Redisplay loop with an unicode overlay-arrow-string

Redisplay loop with an unicode overlay-arrow-string

[Yuri D'Elia]

I'm disabling the left fringe to force using the overlay-arrow-string
(due to the fringe being unreadable on hidpi displays).

At some point I thought of being smart and attempt to:

  (setq overlay-arrow-string "🡆") ; RIGHTWARDS HEAVY ARROW

since it's an uncommon/heavy character (while still being narrower than
"=>"). Or just in case it's not readable for you, a more commonly
available character:

  (setq overlay-arrow-string "⮕") ; RIGHTWARDS BLACK ARROW

This results in an endless/unrecoverable redisplay loop on my current
config, hanging emacs.

To trigger on an emacs -q, evaluate this inside a buffer:

  (fringe-mode 0) ; force-disable the fringe
  (setq overlay-arrow-string "⮕")
  (setq overlay-arrow-position
        (set-marker (or overlay-arrow-position (make-marker))
                    (line-beginning-position)))

Re: Redisplay loop with an unicode overlay-arrow-string

[Eli Zaretskii]

> From: Yuri D'Elia <wavexx@thregr.org>
> Date: Thu, 21 Apr 2022 21:16:46 +0200
> 
>   (setq overlay-arrow-string "⮕") ; RIGHTWARDS BLACK ARROW
> 
> This results in an endless/unrecoverable redisplay loop on my current
> config, hanging emacs.
> 
> To trigger on an emacs -q, evaluate this inside a buffer:
> 
>   (fringe-mode 0) ; force-disable the fringe
>   (setq overlay-arrow-string "⮕")
>   (setq overlay-arrow-position
>         (set-marker (or overlay-arrow-position (make-marker))
>                     (line-beginning-position)))

Thanks, should now be fixed on the release branch.

(And PLEASE always report bugs using "M-x report-emacs-bug".)

Re: Redisplay loop with an unicode overlay-arrow-string

[Yuri D'Elia]

On Thu, Apr 21 2022, Eli Zaretskii wrote:
> Thanks, should now be fixed on the release branch.

Seems to be fixed for the single character case, but if you try with two
unicode chars it doesn't always work as expected:

  (setq overlay-arrow-string "━━") ; only one char shown
  (setq overlay-arrow-string " ━") ; two shown
  (setq overlay-arrow-string "━ ") ; one shown

> (And PLEASE always report bugs using "M-x report-emacs-bug".)

Sorry about this, I wasn't even sure this was a bug at first (I noticed
this due to some error thrown within xref which was using the
arrow-string). Then I wasn't even sure this was supposed to work at
all..

Re: Redisplay loop with an unicode overlay-arrow-string

[Yuan Fu]

> 
>> (And PLEASE always report bugs using "M-x report-emacs-bug".)
> 
> Sorry about this, I wasn't even sure this was a bug at first (I noticed
> this due to some error thrown within xref which was using the
> arrow-string). Then I wasn't even sure this was supposed to work at
> all..
> 

I believe the metric is “if you are unsure if it’s a bug report, it’s a bug report” ;-)

Yuan

Re: Redisplay loop with an unicode overlay-arrow-string

[Eli Zaretskii]

> From: Yuri D'Elia <wavexx@thregr.org>
> Cc: emacs-devel@gnu.org
> Date: Thu, 21 Apr 2022 23:25:52 +0200
> 
> On Thu, Apr 21 2022, Eli Zaretskii wrote:
> > Thanks, should now be fixed on the release branch.
> 
> Seems to be fixed for the single character case, but if you try with two
> unicode chars it doesn't always work as expected:
> 
>   (setq overlay-arrow-string "━━") ; only one char shown
>   (setq overlay-arrow-string " ━") ; two shown
>   (setq overlay-arrow-string "━ ") ; one shown

Fixed, thanks.

> > (And PLEASE always report bugs using "M-x report-emacs-bug".)
> 
> Sorry about this, I wasn't even sure this was a bug at first (I noticed
> this due to some error thrown within xref which was using the
> arrow-string). Then I wasn't even sure this was supposed to work at
> all..

Crashes and infloops in redisplay cannot possibly be anything but
bugs, right?

Re: Redisplay loop with an unicode overlay-arrow-string

[Yuri D'Elia]

On Thu, Apr 21 2022, Yuan Fu wrote:
> I believe the metric is “if you are unsure if it’s a bug report, it’s
> a bug report” ;-)

Heh, true, although after years of "try it within emacs -q" and seeing
the behavior changing _drammatically_, I know better that most of the
time the issue is some of my elisp customization or code, somewhere,
sometimes not even remotely related such as my build ;))

I swear I tried the unicode arrow somehow. How I did it? No idea.. it
hangs in any way I set it now ;)).

Re: Redisplay loop with an unicode overlay-arrow-string

[Eli Zaretskii]

> From: Yuri D'Elia <wavexx@thregr.org>
> Date: Fri, 22 Apr 2022 11:49:37 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, emacs-devel@gnu.org
> 
> I swear I tried the unicode arrow somehow. How I did it? No idea.. it
> hangs in any way I set it now ;)).

It only hangs (well, hanged) if the characters involved use more than
one byte in its internal representation, and then only if the byte
index of some character is greater than the index of the last
characters in the string.  The hang is (was) caused by
get-text-property, called by the code, signaling an error due to
invalid character index, which would then trigger another redisplay
cycle, which will then cause another error, etc. etc.