bug#39977: 28.0.50; Unhelpful stack trace

[martin rudalics]

>> We already disallow deleting the last live or visible frame and the last
>> window on a frame.
>
> Those situations are easy to detect, so we do that.

For some value of easy.

> You are now
> proposing something more sophisticated than that, and I'm afraid that
> doing so is not as straightforward as in those few simple cases we
> already handle.

I'm afraid that we already might mishandle some of those simple cases.

>> So the redisplay code, whenever it runs Lisp in between, could
>> simply set a boolean that will disallow deleting any window or frame
>> as well as setting the window configuration and other dangerous
>> operations that implicitly might kill a window or a buffer.
>
> The problem is how to do this without breaking legitimate code.  For
> example, changing the window configuration temporarily, then changing
> it back is quite legitimate,

Right in the middle of redisplay, while constructing the mode line or
the title format?  I won't object but this is something we should decide
ASAP in order to decide which kind of solution to pursue.

> so summarily disallowing such actions is
> too drastic and will be hard to justify.
[...]
> All we need to do is avoid crashing and keeping the display
> up-to-date; any other outcome: error messages, code that doesn't do
> what the author expected/intended, and any other annoyance -- are
> completely fine, because whoever writes such nasty code will learn a
> lesson.

Hmmm...  I thought we have all those emacs_abort instances to ease
debugging.

martin