, i.e.:

• https://github.com/ipython/ipykernel/blob/8acaee80c83c9a64a22afde522f9df2d0c3638e4/ipykernel/comm/manager.py#L90
• https://github.com/ipython/ipykernel/blob/8acaee80c83c9a64a22afde522f9df2d0c3638e4/ipykernel/comm/manager.py#L111
• https://github.com/ipython/ipykernel/blob/8acaee80c83c9a64a22afde522f9df2d0c3638e4/ipykernel/comm/manager.py#L126

Thoughts?

[minrk]

Probably makes sense.

[oeway]

Any update to this issue?