Avoid infinite recursion while displaying box face

* src/xdisp.c (face_before_or_after_it_pos): Fix calculation of
the previous string/buffer character position under bidi
iteration.  (Bug#21428)

1 files changed, 20 insertions, 13 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 3eff61f3716..34b5ca3cced 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -4014,21 +4014,23 @@ face_before_or_after_it_pos (struct it *it, bool before_p)
               /* With bidi iteration, the character before the current
                  in the visual order cannot be found by simple
                  iteration, because "reverse" reordering is not
-                 supported.  Instead, we need to use the move_it_*
-                 family of functions.  */
+                 supported.  Instead, we need to start from the string
+                 beginning and go all the way to the current string
+                 position, remembering the previous position.  */
               /* Ignore face changes before the first visible
                  character on this display line.  */
               if (it->current_x <= it->first_visible_x)
                 return it->face_id;
               SAVE_IT (it_copy, *it, it_copy_data);
-              /* Implementation note: Since move_it_in_display_line
-                 works in the iterator geometry, and thinks the first
-                 character is always the leftmost, even in R2L lines,
-                 we don't need to distinguish between the R2L and L2R
-                 cases here.  */
-              move_it_in_display_line (&it_copy, SCHARS (it_copy.string),
-                                       it_copy.current_x - 1, MOVE_TO_X);
-              charpos = IT_STRING_CHARPOS (it_copy);
+              IT_STRING_CHARPOS (it_copy) = 0;
+              bidi_init_it (0, 0, FRAME_WINDOW_P (it_copy.f), &it_copy.bidi_it);
+              while (IT_STRING_CHARPOS (it_copy) != IT_STRING_CHARPOS (*it))
+                {
+                  charpos = IT_STRING_CHARPOS (it_copy);
+                  if (charpos >= SCHARS (it->string))
+                    break;
+                  bidi_move_to_visually_next (&it_copy.bidi_it);
+                }
               RESTORE_IT (it, it, it_copy_data);
             }
           else
@@ -4108,11 +4110,15 @@ face_before_or_after_it_pos (struct it *it, bool before_p)
         {
           if (before_p)
             {
+              int current_x;
+
               /* With bidi iteration, the character before the current
                  in the visual order cannot be found by simple
                  iteration, because "reverse" reordering is not
                  supported.  Instead, we need to use the move_it_*
-                 family of functions.  */
+                 family of functions, and move to the previous
+                 character starting from the beginning of the visual
+                 line.  */
               /* Ignore face changes before the first visible
                  character on this display line.  */
               if (it->current_x <= it->first_visible_x)
@@ -4123,8 +4129,9 @@ face_before_or_after_it_pos (struct it *it, bool before_p)
                  character is always the leftmost, even in R2L lines,
                  we don't need to distinguish between the R2L and L2R
                  cases here.  */
-              move_it_in_display_line (&it_copy, ZV,
-                                       it_copy.current_x - 1, MOVE_TO_X);
+              current_x = it_copy.current_x;
+              move_it_vertically_backward (&it_copy, 0);
+              move_it_in_display_line (&it_copy, ZV, current_x - 1, MOVE_TO_X);
               pos = it_copy.current.pos;
               RESTORE_IT (it, it, it_copy_data);
             }