bug#56393: Actually fix the long lines display bug

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 17 Jul 2022 15:37:08 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: larsi@gnus.org, 56393@debbugs.gnu.org
> 
> 
> >
> > I still see a segfault in get_visually_first_element.  To reproduce:
> >
> > emacs -Q
> > C-x C-f long-line.xml
> > C-u 12 M-g g
> > C-s C-q C-j
> >
> > IOW, go to line 12 and search for a newline.  I think this segfaults 
> > because point is very far from window-start, so the narrowing doesn't 
> > include the position where get_visually_first_element is called.
> >
> > Similarly, this also segfaults:
> >
> > emacs -Q
> > C-x C-f long-line.xml
> > C-u 15000 M-g c
> > C-x 2
> >
> 
> Thanks.  I can't reproduce either of these two bugs with a regular build, 
> both of these recipes produce the expected result.  Are these segfaults 
> due to --enable-checking?

No, I don't think so.  --enable-checking produces SIGABRT, not
SIGSEGV.

The segfault happens because it->bidi_it.bytepos is 1, and FETCH_BYTE
cannot be called with a zero argument.  Here's a relevant portion of a
debugging session:

  Thread 1 received signal SIGSEGV, Segmentation fault.
  0x01055028 in get_visually_first_element (it=0x82d250) at xdisp.c:8691
  8691                   && (FETCH_BYTE (it->bidi_it.bytepos - 1) == '\n'
  (gdb) bt
  #0  0x01055028 in get_visually_first_element (it=0x82d250) at xdisp.c:8691
  #1  0x010563d4 in next_element_from_buffer (it=0x82d250) at xdisp.c:9149
  #2  0x01052bd3 in get_next_display_element (it=0x82d250) at xdisp.c:7786
  #3  0x01057166 in move_it_in_display_line_to (it=0x82d250, to_charpos=306655,
      to_x=0, op=(MOVE_TO_X | MOVE_TO_POS)) at xdisp.c:9574
  #4  0x01059ce7 in move_it_to (it=0x82d250, to_charpos=306655, to_x=-1,
      to_y=543, to_vpos=-1, op=10) at xdisp.c:10245
  #5  0x01038364 in pos_visible_p (w=0x75fd640, charpos=306655, x=0x82e23c,
      y=0x82e238, rtop=0x82e24c, rbot=0x82e248, rowh=0x82e244, vpos=0x82e240)
      at xdisp.c:1730
  #6  0x010ca308 in Fpos_visible_in_window_p (pos=XIL(0), window=XIL(0),
      partially=XIL(0x30)) at window.c:2000
  #7  0x01276489 in funcall_subr (subr=0x173c8c0 <Spos_visible_in_window_p>,
      numargs=3, args=0x68b0248) at eval.c:3000
  #8  0x012e9634 in exec_byte_code (fun=XIL(0xa000000005db8a1c),
      args_template=768, nargs=3, args=0x68b0208) at bytecode.c:809
  #9  0x012769cd in fetch_and_exec_byte_code (fun=XIL(0xa000000005db9bdc),
      args_template=256, nargs=1, args=0x82ebd0) at eval.c:3045
  #10 0x01276f2c in funcall_lambda (fun=XIL(0xa000000005db9bdc), nargs=1,
      arg_vector=0x82ebd0) at eval.c:3117
  #11 0x01275ee2 in funcall_general (fun=XIL(0xa000000005db9bdc), numargs=1,
      args=0x82ebd0) at eval.c:2908
  #12 0x0127623e in Ffuncall (nargs=2, args=0x82ebc8) at eval.c:2958
  #13 0x01265666 in Ffuncall_interactively (nargs=2, args=0x82ebc8)
      at callint.c:291
  #14 0x0127687e in funcall_subr (subr=0x1747440 <Sfuncall_interactively>,
      numargs=2, args=0x82ebc8) at eval.c:3023
  #15 0x01275e7b in funcall_general (fun=XIL(0xa000000001747440), numargs=2,
      args=0x82ebc8) at eval.c:2904
  #16 0x0127623e in Ffuncall (nargs=3, args=0x82ebc0) at eval.c:2958
  #17 0x01268869 in Fcall_interactively (function=XIL(0x452974c),
      record_flag=XIL(0), keys=XIL(0xa0000000076986d8)) at callint.c:829
  #18 0x01276489 in funcall_subr (subr=0x1747480 <Scall_interactively>,
      numargs=3, args=0x68b0078) at eval.c:3000
  #19 0x012e9634 in exec_byte_code (fun=XIL(0xa000000005dff734),
      args_template=1025, nargs=1, args=0x82f5b0) at bytecode.c:809
  #20 0x012769cd in fetch_and_exec_byte_code (fun=XIL(0xa000000005dff734),
      args_template=1025, nargs=1, args=0x82f5a8) at eval.c:3045
  #21 0x01276f2c in funcall_lambda (fun=XIL(0xa000000005dff734), nargs=1,
      arg_vector=0x82f5a8) at eval.c:3117
  #22 0x01275ee2 in funcall_general (fun=XIL(0xa000000005dff734), numargs=1,
      args=0x82f5a8) at eval.c:2908
  #23 0x0127623e in Ffuncall (nargs=2, args=0x82f5a0) at eval.c:2958
  #24 0x0116b50a in call1 (fn=XIL(0x4440), arg1=XIL(0x452974c)) at lisp.h:3239
  #25 0x0116f73c in command_loop_1 () at keyboard.c:1510
  #26 0x01270662 in internal_condition_case (bfun=0x116eab0 <command_loop_1>,
      handlers=XIL(0x90), hfun=0x116da7e <cmd_error>) at eval.c:1485
  #27 0x0116e51d in command_loop_2 (handlers=XIL(0x90)) at keyboard.c:1132
  #28 0x0126f4e9 in internal_catch (tag=XIL(0xfe70),
      func=0x116e4e6 <command_loop_2>, arg=XIL(0x90)) at eval.c:1208
  #29 0x0116e488 in command_loop () at keyboard.c:1110
  #30 0x0116d4de in recursive_edit_1 () at keyboard.c:719
  #31 0x0116d77c in Frecursive_edit () at keyboard.c:802
  #32 0x011686e6 in main (argc=2, argv=0xa42a10) at emacs.c:2517

  Lisp Backtrace:
  "pos-visible-in-window-p" (0x68b0248)
  "pos-visible-in-window-group-p" (0x68b01f0)
  "isearch-update" (0x68b01a0)
  "isearch-search-and-update" (0x68b0178)
  "isearch-process-search-string" (0x68b0138)
  "isearch-process-search-char" (0x68b00d8)
  "isearch-quote-char" (0x82ebd0)
  "funcall-interactively" (0x82ebc8)
  "call-interactively" (0x68b0078)
  "command-execute" (0x82f5a8)
  (gdb) p it.bidi_it.bytepos
  $1 = 1
  (gdb) p bob
  $2 = 293760
  (gdb) p it->current
  $3 = {
    pos = {
      charpos = 1,
      bytepos = 1
    },
    overlay_string_index = -1,
    string_pos = {
      charpos = -1,
      bytepos = -1
    },
    dpvec_index = -1
  }