bug#56393: Actually fix the long lines display bug

[Eli Zaretskii <eliz@gnu.org>]

> Cc: gerd.moellmann@gmail.com, larsi@gnus.org, 56393@debbugs.gnu.org
> Date: Sat, 09 Jul 2022 13:54:24 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> Here's one problem caused by using PT in get_narrowed_begv:
> 
>   emacs -Q
>   C-x C-f src/xdisp.c RET
>   C-x 2
>   M->

Here's another:

  emacs -Q
  C-x C-f src/xdisp.c
  C-x *of

After I type 'f' of "*of", Emacs segfaults:

  Thread 1 received signal SIGSEGV, Segmentation fault.
  0x01054f5c in get_visually_first_element (it=0x82d210) at xdisp.c:8667
  8667                   && (FETCH_BYTE (it->bidi_it.bytepos - 1) == '\n'
  (gdb) bt
  #0  0x01054f5c in get_visually_first_element (it=0x82d210) at xdisp.c:8667
  #1  0x010562b8 in next_element_from_buffer (it=0x82d210) at xdisp.c:9125
  #2  0x01052b35 in get_next_display_element (it=0x82d210) at xdisp.c:7762
  #3  0x0105704a in move_it_in_display_line_to (it=0x82d210, to_charpos=48949,
      to_x=0, op=(MOVE_TO_X | MOVE_TO_POS)) at xdisp.c:9550
  #4  0x01059bcb in move_it_to (it=0x82d210, to_charpos=48949, to_x=-1,
      to_y=543, to_vpos=-1, op=10) at xdisp.c:10221
  #5  0x01038364 in pos_visible_p (w=0x75fd230, charpos=48949, x=0x82e1fc,
      y=0x82e1f8, rtop=0x82e20c, rbot=0x82e208, rowh=0x82e204, vpos=0x82e200)
      at xdisp.c:1730
  #6  0x010c9fcc in Fpos_visible_in_window_p (pos=XIL(0), window=XIL(0),
      partially=XIL(0x30)) at window.c:2000
  #7  0x0127608d in funcall_subr (subr=0x173c8c0 <Spos_visible_in_window_p>,
      numargs=3, args=0x68b0240) at eval.c:3000
  #8  0x012e9238 in exec_byte_code (fun=XIL(0xa000000005db8a1c),
      args_template=768, nargs=3, args=0x68b0200) at bytecode.c:809
  #9  0x012765d1 in fetch_and_exec_byte_code (fun=XIL(0xa000000005e00f1c),
      args_template=512, nargs=2, args=0x82eb60) at eval.c:3045
  #10 0x01276b30 in funcall_lambda (fun=XIL(0xa000000005e00f1c), nargs=2,
      arg_vector=0x82eb60) at eval.c:3117
  #11 0x01275ae6 in funcall_general (fun=XIL(0xa000000005e00f1c), numargs=2,
      args=0x82eb60) at eval.c:2908
  #12 0x01275e42 in Ffuncall (nargs=3, args=0x82eb58) at eval.c:2958
  #13 0x0126526a in Ffuncall_interactively (nargs=3, args=0x82eb58)
      at callint.c:291
  #14 0x01276482 in funcall_subr (subr=0x1747440 <Sfuncall_interactively>,
      numargs=3, args=0x82eb58) at eval.c:3023
  #15 0x01275a7f in funcall_general (fun=XIL(0xa000000001747440), numargs=3,
      args=0x82eb58) at eval.c:2904
  #16 0x01275e42 in Ffuncall (nargs=4, args=0x82eb50) at eval.c:2958
  #17 0x01274d39 in Fapply (nargs=3, args=0x82ecc8) at eval.c:2629
  #18 0x0126599c in Fcall_interactively (function=XIL(0x4570a8c),
      record_flag=XIL(0), keys=XIL(0xa000000007a59e70)) at callint.c:384
  #19 0x0127608d in funcall_subr (subr=0x1747480 <Scall_interactively>,
      numargs=3, args=0x68b0078) at eval.c:3000
  #20 0x012e9238 in exec_byte_code (fun=XIL(0xa000000005dff734),
      args_template=1025, nargs=1, args=0x82f5b0) at bytecode.c:809
  #21 0x012765d1 in fetch_and_exec_byte_code (fun=XIL(0xa000000005dff734),
      args_template=1025, nargs=1, args=0x82f5a8) at eval.c:3045
  #22 0x01276b30 in funcall_lambda (fun=XIL(0xa000000005dff734), nargs=1,
      arg_vector=0x82f5a8) at eval.c:3117
  #23 0x01275ae6 in funcall_general (fun=XIL(0xa000000005dff734), numargs=1,
      args=0x82f5a8) at eval.c:2908
  #24 0x01275e42 in Ffuncall (nargs=2, args=0x82f5a0) at eval.c:2958
  #25 0x0116b1ce in call1 (fn=XIL(0x4440), arg1=XIL(0x4570a8c)) at lisp.h:3239
  #26 0x0116f400 in command_loop_1 () at keyboard.c:1510
  #27 0x01270266 in internal_condition_case (bfun=0x116e774 <command_loop_1>,
      handlers=XIL(0x90), hfun=0x116d742 <cmd_error>) at eval.c:1485
  #28 0x0116e1e1 in command_loop_2 (handlers=XIL(0x90)) at keyboard.c:1132
  #29 0x0126f0ed in internal_catch (tag=XIL(0xfe70),
      func=0x116e1aa <command_loop_2>, arg=XIL(0x90)) at eval.c:1208
  #30 0x0116e14c in command_loop () at keyboard.c:1110
  #31 0x0116d1a2 in recursive_edit_1 () at keyboard.c:719
  #32 0x0116d440 in Frecursive_edit () at keyboard.c:802
  #33 0x011683aa in main (argc=2, argv=0xa42a10) at emacs.c:2517

  Lisp Backtrace:
  "pos-visible-in-window-p" (0x68b0240)
  "pos-visible-in-window-group-p" (0x68b01e8)
  "isearch-update" (0x68b0198)
  "isearch-search-and-update" (0x68b0170)
  "isearch-process-search-string" (0x68b0130)
  "isearch-process-search-char" (0x68b00e0)
  "isearch-printing-char" (0x82eb60)
  "funcall-interactively" (0x82eb58)
  "call-interactively" (0x68b0078)
  "command-execute" (0x82f5a8)
  (gdb) p it->current
  $1 = {
    pos = {
      charpos = 1,
      bytepos = 1
    },
    overlay_string_index = -1,
    string_pos = {
      charpos = -1,
      bytepos = -1
    },
    dpvec_index = -1
  }
  (gdb) p it->bidi_it.bytepos
  $2 = 1
  (gdb) p bob
  $3 = 20000
  (gdb) p it->narrowed_begv
  $4 = 20000

This is because start_display was called to start from the window's
start point:

  #5  0x01038364 in pos_visible_p (w=0x75fd230, charpos=48949, x=0x82e1fc,
      y=0x82e1f8, rtop=0x82e20c, rbot=0x82e208, rowh=0x82e204, vpos=0x82e200)
      at xdisp.c:1730
  1730      move_it_to (&it, charpos, -1, it.last_visible_y - 1, -1,
  (gdb) p top
  $6 = {
    charpos = 1,
    bytepos = 1
  }
  (gdb) p PT
  $7 = 48949
  (gdb) p w->start
  $8 = XIL(0xa00000000760a9d0)
  (gdb) xmarker
  $9 = (struct Lisp_Marker *) 0x760a9d0
  (gdb) p *$
  $10 = {
    header = {
      size = 1124081664
    },
    buffer = 0x76be1b8,
    need_adjustment = 0,
    insertion_type = 0,
    next = 0x7621158,
    charpos = 1,
    bytepos = 1
  }

In the source code it looks like this:

  SET_TEXT_POS_FROM_MARKER (top, w->start);
  [...]
  start_display (&it, w, top);
  move_it_to (&it, charpos, -1, it.last_visible_y - 1, -1,
	      (charpos >= 0 ? MOVE_TO_POS : 0) | MOVE_TO_Y);