bug#39987: 27.0.90; `checkdoc-file-comments-engine' occasionally returns invalid buffer positions

[Philipp Stephani]

Assume /tmp/empty.el is an empty file.

$ emacs -Q -batch -l checkdoc -eval '(let (checkdoc-autofix-flag 
checkdoc-bouncy-flag (checkdoc-create-error-function (apply-partially (function 
message) "text=%S beg=%s end=%s")) (debug-on-error t)) (with-temp-buffer 
(insert-file-contents "/tmp/empty.el" t) (checkdoc-file-comments-engine)))'
text="The first line should be of the form: \";;; package --- Summary\"" beg=1 
end=1
text="You should have a summary line (\";;; .* --- .*\")" beg=nil end=nil
text="You should have a section marked \";;; Commentary:\"" beg=nil end=nil
text="You should have a section marked \";;; Code:\"" beg=1 end=nil
text="The footer should be: (provide 'empty)\\n;;; empty.el ends here" beg=0 
end=1

The problem is `beg=0' in the last line, which is not a valid buffer
position.  Probably this should be `beg=1' (or `beg=end=nil').


In GNU Emacs 27.0.90 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.24.12)
 of 2020-03-08
Repository revision: 0a3682a566d5563e3d57defe49359cee236e0274
Repository branch: emacs-27
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux rodete

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --enable-gcc-warnings=warn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking=all --enable-check-lisp-object-type --with-modules
 'CFLAGS=-O1 -ggdb3 -fsanitize=address -fsanitize=pointer-compare
 -fsanitize=pointer-subtract -fno-sanitize=leak -fsanitize=undefined
 -fno-omit-frame-pointer''

Configured features:
XPM JPEG TIFF GIF PNG SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS FREETYPE HARFBUZZ XFT ZLIB TOOLKIT_SCROLL_BARS GTK3
X11 XDBE XIM MODULES THREADS PDUMPER GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec epa epg epg-config gnus-util
rmail rmail-loaddefs text-property-search time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils phst skeleton
derived edmacro kmacro pcase ffap thingatpt url-parse auth-source cl-seq
eieio eieio-core cl-macs eieio-loaddefs password-cache json map url-vars
subr-x rx gnutls puny seq byte-opt gv bytecomp byte-compile cconv dbus
xml cl-loaddefs cl-lib tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type mwheel term/x-win x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
tab-bar menu-bar rfn-eshadow isearch timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core term/tty-colors frame
minibuffer cl-generic cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite charscript charprop case-table epa-hook jka-cmpr-hook help
simple abbrev obarray cl-preloaded nadvice loaddefs button faces
cus-face macroexp files text-properties overlay sha1 md5 base64 format
env code-pages mule custom widget hashtable-print-readable backquote
threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 59584 7147)
 (symbols 48 8083 1)
 (strings 32 20931 1947)
 (string-bytes 1 682023)
 (vectors 16 12071)
 (vector-slots 8 165012 7952)
 (floats 8 25 38)
 (intervals 56 206 0)
 (buffers 1000 11))