bug#25611: 26.0.50; dired-do-compress unpacks .tgz files

[Mike Kupfer]

Suppose I have a .tar file, I move point to that file in a dired buffer,
and I press "Z".  My file gets converted to a .tgz file.  Okay so far.

If I then press "Z" again, I expect to get the original .tar file back.
Instead, the contents of the .tar file are extracted, and the .tgz file
is left where it was.

First problem: no .tar file.

Second problem: this violates the principle of Least Surprise.  The
prompt just talked about (de)compressing the file, it didn't say
anything about unpacking it.  (Not to mention this is a departure from
earlier behavior of "Z", and it's different behavior than what you'd get
with a plain old text file.)

Corollary to second problem: this can overwrite the user's existing
files, causing the user to lose data.

I can see the utility of the new behavior, but I think it should either
be non-default behavior or bound to a different character in dired.  (I
suppose you could just fix the prompt, but you'd lose the invertibility
of the old "Z" behavior, which seems like a step backward.)

In GNU Emacs 26.0.50.6 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2017-02-02 built on alto
Repository revision: 7cb7a582f44db94292709d35f4f5474f891f03b0
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.7 (jessie)

Recent messages:
(Shell command succeeded with no output)
Deleting...done
Move: 1 of 1
Move: 1 file
Directory has changed on disk; type g to update Dired
Quit
Type C-x 1 to delete the help window.
tar: This does not look like a tar archive
tar: office-mail-20161212.tar: Not found in archive
tar: Exiting with failure status due to previous errors

Configured using:
 'configure --prefix=/usr/new'

Configured features:
XPM JPEG TIFF GIF PNG SOUND NOTIFY GNUTLS LIBXML2 FREETYPE XFT ZLIB
TOOLKIT_SCROLL_BARS LUCID X11

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

Major mode: Dired by name

Minor modes in effect:
  shell-dirtrack-mode: t
  delete-selection-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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emacsbug view skeleton misearch multi-isearch sgml-mode dom pcmpl-unix
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gnus-kill gnus-dup gnus-ml disp-table url-http url-gw url-cache url-auth
url-handlers nnrss xml mm-url url url-proxy url-privacy url-expand
url-methods url-history url-cookie url-domsuf nndraft nnmh
network-stream nsm starttls gnus-agent gnus-srvr gnus-score score-mode
nnvirtual gnus-msg nntp gnus-cache dired-aux dired-x mh-funcs mh-thread
mh-identity mh-letter mh-comp url-util url-parse url-vars org-element
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mail-extr mh-mime mh-gnus mh-acros mh-show goto-addr thingatpt gnus-cite
gnus-art mm-uu mml2015 mm-view mml-smime smime dig mailcap gnus-sum
gnus-group gnus-undo gnus-start gnus-cloud nnimap nnmail mail-source tls
gnutls utf7 netrc nnoo parse-time gnus-spec gnus-int gnus-win gnus-range
gnus nnheader wid-edit mh-inc hl-line mh-tool-bar mh-seq mh-xface
mh-utils mh-folder which-func imenu mh-scan mh-e mh-compat mh-buffers
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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
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european ethiopic indian cyrillic chinese composite charscript
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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 inotify dynamic-setting
font-render-setting x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 334531 42157)
 (symbols 48 44642 1)
 (miscs 40 429 941)
 (strings 32 97784 18745)
 (string-bytes 1 3083356)
 (vectors 16 45101)
 (vector-slots 8 1532783 157208)
 (floats 8 525 644)
 (intervals 56 2775 535)
 (buffers 976 41)
 (heap 1024 70224 44977))