Re: Test to determine character left-overhangs a tab stretch.

[Eli Zaretskii]

> Date:  Tue, 13 Nov 2018 12:57:08 -0800
> From:  Keith David Bershatsky <address@hidden>
> Cc:  address@hidden
> 
> Thank you Eli for helping out with this particular issue.  I think the 
> problem is that first_glyph->type (in the following test) is sometimes 
> invalid and that is what causes the crash.  The test is designed to skip over 
> the area of code when a tab stretch would normally be drawn anew due to the 
> next glyph having a left overhang.
> 
> Last Commit : Tue Nov 13 22:01:57 2018 +0200
>             : Eli Zaretskii <address@hidden>
>             : 4a5a17507fe1e12ee02c174350edc479fb01ac01
>             : Fix recent change in fileio.c
> 
> diff --git a/src/xdisp.c b/src/xdisp.c
> index fa7691c..198a516 100644
> --- a/src/xdisp.c
> +++ b/src/xdisp.c
> @@ -26841,7 +26841,45 @@ draw_glyphs (struct window *w, int x, struct 
> glyph_row *row,
>        prepended must be drawn because the first glyph string
>        draws over it.  */
>        i = left_overwritten (head);
> -      if (i >= 0)

left_overwritten can return a negative value, in which case this:

> +      bool skip_p = false;
> +      int beg = i;
> +
> +      if (hl == DRAW_CURSOR)
> +        while (beg < start)
> +          {
> +            struct glyph *first_glyph = (row)->glyphs[area] + beg;
> +            if (first_glyph != NULL
> +                && first_glyph->type != NULL)

attempts to dereference a pointer that is before the beginning of
row->glyphs[area], and you get a segfault.  I think your code should
be inside the "if (i >= 0)" clause.

Stepping back, I'm not sure what you do is a good idea.  Redrawing
neighboring glyphs due to overhanging is done for a reason, so
skipping it might well produce incorrect display.  I advice against
that.