Re: non-local exits in redisplay

[Akira Kyle]

On Mon, Oct 12, 2020 at 11:01 AM, Eli Zaretskii <eliz@gnu.org> 
wrote:

> As Stefan mentions, you are well advised not to signal errors 
> inside
> redisplay.  The reason is simple: signaling an error always 
> reenters
> redisplay immediately, because we need to display the error 
> message.
> While that is a special kind of redisplay, it usually redraws at 
> least
> one window, and can redraw more, depending on what exactly is on
> display.  It is virtually impossible to predict when displaying 
> the
> error signal message will hit the same error again; even if you 
> seem
> to be able to get away with that in some situation, another user 
> of
> your code might not be so lucky, if they use some specialized 
> feature,
> like post-command-hooks that affect the display (it could be as 
> simple
> as hl-line-mode or similar), or functions that run off timers 
> that
> display something, or any number of other optional features, of 
> which
> Emacs has  a gazillion.

Yes as I'm learning more about the interplay between the lisp 
interpreter and the complicated machinery of redisplay, I'm 
realizing there are many ways to shoot yoursef in the foot.

> So you really shouldn't signal errors from redisplay.  the most 
> you
> can do is silently deposit an error message in the *Messages* 
> buffer
> (that's what the safe_call and friends do, btw) and hope that 
> the user
> will look there at some point, e.g. because the display doesn't 
> look
> "right".

Which isn't very satisfactory from a UI perspective. I suppose 
setting some flag in redisplay which could then be checked later 
in lisp to signal the error would be the obvious work around.

> Can you explain why you'd need to signal an error from the 
> display
> code?  Maybe we can find some way of dealing with your problem 
> that is
> better than just inserting a message in *Messages*.
>
> And one more thing: please don't over-use the safe_call facility 
> as
> well, i.e. try not to call too much Lisp from the display 
> engine.  As
> convenient as that may sound, calling Lisp from display has its
> downsides.  For starters, it produces more garbage, which 
> triggers
> more frequent GCs, which slows down Emacs.
>
> So if you are tempted to call Lisp, try first to find an 
> equivalent
> way of doing that in C (don't hesitate to ask questions if you 
> cannot
> find it or are unsure how to do that), and only use Lisp if the
> alternative is not reasonable.

Perhaps I'm really getting ahead of myself here...
I'll describe what my goal is for the xwidgets code, which is what 
ultimately prompted this question, in a new thread in case others 
might be interested in commenting on it.

Thanks!
Akira