bug#12867: 24.3.50; easy-to-repro crash involving mode line

[Eli Zaretskii <eliz@gnu.org>]

> From: "Drew Adams" <drew.adams@oracle.com>
> Cc: <12867@debbugs.gnu.org>
> Date: Mon, 12 Nov 2012 10:16:55 -0800
> 
> > Yes, post-command-hook is ugly and expensive.  But you should know
> > that %l is not ideal either: e.g., if display of the line numbers is
> > disabled because the file is too large or the lines are too long, and
> > Emacs displays "???" instead of the number, I think mode line is not
> > redisplayed when the current line changes.  Again, because Emacs tries
> > very hard to avoid this costly redisplay.
> 
> I see - good to know.  I don't expect it will be a problem here, but I'll keep
> an eye out.  Any idea what size buffers lead to such behavior?

Buffers that have more lines than line-number-display-limit or lines
wider than line-number-display-limit-width.

> > > I will be glad to find a way to simplify the code and 
> > > remove this ugly little hack.  Suggestions welcome.
> > 
> > A user option sounds like the right approach.
> 
> Do you mean at the level of my code, or are you thinking about an enhancement
> for Emacs?  The rest of your text leads me to think the latter.  If not, I'm not
> sure what kind of option you have in mind here.

I mean a new option, an enhancement.

> > But we should first formulate the conditions under which this
> > redisplay will be performed.
> 
> If we're talking about my use case then it is each time the current line
> changes.

Would it be good enough to redisplay whenever point moves, and let
your code you run from :eval decide whether the text on the mode line
needs to be changed?  I think this will be a more general solution.

> > Anyway, it looks like this discussion should continue in another bug
> > report, as the crash is solved.
> 
> OK.  Do you want to formulate the bug subject?

Done: bug #12872.  Let's continue there.