bug#36771: 27.0.50; mailcap documentation and effect missmatch

[Tomas Nordin]

Hello Emacs

emacs -Q

In the scratch buffer, eval each of the following:

(require 'mailcap)
(mailcap-mime-info "text/html") ;; -> "view %s"
mailcap-prefer-mailcap-viewers ;; -> t
(setq mailcap-prefer-mailcap-viewers nil) ;; -> nil
(mailcap-mime-info "text/html") ;; -> "/usr/bin/firefox-esr %s"

I expect the result of mailcap-mime-info to be "/usr/bin/firefox-esr %s"
when mailcap-prefer-mailcap-viewers is t because the doc of that
variable says

   If non-nil, prefer viewers specified in ~/.mailcap.
   If nil, the most specific viewer will be chosen, even if there is
   a general override in ~/.mailcap.  For instance, if /etc/mailcap
   has an entry for "image/gif", that one will be chosen even if
   you have an entry for "image/*" in your ~/.mailcap file.

and

$ cat ~/.mailcap | grep '^text/html'
text/html; /usr/bin/firefox-esr %s; description=HTML Text; test=test -n 
"$DISPLAY";  nametemplate=%s.html

What do you think? 

If you agree this is a bug, I could provide that a bisect session gave
me that it started to behave this way by commit
7e47d44da4b54c518c5e09b4f3d58dafdd43033d

Before that I think I got "/usr/bin/firefox-esr %s" by default. It was
after a rebuild of emacs I noted a difference using notmuch trying to
view some html part in external viewer.

In addition I am confused why mailcap-mime-info produces "view %s" in
any case because

$ cat /etc/mailcap | grep '^text/html'
text/html; /usr/bin/sensible-browser %s; description=HTML Text; 
nametemplate=%s.html
text/html; /usr/bin/firefox-esr %s; description=HTML Text; test=test -n 
"$DISPLAY";  nametemplate=%s.html
text/html; gnome-builder %s; test=test -n "$DISPLAY"
text/html; /usr/bin/w3m -T text/html %s; needsterminal; description=HTML Text; 
nametemplate=%s.html
text/html; /usr/bin/w3m -I %{charset} -dump -T text/html %s; copiousoutput; 
description=HTML Text; nametemplate=%s.html

I would naively think that the above would result in sensible-browser maybe.

In GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-07-16 built on fliptop
Repository revision: 6253541c76a449780815f4a8fd75a9aa70b931ae
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
Mark saved where search started
tn-eval-insert-last-sexp
mailcap
"view %s"
t
nil
"/usr/bin/firefox-esr %s"

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS PDUMPER LCMS2 GMP

Important settings:
  value of $LC_TIME: sv_SE.UTF-8
  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 puny dired dired-loaddefs
format-spec subr-x rfc822 mml easymenu mml-sec password-cache epa
derived epg epg-config gnus-util rmail rmail-loaddefs
text-property-search time-date seq byte-opt gv bytecomp byte-compile
cconv mm-decode mm-bodies mm-encode mailabbrev gmm-utils mailheader
cl-loaddefs cl-lib sendmail mail-utils mail-parse rfc2231 rfc2047
rfc2045 mm-util ietf-drums mail-prsvr mailcap misearch multi-isearch
elec-pair 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 menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors 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 composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray 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 threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 56375 8481)
 (symbols 48 6019 1)
 (strings 32 20246 1857)
 (string-bytes 1 576796)
 (vectors 16 9978)
 (vector-slots 8 127710 7016)
 (floats 8 18 40)
 (intervals 56 431 1)
 (buffers 992 12))