bug#60096: 29.0.60; Crash in format_mode_line_unwind_data

[martin rudalics <rudalics@gmx.at>]

 >> 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