bug#60096: 29.0.60; Crash in format_mode_line_unwind_data

[martin rudalics]

>> Alternatively, we could exclude windows with a nil buffer in
>> add_window_to_list (think of the case that within the blurb
>> producing code someone wants to consult the window list).
>
> Maybe we should try this on master.  I indeed expected
> add_window_to_list to filter such invalid windows and was surprised
> that it didn't.  Basically, I don't understand how we never had such
> windows in the list before, since there's no code which actively
> removes them and thus protects the list from holding such windows.  I
> think we just have been lucky.

Probably so far we never tried to call 'kill-buffer' from within
'set-window-configuration'.  If the only "live" window shows *scratch*,
*scratch* gets killed and we kill a temporary buffer before we were able
to recreate *scratch*, window_list will return the empty list.

>> Principally, we should never run 'replace-buffer-in-windows' from within
>> 'set-window-configuration'.
>
> This can no longer be guaranteed, given that other_buffer_safely calls
> into Lisp, and so do a few other primitives.

What if such a call into Lisp tries to run 'set-window-configuration'?

> You are right in principle, but other_buffer_safely was doing the
> above for many years before we acquired get-scratch-buffer-create and
> started calling it from here.  So I think we are relatively safe
> (again, maybe by pure chance).

Then not calling 'get-scratch-buffer-create' from other_window_safely
would be more safe.

martin