bug#19200: Point adjustemnt moves *into* invisible text

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: 19200@debbugs.gnu.org, michael_heerdegen@web.de, jonas@bernoul.li
> Date: Wed, 23 Mar 2016 12:10:26 -0400
> 
> So someone needs to step through the code and figure out why this
> doesn't happen.

I guess you expected me to be that Someone...

> E.g. maybe it doesn't happen because adjust_point_for_property is
> not called at all.

It _is_ called.

> >> The issue of the main bug is not so much that we don't know how to fix
> >> it, but that noone has bothered to investigate it to try and figure out
> >> what is actually happening.
> > Didn't I do that?  Doesn't the fact that the relevant code calls
> > get-char-property-and-overlay explain what happens?
> 
> No: the get-char-property-and-overlay calls will only determine the
> boundaries of the invisible text (i.e. they should find that the
> invisible chunk goes between 3 and 5).

The function is entered with point at 5, so 'beg' and 'end' start with
that value.

Then get_char_property_and_overlay in the "while (end < ZV" loop
returns nil for position 5, so that loop is exited immediately.

Then a similar call in the "while (beg > BEGV" loop returns t for
position 5 - 1 = 4.  Then previous-single-char-property-change returns
3, so 'beg' becomes 3.  Then another call to
get_char_property_and_overlay returns nil for position 3 - 1 = 2, and
the while loop is exited with beg = 3 and end = 5.  Since point is 5,
we land here:

          /* Pretend the area doesn't exist if the buffer is not
             modified.  */
          if (!modified && !ellipsis && beg < end)
            {
              if (last_pt == beg && PT == end && end < ZV)
                (check_composition = check_display = true, SET_PT (end + 1));
              else if (last_pt == end && PT == beg && beg > BEGV)
                (check_composition = check_display = true, SET_PT (beg - 1));
              else if (PT == ((PT < last_pt) ? beg : end))
                /* We've already moved as far as we can.  Trying to go
                   to the other end would mean moving backwards and thus
                   could lead to an infinite loop.  */
                ;
              else if (val = Fget_pos_property (make_number (PT),
                                                Qinvisible, Qnil),
                       TEXT_PROP_MEANS_INVISIBLE (val)
                       && (val = (Fget_pos_property
                                  (make_number (PT == beg ? end : beg),
                                   Qinvisible, Qnil)),
                           !TEXT_PROP_MEANS_INVISIBLE (val)))
                (check_composition = check_display = true,
                 SET_PT (PT == beg ? end : beg));
            }

last_pt is 1, so we wind up in this branch:

              else if (PT == ((PT < last_pt) ? beg : end))
                /* We've already moved as far as we can.  Trying to go
                   to the other end would mean moving backwards and thus
                   could lead to an infinite loop.  */
                ;

which does nothing.  So point never moves and stays at 5.

> After that, adjust_point_for_property should start by moving point to
> position 3 (because last_pt should be < 3).

It doesn't.

> And after that it should use Fget_pos_property to decide whether to stay
> at position 3 or to move to position 5, and in this case it should
> choose to stay at position 3.

It doesn't get there.