bug#41005: problem with rendering Persian text in Emacs 27

[Pip Cet]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: hossein valizadeh <valizadeh.ho@gmail.com>
>> Date: Fri, 5 Jun 2020 17:02:55 +0430
>> Cc: pipcet@gmail.com, 41005@debbugs.gnu.org, 
>>      Nicholas Drozd <nicholasdrozd@gmail.com>
>> 
>> I'm sorry, my English is so bad, at least in writing. That's why it's
>> hard for me to give a full explanation.
>
> Your English is entirely adequate, there's nothing for you to be
> ashamed of.
>
> Thanks for the details, I think they point to the code identified by
> Pip Cet, which is called from current-column and similar APIs.  A fix
> will probably be available soon.

I think the attached patch is a fairly minimal fix; it's against master,
applies to emacs-27 but I haven't tested it there.

Given these two bugs, I wonder whether it wouldn't be more reasonable
always to let HarfBuzz guess the direction, at least for Emacs-27:
scripts which change direction, if they are supported by HarfBuzz, won't
work anyway.

Or am I missing something?

>From 18d0e15ac298f40951ddeeec56e9d87c01f51798 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@gmail.com>
Date: Fri, 5 Jun 2020 12:54:01 +0000
Subject: [PATCH] Test patch for bug#41005

---
 src/composite.c | 4 +++-
 src/indent.c    | 4 ++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/composite.c b/src/composite.c
index 2c589e4f3a..13421a80da 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -1213,7 +1213,9 @@ composition_reseat_it (struct composition_it *cmp_it, 
ptrdiff_t charpos,
        continue;
       if (charpos < endpos)
        {
-         if ((bidi_level & 1) == 0)
+         if (bidi_level < 0)
+           direction = Qnil;
+         else if ((bidi_level & 1) == 0)
            direction = QL2R;
          else
            direction = QR2L;
diff --git a/src/indent.c b/src/indent.c
index c0b4c13b2c..581323b91e 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -596,7 +596,7 @@ scan_for_column (ptrdiff_t *endpos, EMACS_INT *goalcol, 
ptrdiff_t *prevcol)
       if (cmp_it.id >= 0
          || (scan == cmp_it.stop_pos
              && composition_reseat_it (&cmp_it, scan, scan_byte, end,
-                                       w, NEUTRAL_DIR, NULL, Qnil)))
+                                       w, -1, NULL, Qnil)))
        composition_update_it (&cmp_it, scan, scan_byte, Qnil);
       if (cmp_it.id >= 0)
        {
@@ -1504,7 +1504,7 @@ compute_motion (ptrdiff_t from, ptrdiff_t frombyte, 
EMACS_INT fromvpos,
          if (cmp_it.id >= 0
              || (pos == cmp_it.stop_pos
                  && composition_reseat_it (&cmp_it, pos, pos_byte, to, win,
-                                           NEUTRAL_DIR, NULL, Qnil)))
+                                           -1, NULL, Qnil)))
            composition_update_it (&cmp_it, pos, pos_byte, Qnil);
          if (cmp_it.id >= 0)
            {
-- 
2.27.0.rc0