bug#31406: 25.3; GUD not listed when describe-mode on source code buffer

[Sébastien Gendre]

After 'pdb' Emacs function, and switch to the buffer where the source
code debbuged are, the Emacs function 'discribe-mode' doesn't say
anything about GUD and its (complex) commands available on this
buffer. I can still execute them (like C-x C-a C-b), but I need to go to
the manual to know wich commands I can run and what their key binding
are.

When I return on the GUD buffer, Emacs fuction 'discribe-mode' talk
about GUD buffer commands (like C-c C-b) but don't give anything about the
corresponding command on the code source buffer.

Tested with 'emacs -q'.







Emacs information:
In GNU Emacs 25.3.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.22.26)
 of 2018-01-12 built on buildhw-08.phx2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.11906000
Configured using:
 'configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
 --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr
 --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
 --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
 --libexecdir=/usr/libexec --localstatedir=/var
 --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-dbus --with-gif --with-jpeg --with-png
 --with-rsvg --with-tiff --with-xft --with-xpm --with-x-toolkit=gtk3
 --with-gpm=no --with-xwidgets --with-modules
 build_alias=x86_64-redhat-linux-gnu host_alias=x86_64-redhat-linux-gnu
 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g -pipe -Wall -Werror=format-security
 -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong
 --param=ssp-buffer-size=4 -grecord-gcc-switches
 -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic'
 LDFLAGS=-Wl,-z,relro
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GCONF GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES XWIDGETS

Important settings:
  value of $LANG: fr_CH.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Debugger

Minor modes in effect:
  diff-auto-refine-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-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

Recent messages:
apply: Searching for program: Aucun fichier ou dossier de ce type, pdb
Type "q" in help window to restore its previous buffer, C-M-v to scroll help.
mwheel-scroll: Beginning of buffer [9 times]
Type "q" in help window to restore its previous buffer, C-M-v to scroll help. 
[2 times]
Quit
C-x C-g is undefined
Quit
Type "q" in help window to restore its previous buffer.
Mark set
Making completion list... [2 times]

Load-path shadows:
/usr/share/emacs/site-lisp/gnus-bonus/spam-stat hides 
/usr/share/emacs/25.3/lisp/gnus/spam-stat
/usr/share/emacs/site-lisp/gnus-bonus/nnir hides 
/usr/share/emacs/25.3/lisp/gnus/nnir
/usr/share/emacs/site-lisp/gnus-bonus/nnnil hides 
/usr/share/emacs/25.3/lisp/gnus/nnnil

Features:
(shadow sort mail-extr emacsbug message idna dired rfc822 mml mml-sec
epg epg-config mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mail-utils eieio-opt speedbar sb-image ezimage dframe find-func misearch
multi-isearch gdb-mi bindat gud vc-git diff-mode python tramp-sh tramp
tramp-compat auth-source cl-seq eieio eieio-core gnus-util mm-util
help-fns mail-prsvr password-cache tramp-loaddefs trampver ucs-normalize
shell pcomplete format-spec advice json map seq byte-opt bytecomp
byte-compile cl-extra help-mode cconv comint ring ansi-color
preview-latex emacs-goodies-loaddefs easy-mmode color-theme easymenu
wid-edit cl clang-format cl-macs gv xml cl-loaddefs pcase cl-lib
auto-loads tex-site time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment
elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
frame 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 charscript
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer
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 dbusbind inotify
dynamic-setting system-font-setting font-render-setting xwidget-internal
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 246105 11807)
 (symbols 48 25994 0)
 (miscs 40 203 454)
 (strings 32 34188 6794)
 (string-bytes 1 1096072)
 (vectors 16 41109)
 (vector-slots 8 745484 3875)
 (floats 8 255 321)
 (intervals 56 393 0)
 (buffers 976 22))