Re: "Final" version of tty child frames

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Emacs Devel <emacs-devel@gnu.org>
> Date: Wed, 23 Oct 2024 07:00:03 +0200
> 
> Another question is if there is something lurking with R2L? I have no
> idea what terminals do in that case, but I'd guess Eli knows :-).

Terminals should do nothing, and if your terminal emulator has a "bidi
reordering" feature, you should disable it, because Emacs does
everything by itself, and needs the terminal to be "dumb" in this
aspect.

Basically, we produce a glyph row where all the glyphs are flushed to
the right window edge, and there's empty space on the left of each
screen line to make it a full-width screen line.  The low-level write
code that writes to the terminal then works normally, writing glyphs
left to right.

The current branch hits an assertion violation when showing RTL text
in a child window.  (Showing RTL text in the parent window works
fine.)  Here's the recipe:

 emacs -Q -nw
 Evaluate in *scratch:

    (defun my-make-child ()
      (interactive)
      (make-frame `((parent-frame . ,(selected-frame))
		    (background-color . "gray10")
		    (foreground-color . "white")
		    (internal-border-width . 1)
		    (top . 15)
		    (left . 40)
		    (width . 80)
		    (height . 25))))

 M-x my-make-child RET
 C-x 5 o
 C-x C-f etc/tutorials/TUTORIAL.he RET

The backtrace from the assertion is below, including the data which is
involved.  As you see, X is 119 whereas the matrix_w is 108, which
violates the assertion.

Let me know if you want me to try something or show more data or maybe
tell you more about how RTL display works in TTY case.

  Thread 1 "emacs" received signal SIGABRT, Aborted.
  0x00007ff5201959fc in pthread_kill () from /lib/x86_64-linux-gnu/libc.so.6
  (gdb) bt
  #0  0x00007ff5201959fc in pthread_kill () at /lib/x86_64-linux-gnu/libc.so.6
  #1  0x00007ff520141476 in raise () at /lib/x86_64-linux-gnu/libc.so.6
  #2  0x0000559f4892c840 in terminate_due_to_signal
      (sig=sig@entry=6, backtrace_limit=backtrace_limit@entry=2147483647)
      at emacs.c:469
  #3  0x0000559f48933221 in die
      (msg=msg@entry=0x559f48c2ceb8 "x < root->current_matrix->matrix_w", file=file@entry=0x559f48c2c004 "dispnew.c", line=line@entry=3774) at alloc.c:8059
  #4  0x0000559f4891c4cb in is_cursor_obscured () at dispnew.c:3774
  #5  terminal_cursor_magic (topmost_child=0x559f4b07b9b0, root=0x559f4afcd050)
      at dispnew.c:3790
  #6  combine_updates_for_frame
      (f=<optimized out>, force_p=<optimized out>, inhibit_scrolling=<optimized out>) at dispnew.c:3840
  #7  0x0000559f48954656 in combine_updates
      (roots=roots@entry=0x7ff51c751723, force_p=force_p@entry=false, inhibit_scrolling=inhibit_scrolling@entry=false) at dispnew.c:3867
  #8  0x0000559f489af858 in redisplay_internal () at xdisp.c:17612
  #9  0x0000559f48abe24a in read_char
      (commandflag=1, map=0x7ff51c752503, prev_event=0x0, used_mouse_menu=0x7fff251757eb, end_time=0x0) at keyboard.c:2673
  #10 0x0000559f48abf794 in read_key_sequence
      (keybuf=0x7fff25175940, prompt=0x0, dont_downcase_last=<optimized out>, can_return_switch_frame=true, fix_current_buffer=true, prevent_redisplay=<optimized out>, disable_text_conversion_p=false) at keyboard.c:10747
  #11 0x0000559f48ac14f1 in command_loop_1 () at keyboard.c:1424
  #12 0x0000559f48b4fee7 in internal_condition_case
      (bfun=bfun@entry=0x559f48ac1280 <command_loop_1>, handlers=handlers@entry=0x90, hfun=hfun@entry=0x559f48ab4d50 <cmd_error>) at eval.c:1598
  #13 0x0000559f48aaab8e in command_loop_2 (handlers=handlers@entry=0x90)
      at keyboard.c:1163
  #14 0x0000559f48b4fd69 in internal_catch
      (tag=tag@entry=0x12360, func=func@entry=0x559f48aaab60 <command_loop_2>, arg=arg@entry=0x90) at eval.c:1277
  #15 0x0000559f48aaab29 in command_loop () at keyboard.c:1141
  #16 0x0000559f48ab4805 in recursive_edit_1 () at keyboard.c:749
  #17 0x0000559f48ab4bb8 in Frecursive_edit () at keyboard.c:832
  #18 0x0000559f489479e6 in main (argc=<optimized out>, argv=<optimized out>)
      at emacs.c:2624
  (gdb) up
  #1  0x00007fd8d9b21476 in raise () from /lib/x86_64-linux-gnu/libc.so.6
  (gdb)
  #2  0x000055ae22b9a840 in terminate_due_to_signal (sig=sig@entry=6,
      backtrace_limit=backtrace_limit@entry=2147483647) at emacs.c:469
  469       emacs_raise (sig);
  (gdb)
  #3  0x000055ae22ba1221 in die (
      msg=msg@entry=0x55ae22e9aeb8 "x < root->current_matrix->matrix_w",
      file=file@entry=0x55ae22e9a004 "dispnew.c", line=line@entry=3774)
      at alloc.c:8059
  8059      terminate_due_to_signal (SIGABRT, INT_MAX);
  (gdb)
  #4  0x000055ae22b8a4cb in is_cursor_obscured () at dispnew.c:3774
  3774      eassert (x < root->current_matrix->matrix_w);
  (gdb) p x
  $1 = 119
  (gdb) p root->current_matrix->matrix_w
  value has been optimized out
  (gdb) p root->current_matrix
  value has been optimized out
  (gdb) p root
  $2 = <optimized out>
  (gdb) up
  #5  terminal_cursor_magic (topmost_child=0x55ae245e89e8, root=0x55ae245c3050)
      at dispnew.c:3790
  3790      if (is_cursor_obscured ())
  (gdb) p root->current_matrix->matrix_w
  $3 = 108