"Two collaborative sessions" warning when one user opens the same file twice, with different schemas, in the same session

[JasonWeill]

Description

When I open a file twice, with two different schemas in the same Lab session with Collaboration enabled, I see a warning:

Two collaborative sessions are accessing the file (name) simultaneously. Opening a document with multiple views simultaneously is not supported. Please close one view; otherwise, you might lose some of your progress.

Reproduce

1. Create a notebook, foo.ipynb. Save it. Leave it open.
2. Right-click on foo.ipynb in the file browser. Hover over "Open With", and click "Editor."
3. The notebook opens in what looks like a text file editor. The warning message above appears.

Expected behavior

Either the open operation fails entirely with a message that the file is already open, or the file opens without a warning of any sort.

Personally, I think it would be acceptable to, when Jupyter Collaboration is installed, reject opening the same file more than once, with different schemas.

Context

This warning was first seen in #245, when creating a new file.

Opening the same notebook in two separate browser tabs/windows does not cause this error.

[davidbrochart]

I don't think this is a bug. It's fine to open the same file twice as long as changes are made on only one view.

[JasonWeill]

When opening the same file a second time, is it feasible to detect that the file is already open in a read/write editor, and force the second editor to be read-only?

[davidbrochart]

I think that would complicate the picture. I would rather make it possible to collaborate between different views, as I tried to explain in jupyter-server/jupyter_ydoc#15.