Inconsistent readonly status detection on RTC and after file status changes

[krassowski]

Description

#14600 implements a read-only indicator (see #7980 for design discussion) which works well when opening a read-only file:

There are two situations when this does not work:

• (a) when file was opened as read + write but was later changed to read-only
• (b) when on RTC

Reproduce

With RTC:

• open jupyterlab, create notebook, close it
• run chmod 444 notebook.ipynb
• open the notebook, change, save
• see error in log console on status bar

Without RTC:

• open jupyterlab, create notebook
• run chmod 444 notebook.ipynb
• change, save
• see error dialog

File write access restricted after opening	RTC

Expected behaviour

The save behaviour is consistent between these situations:

• content model gets a proper writable flag on RTC
• change of file permission status is propagated from server to frontend to modelStateChanged signal

Context

• JupyterLab version: main at 4.1

[krassowski]

Note: in our codebase, there are both context.model.readOnly and context.contentsModel.writable but they are not documented and it is not clear how these differ. This duplication could contribute to the problem.