path: root/lisp/tar-mode.el

;;; tar-mode.el --- simple editing of tar files from GNU Emacs  -*- lexical-binding:t -*-

;; Copyright (C) 1990-1991, 1993-2021 Free Software Foundation, Inc.

;; Author: Jamie Zawinski <jwz@lucid.com>
;; Maintainer: emacs-devel@gnu.org
;; Created: 04 Apr 1990
;; Keywords: unix

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;; This package attempts to make dealing with Unix 'tar' archives easier.
;; When this code is loaded, visiting a file whose name ends in '.tar' will
;; cause the contents of that archive file to be displayed in a Dired-like
;; listing.  It is then possible to use the customary Dired keybindings to
;; extract sub-files from that archive, either by reading them into their own
;; editor buffers, or by copying them directly to arbitrary files on disk.
;; It is also possible to delete sub-files from within the tar file and write
;; the modified archive back to disk, or to edit sub-files within the archive
;; and re-insert the modified files into the archive.  See the documentation
;; string of tar-mode for more info.

;; This code now understands the extra fields that GNU tar adds to tar files.

;; Do not attempt to use tar-mode.el with crypt.el, you will lose.

;;    ***************   TO DO   ***************
;;
;; o  chmod should understand "a+x,og-w".
;;
;; o  The code is less efficient that it could be - in a lot of places, I
;;    pull a 512-character string out of the buffer and parse it, when I could
;;    be parsing it in place, not garbaging a string.  Should redo that.
;;
;; o  I'd like a command that searches for a string/regexp in every subfile
;;    of an archive, where <esc> would leave you in a subfile-edit buffer.
;;    (Like the Meta-R command of the Zmacs mail reader.)
;;
;; o  Sometimes (but not always) reverting the tar-file buffer does not
;;    re-grind the listing, and you are staring at the binary tar data.
;;    Typing 'g' again immediately after that will always revert and re-grind
;;    it, though.  I have no idea why this happens.
;;
;; o  Tar-mode interacts poorly with crypt.el and zcat.el because the tar
;;    write-file-hook actually writes the file.  Instead it should remove the
;;    header (and conspire to put it back afterwards) so that other write-file
;;    hooks which frob the buffer have a chance to do their dirty work.  There
;;    might be a problem if the tar write-file-hook does not come *first* on
;;    the list.
;;
;; o  Block files, sparse files, continuation files, and the various header
;;    types aren't editable.  Actually I don't know that they work at all.

;; Rationale:

;; Why does tar-mode edit the file itself instead of using tar?

;; That means that you can edit tar files which you don't have room for
;; on your local disk.

;; I don't know about recent features in gnu tar, but old versions of tar
;; can't replace a file in the middle of a tar file with a new version.
;; Tar-mode can.  I don't think tar can do things like chmod the subfiles.
;; An implementation which involved unpacking and repacking the file into
;; some scratch directory would be very wasteful, and wouldn't be able to
;; preserve the file owners.

;;; Bugs:

;; - Rename on ././@LongLink files
;; - Revert confirmation displays the raw data temporarily.

;;; Code:

(eval-when-compile (require 'cl-lib))
(require 'arc-mode)

(defgroup tar nil
  "Simple editing of tar files."
  :prefix "tar-"
  :group 'data)

(defcustom tar-anal-blocksize 20
  "The blocksize of tar files written by Emacs, or nil, meaning don't care.
The blocksize of a tar file is not really the size of the blocks; rather, it is
the number of blocks written with one system call.  When tarring to a tape,
this is the size of the *tape* blocks, but when writing to a file, it doesn't
matter much.  The only noticeable difference is that if a tar file does not
have a blocksize of 20, tar will tell you that; all this really controls is
how many null padding bytes go on the end of the tar file."
  :type '(choice integer (const nil)))

(defcustom tar-update-datestamp nil
  "Non-nil means Tar mode should play fast and loose with sub-file datestamps.
If this is true, then editing and saving a tar file entry back into its
tar file will update its datestamp.  If false, the datestamp is unchanged.
You may or may not want this - it is good in that you can tell when a file
in a tar archive has been changed, but it is bad for the same reason that
editing a file in the tar archive at all is bad - the changed version of
the file never exists on disk."
  :type 'boolean)

(defcustom tar-mode-show-date nil
  "Non-nil means Tar mode should show the date/time of each subfile.
This information is useful, but it takes screen space away from file names."
  :type 'boolean)

(defcustom tar-copy-preserve-time nil
  "Non-nil means that Tar mode preserves the timestamp when copying files."
  :type 'boolean
  :version "27.1")

(defvar tar-parse-info nil)
(defvar tar-superior-buffer nil
  "Buffer containing the tar archive from which a member was extracted.")
(defvar tar-superior-descriptor nil
  "Tar descriptor for a member extracted from an archive.")
(defvar tar-file-name-coding-system nil)

(put 'tar-superior-buffer 'permanent-local t)
(put 'tar-superior-descriptor 'permanent-local t)

;; The Tar data is made up of bytes and better manipulated as bytes
;; and can be very large, so insert/delete can be costly.  The summary we
;; want to display may contain non-ascii chars, of course, so we'd like it
;; to be multibyte.  We used to keep both in the same buffer and switch
;; from/to uni/multibyte.  But this had several downsides:
;; - set-buffer-multibyte has an O(N^2) worst case that tends to be triggered
;;   here, so it gets atrociously slow on large Tar files.
;; - need to widen/narrow the buffer to show/hide the raw data, and need to
;;   maintain a tar-header-offset that keeps track of the boundary between
;;   the two.
;; - can't use markers because they're not preserved by set-buffer-multibyte.
;; So instead, we now keep the two pieces of data in separate buffers, and
;; use the new buffer-swap-text primitive when we need to change which data
;; is associated with "the" buffer.
(defvar tar-data-buffer nil "Buffer that holds the actual raw tar bytes.")
(make-variable-buffer-local 'tar-data-buffer)

(defvar tar-data-swapped nil
  "If non-nil, `tar-data-buffer' indeed holds raw tar bytes.")
(make-variable-buffer-local 'tar-data-swapped)

(defun tar-data-swapped-p ()
  "Return non-nil if the tar-data is in `tar-data-buffer'."
  (and (buffer-live-p tar-data-buffer)
       ;; Sanity check to try and make sure tar-data-swapped tracks the swap
       ;; state correctly: the raw data is expected to be always larger than
       ;; the summary.
       (progn
	 (cl-assert (or (= (buffer-size tar-data-buffer) (buffer-size))
                     (eq tar-data-swapped
                         (> (buffer-size tar-data-buffer) (buffer-size)))))
	 tar-data-swapped)))

(defun tar-swap-data ()
  "Swap buffer contents between current buffer and `tar-data-buffer'.
Preserve the modified states of the buffers and set `buffer-swapped-with'."
  (let ((data-buffer-modified-p (buffer-modified-p tar-data-buffer))
	(current-buffer-modified-p (buffer-modified-p)))
    (buffer-swap-text tar-data-buffer)
    (setq tar-data-swapped (not tar-data-swapped))
    (restore-buffer-modified-p data-buffer-modified-p)
    (with-current-buffer tar-data-buffer
      (restore-buffer-modified-p current-buffer-modified-p))))

;;; down to business.

(cl-defstruct (tar-header
            (:constructor nil)
            (:type vector)
            :named
            (:constructor
             make-tar-header (data-start name mode uid gid size date checksum
                              link-type link-name magic uname gname dmaj dmin)))
  data-start name mode uid gid size date checksum link-type link-name
  magic uname gname dmaj dmin
  ;; Start of the header can be nil (meaning it's 512 bytes before data-start)
  ;; or a marker (in case the header uses LongLink thingies).
  header-start)

(defconst tar-name-offset 0)
(defconst tar-mode-offset (+ tar-name-offset 100))
(defconst tar-uid-offset  (+ tar-mode-offset 8))
(defconst tar-gid-offset  (+ tar-uid-offset 8))
(defconst tar-size-offset (+ tar-gid-offset 8))
(defconst tar-time-offset (+ tar-size-offset 12))
(defconst tar-chk-offset  (+ tar-time-offset 12))
(defconst tar-linkp-offset (+ tar-chk-offset 8))
(defconst tar-link-offset (+ tar-linkp-offset 1))
;;; GNU-tar specific slots.
(defconst tar-magic-offset (+ tar-link-offset 100))
(defconst tar-uname-offset (+ tar-magic-offset 8))
(defconst tar-gname-offset (+ tar-uname-offset 32))
(defconst tar-dmaj-offset (+ tar-gname-offset 32))
(defconst tar-dmin-offset (+ tar-dmaj-offset 8))
(defconst tar-prefix-offset (+ tar-dmin-offset 8))
(defconst tar-end-offset (+ tar-prefix-offset 155))

(defun tar-roundup-512 (s)
  "Round S up to the next multiple of 512."
  (ash (ash (+ s 511) -9) 9))

(defun tar-header-block-tokenize (pos coding &optional disable-slash)
  "Return a `tar-header' structure.
This is a list of name, mode, uid, gid, size,
write-date, checksum, link-type, and link-name.
CODING is our best guess for decoding non-ASCII file names.
DISABLE-SLASH, if non-nil, means don't decide an entry is a directory
based on the trailing slash, only based on the \"link-type\" field
of the file header.  This is used for \"old GNU\" Tar format."
  (if (> (+ pos 512) (point-max)) (error "Malformed Tar header"))
  (cl-assert (zerop (mod (- pos (point-min)) 512)))
  (cl-assert (not enable-multibyte-characters))
  (let ((string (buffer-substring pos (setq pos (+ pos 512)))))
    (when      ;(some 'plusp string)		 ; <-- oops, massive cycle hog!
        (or (not (= 0 (aref string 0))) ; This will do.
            (not (= 0 (aref string 101))))
      (let* ((name-end tar-mode-offset)
             (link-end (1- tar-magic-offset))
             (uname-end (1- tar-gname-offset))
             (gname-end (1- tar-dmaj-offset))
             (link-p (aref string tar-linkp-offset))
             (magic-str (substring string tar-magic-offset
				   ;; The magic string is actually 6bytes
				   ;; of magic string plus 2bytes of version
				   ;; which we here ignore.
                                   (- tar-uname-offset 2)))
	     ;; The magic string is "ustar\0" for POSIX format, and
	     ;; "ustar " for GNU Tar's format.
             (uname-valid-p (car (member magic-str '("ustar " "ustar\0"))))
             name linkname
             (nulsexp   "[^\000]*\000"))
        (when (string-match nulsexp string tar-name-offset)
          (setq name-end (min name-end (1- (match-end 0)))))
        (when (string-match nulsexp string tar-link-offset)
          (setq link-end (min link-end (1- (match-end 0)))))
        (when (string-match nulsexp string tar-uname-offset)
          (setq uname-end (min uname-end (1- (match-end 0)))))
        (when (string-match nulsexp string tar-gname-offset)
          (setq gname-end (min gname-end (1- (match-end 0)))))
        (setq name (substring string tar-name-offset name-end)
              link-p (if (or (= link-p 0) (= link-p ?0))
                         nil
                       (- link-p ?0)))
        (setq linkname (substring string tar-link-offset link-end))
        (when (and (equal uname-valid-p "ustar\0")
                   (string-match nulsexp string tar-prefix-offset)
                   (> (match-end 0) (1+ tar-prefix-offset)))
          (setq name (concat (substring string tar-prefix-offset
                                        (1- (match-end 0)))
                             "/" name)))
        (setq name
              (decode-coding-string name coding)
              linkname
              (decode-coding-string linkname coding))
        (if (and (null link-p) (null disable-slash) (string-match "/\\'" name))
            (setq link-p 5))            ; directory

        (if (and (equal name "././@LongLink")
                 ;; Supposedly @LongLink is only used for GNUTAR
                 ;; format (i.e. "ustar ") but some POSIX Tar files
                 ;; (with "ustar\0") have been seen using it as well.
                 (member magic-str '("ustar " "ustar\0")))
            ;; This is a GNU Tar long-file-name header.
            (let* ((size (tar-parse-octal-integer
                          string tar-size-offset tar-time-offset))
                   ;; The long name is in the next 512-byte block.
                   ;; We've already moved POS there, when we computed
                   ;; STRING above.
		   (name (decode-coding-string
                          ;; -1 so as to strip the terminating 0 byte.
			  (buffer-substring pos (+ pos size -1)) coding))
                   ;; Tokenize the header of the _real_ file entry,
                   ;; which is further 512 bytes into the archive.
                   (descriptor (tar-header-block-tokenize
                                (+ pos (tar-roundup-512 size)) coding
                                ;; Don't intuit directories from
                                ;; the trailing slash, because the
                                ;; truncated name might by chance end
                                ;; in a slash.
				'ignore-trailing-slash)))
              ;; Fix the descriptor of the real file entry by using
              ;; the information from the long name entry.
              (cond
               ((eq link-p (- ?L ?0))      ;GNUTYPE_LONGNAME.
                (setf (tar-header-name descriptor) name))
               ((eq link-p (- ?K ?0))      ;GNUTYPE_LONGLINK.
                (setf (tar-header-link-name descriptor) name))
               (t
                (message "Unrecognized GNU Tar @LongLink format")))
              ;; Fix the "link-type" attribute, based on the long name.
              (if (and (null (tar-header-link-type descriptor))
                       (string-match "/\\'" name))
                  (setf (tar-header-link-type descriptor) 5)) ; directory
              (setf (tar-header-header-start descriptor)
                    (copy-marker (- pos 512) t))
              descriptor)

          (make-tar-header
           (copy-marker pos nil)
           name
           (tar-parse-octal-integer string tar-mode-offset tar-uid-offset)
           (tar-parse-octal-integer string tar-uid-offset tar-gid-offset)
           (tar-parse-octal-integer string tar-gid-offset tar-size-offset)
           (tar-parse-octal-integer string tar-size-offset tar-time-offset)
           (tar-parse-octal-integer string tar-time-offset tar-chk-offset)
           (tar-parse-octal-integer string tar-chk-offset tar-linkp-offset)
           link-p
           linkname
           uname-valid-p
           (when uname-valid-p
             (decode-coding-string
              (substring string tar-uname-offset uname-end) coding))
           (when uname-valid-p
             (decode-coding-string
              (substring string tar-gname-offset gname-end) coding))
           (tar-parse-octal-integer string tar-dmaj-offset tar-dmin-offset)
           (tar-parse-octal-integer string tar-dmin-offset tar-prefix-offset)
           ))))))

;; Pseudo-field.
(defun tar-header-data-end (descriptor)
  (let* ((data-start (tar-header-data-start descriptor))
         (link-type (tar-header-link-type descriptor))
         (size (tar-header-size descriptor)))
    (+ data-start
       ;; Ignore size for files of type 1-6
       (if (and (not (memq link-type '(1 2 3 4 5 6))) (> size 0))
           (tar-roundup-512 size)
         0))))

(defun tar-parse-octal-integer (string &optional start end)
  (if (null start) (setq start 0))
  (if (null end) (setq end (length string)))
  (if (= (aref string start) 0)
      0
    (let ((n 0))
      (while (< start end)
	(setq n (if (< (aref string start) ?0) n
		  (+ (* n 8) (- (aref string start) ?0)))
	      start (1+ start)))
      n)))

(define-obsolete-function-alias 'tar-parse-octal-long-integer
  #'tar-parse-octal-integer "27.1")

(defun tar-parse-octal-integer-safe (string)
  (if (zerop (length string)) (error "empty string"))
  (mapc (lambda (c)
	  (if (or (< c ?0) (> c ?7))
	      (error "`%c' is not an octal digit" c)))
	string)
  (tar-parse-octal-integer string))

(defun tar-new-regular-file-header (filename &optional size time)
  "Return a Tar header for a regular file.
The header will lack a proper checksum; use `tar-header-block-checksum'
to compute one, or request `tar-header-serialize' to do that.

Other tar-mode facilities may also require the data-start header
field to be set to a valid value.

If SIZE is not given or nil, it defaults to 0.
If TIME is not given or nil, assume now."
  (make-tar-header
   nil
   filename
   #o644 0 0 (or size 0)
   (or time (current-time))
   nil				; checksum
   nil nil
   nil nil nil nil nil))

(defun tar--pad-to (pos)
  (make-string (+ pos (- (point)) (point-min)) 0))

(defun tar--put-at (pos val &optional fmt mask)
  (when val
    (insert (tar--pad-to pos)
	    (if fmt
		(format fmt (if mask (logand mask val) val))
	      val))))

(defun tar-header-serialize (header &optional update-checksum)
  "Return the serialization of a Tar HEADER as a string.
This function calls `tar-header-block-check-checksum' to ensure the
checksum is correct.

If UPDATE-CHECKSUM is non-nil, update HEADER with the newly-computed
checksum before doing the check."
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (let ((encoded-name
	   (encode-coding-string (tar-header-name header)
				 tar-file-name-coding-system)))
      (unless (< (length encoded-name) 99)
	;; FIXME: Implement it.
	(error "Long file name support is not implemented"))
      (insert encoded-name))
    (tar--put-at tar-mode-offset (tar-header-mode header) "%6o\0 " #o777777)
    (tar--put-at tar-uid-offset  (tar-header-uid  header) "%6o\0 " #o777777)
    (tar--put-at tar-gid-offset  (tar-header-gid  header) "%6o\0 " #o777777)
    (tar--put-at tar-size-offset (tar-header-size header) "%11o ")
    (insert (tar--pad-to tar-time-offset)
	    (tar-octal-time (tar-header-date header))
	    " ")
    ;; Omit tar-header-checksum (tar-chk-offset) for now.
    (tar--put-at   tar-linkp-offset (tar-header-link-type header))
    (tar--put-at   tar-link-offset  (tar-header-link-name header))
    (when (tar-header-magic header)
      (tar--put-at tar-magic-offset (tar-header-magic header))
      (tar--put-at tar-uname-offset (tar-header-uname header))
      (tar--put-at tar-gname-offset (tar-header-gname header))
      (tar--put-at tar-dmaj-offset (tar-header-dmaj header) "%7o\0" #o7777777)
      (tar--put-at tar-dmin-offset (tar-header-dmin header) "%7o\0" #o7777777))
    (tar--put-at 512 "")
    (let ((ck (tar-header-block-checksum (buffer-string))))
      (goto-char (+ (point-min) tar-chk-offset))
      (delete-char 8)
      (insert (format "%6o\0 " ck))
      (when update-checksum
	(setf (tar-header-checksum header) ck))
      (tar-header-block-check-checksum (buffer-string)
				       (tar-header-checksum header)
				       (tar-header-name header)))
    ;; .
    (buffer-string)))


(defun tar-header-block-checksum (string)
  "Compute and return a tar-acceptable checksum for this block."
  (cl-assert (not (multibyte-string-p string)))
  (let* ((chk-field-start tar-chk-offset)
	 (chk-field-end (+ chk-field-start 8))
	 (sum 0)
	 (i 0))
    ;; Add up all of the characters except the ones in the checksum field.
    ;; Add that field as if it were filled with spaces.
    (while (< i chk-field-start)
      (setq sum (+ sum (aref string i))
	    i (1+ i)))
    (setq i chk-field-end)
    (while (< i 512)
      (setq sum (+ sum (aref string i))
	    i (1+ i)))
    (+ sum (* 32 8))))

(defun tar-header-block-check-checksum (hblock desired-checksum file-name)
  "Beep and print a warning if the checksum doesn't match."
  (if (not (= desired-checksum (tar-header-block-checksum hblock)))
      (progn (beep) (message "Invalid checksum for file %s!" file-name))))

(defun tar-clip-time-string (time)
  (declare (obsolete format-time-string "27.1"))
  (let ((str (current-time-string time)))
    (concat " " (substring str 4 16) (format-time-string " %Y" time))))

(defun tar-grind-file-mode (mode)
  "Construct a `rw-r--r--' string indicating MODE.
MODE should be an integer which is a file mode value.
For instance, if mode is #o700, then it produces `rwx------'."
  (substring (file-modes-number-to-symbolic mode) 1))

(defun tar-header-block-summarize (tar-hblock &optional mod-p)
  "Return a line similar to the output of `tar -vtf'."
  (let ((name (tar-header-name tar-hblock))
	(mode (tar-header-mode tar-hblock))
	(uid (tar-header-uid tar-hblock))
	(gid (tar-header-gid tar-hblock))
	(uname (tar-header-uname tar-hblock))
	(gname (tar-header-gname tar-hblock))
	(size (tar-header-size tar-hblock))
	(time (tar-header-date tar-hblock))
	;; (ck (tar-header-checksum tar-hblock))
	(type (tar-header-link-type tar-hblock))
	(link-name (tar-header-link-name tar-hblock)))
    (format "%c%c%s %7s/%-7s %7s%s %s%s"
	    (if mod-p ?* ? )
	    (cond ((or (eq type nil) (eq type 0)) ?-)
		  ((eq type 1) ?h)	; link
		  ((eq type 2) ?l)	; symlink
		  ((eq type 3) ?c)	; char special
		  ((eq type 4) ?b)	; block special
		  ((eq type 5) ?d)	; directory
		  ((eq type 6) ?p)	; FIFO/pipe
		  ((eq type 20) ?*)	; directory listing
		  ((eq type 28) ?L)	; next has longname
		  ((eq type 29) ?M)	; multivolume continuation
		  ((eq type 35) ?S)	; sparse
		  ((eq type 38) ?V)	; volume header
		  ((eq type 55) ?H)	; pax global extended header
		  ((eq type 72) ?X)	; pax extended header
		  (t ?\s)
		  )
	    (tar-grind-file-mode mode)
	    (if (= 0 (length uname)) uid uname)
	    (if (= 0 (length gname)) gid gname)
	    size
	    (if tar-mode-show-date
                (format-time-string " %Y-%m-%d %H:%M" time)
              "")
	    (propertize name
			'mouse-face 'highlight
			'help-echo "mouse-2: extract this file into a buffer")
	    (if (or (eq type 1) (eq type 2))
		(concat (if (= type 1) " ==> " " --> ") link-name)
	      ""))))

(defun tar-untar-buffer ()
  "Extract all archive members in the tar-file into the current directory."
  (interactive)
  ;; FIXME: make it work even if we're not in tar-mode.
  (let ((data-buf (if (tar-data-swapped-p) tar-data-buffer
                    (current-buffer)))
        (reporter (make-progress-reporter "Extracting")))
    (with-current-buffer data-buf
      (cl-assert (not enable-multibyte-characters)))
    (dolist (descriptor tar-parse-info)
      (let* ((orig (tar-header-name descriptor))
	     ;; Note that default-directory may have different values
	     ;; in the tar-mode and data buffers, so we stick to the
	     ;; absolute file name from now on.
	     (name (expand-file-name orig))
             (dir (if (eq (tar-header-link-type descriptor) 5)
                      name
                    (file-name-directory name)))
             (link-desc (tar--describe-as-link descriptor))
             (start (tar-header-data-start descriptor))
             (end (+ start (tar-header-size descriptor))))
        (unless (file-directory-p name)
          (progress-reporter-update reporter name)
          (if (and dir (not (file-exists-p dir)))
              (make-directory dir t))
          (unless (file-directory-p name)
	    (with-current-buffer data-buf
              (let ((coding-system-for-write 'no-conversion)
                    (write-region-inhibit-fsync t))
                (when link-desc
                  (lwarn '(tar link) :warning
                         "Extracted `%s', %s, as a normal file"
                         name link-desc))
                (write-region start end name nil :nomessage)))
            (set-file-modes name (tar-header-mode descriptor))))))
    (progress-reporter-done reporter)))

(defun tar-summarize-buffer ()
  "Parse the contents of the tar file in the current buffer."
  (cl-assert (tar-data-swapped-p))
  (let* ((modified (buffer-modified-p))
         (result '())
         (pos (point-min))
	 (coding tar-file-name-coding-system)
         (progress-reporter
          (with-current-buffer tar-data-buffer
            (make-progress-reporter "Parsing tar file..."
                                    (point-min) (point-max))))
         descriptor)
    (with-current-buffer tar-data-buffer
      (while (and (< pos (point-max))
                  (setq descriptor (tar-header-block-tokenize pos coding)))
        (let ((size (tar-header-size descriptor)))
          (if (< size 0)
              (error "%s has size %s - corrupted"
                     (tar-header-name descriptor) size)))
        ;;
        ;; This is just too slow.  Don't really need it anyway....
        ;;(tar-header-block-check-checksum
        ;;  hblock (tar-header-block-checksum hblock)
        ;;  (tar-header-name descriptor))

        (push descriptor result)
        (setq pos (tar-header-data-end descriptor))
        (progress-reporter-update progress-reporter pos)))

    (setq-local tar-parse-info (nreverse result))
    ;; A tar file should end with a block or two of nulls,
    ;; but let's not get a fatal error if it doesn't.
    (if (null descriptor)
        (progress-reporter-done progress-reporter)
      (message "Warning: premature EOF parsing tar file"))
    (goto-char (point-min))
    (let ((create-lockfiles nil) ; avoid changing dir mtime by lock_file
	  (inhibit-read-only t)
          (total-summaries
           (mapconcat #'tar-header-block-summarize tar-parse-info "\n")))
      (insert total-summaries "\n")
      (goto-char (point-min))
      (restore-buffer-modified-p modified))))

(defvar tar-mode-map
  (let ((map (make-keymap)))
    (suppress-keymap map)
    (define-key map " " 'tar-next-line)
    (define-key map "C" 'tar-copy)
    (define-key map "d" 'tar-flag-deleted)
    (define-key map "\^D" 'tar-flag-deleted)
    (define-key map "e" 'tar-extract)
    (define-key map "f" 'tar-extract)
    (define-key map "\C-m" 'tar-extract)
    (define-key map [mouse-2] 'tar-mouse-extract)
    (define-key map "g" 'revert-buffer)
    (define-key map "n" 'tar-next-line)
    (define-key map "\^N" 'tar-next-line)
    (define-key map [down] 'tar-next-line)
    (define-key map "o" 'tar-extract-other-window)
    (define-key map "p" 'tar-previous-line)
    (define-key map "\^P" 'tar-previous-line)
    (define-key map [up] 'tar-previous-line)
    (define-key map "I" 'tar-new-entry)
    (define-key map "R" 'tar-rename-entry)
    (define-key map "u" 'tar-unflag)
    (define-key map "v" 'tar-view)
    (define-key map "w" 'woman-tar-extract-file)
    (define-key map "x" 'tar-expunge)
    (define-key map "\177" 'tar-unflag-backwards)
    (define-key map "E" 'tar-extract-other-window)
    (define-key map "M" 'tar-chmod-entry)
    (define-key map "G" 'tar-chgrp-entry)
    (define-key map "O" 'tar-chown-entry)
    ;; Let mouse-1 follow the link.
    (define-key map [follow-link] 'mouse-face)

    ;; Make menu bar items.

    ;; Get rid of the Edit menu bar item to save space.
    (define-key map [menu-bar edit] 'undefined)

    (define-key map [menu-bar immediate]
      (cons "Immediate" (make-sparse-keymap "Immediate")))

    (define-key map [menu-bar immediate woman]
      '("Read Man Page (WoMan)" . woman-tar-extract-file))
    (define-key map [menu-bar immediate view]
      '("View This File" . tar-view))
    (define-key map [menu-bar immediate display]
      '("Display in Other Window" . tar-display-other-window))
    (define-key map [menu-bar immediate find-file-other-window]
      '("Find in Other Window" . tar-extract-other-window))
    (define-key map [menu-bar immediate find-file]
      '("Find This File" . tar-extract))

    (define-key map [menu-bar mark]
      (cons "Mark" (make-sparse-keymap "Mark")))

    (define-key map [menu-bar mark unmark-all]
      '("Unmark All" . tar-clear-modification-flags))
    (define-key map [menu-bar mark deletion]
      '("Flag" . tar-flag-deleted))
    (define-key map [menu-bar mark unmark]
      '("Unflag" . tar-unflag))

    (define-key map [menu-bar operate]
      (cons "Operate" (make-sparse-keymap "Operate")))

    (define-key map [menu-bar operate chown]
      '("Change Owner..." . tar-chown-entry))
    (define-key map [menu-bar operate chgrp]
      '("Change Group..." . tar-chgrp-entry))
    (define-key map [menu-bar operate chmod]
      '("Change Mode..." . tar-chmod-entry))
    (define-key map [menu-bar operate rename]
      '("Rename to..." . tar-rename-entry))
    (define-key map [menu-bar operate copy]
      '("Copy to..." . tar-copy))
    (define-key map [menu-bar operate expunge]
      '("Expunge Marked Files" . tar-expunge))
    
    map)
  "Local keymap for Tar mode listings.")


;; tar mode is suitable only for specially formatted data.
(put 'tar-mode 'mode-class 'special)
(put 'tar-subfile-mode 'mode-class 'special)

(defun tar-change-major-mode-hook ()
  ;; Bring the actual Tar data back into the main buffer.
  (when (tar-data-swapped-p) (tar-swap-data))
  ;; Throw away the summary.
  (when (buffer-live-p tar-data-buffer) (kill-buffer tar-data-buffer)))

(defun tar-mode-kill-buffer-hook ()
  (if (buffer-live-p tar-data-buffer) (kill-buffer tar-data-buffer)))

;;;###autoload
(define-derived-mode tar-mode special-mode "Tar"
  "Major mode for viewing a tar file as a dired-like listing of its contents.
You can move around using the usual cursor motion commands.
Letters no longer insert themselves.
Type `e' to pull a file out of the tar file and into its own buffer;
or click mouse-2 on the file's line in the Tar mode buffer.
Type `c' to copy an entry from the tar file into another file on disk.

If you edit a sub-file of this archive (as with the `e' command) and
save it with \\[save-buffer], the contents of that buffer will be
saved back into the tar-file buffer; in this way you can edit a file
inside of a tar archive without extracting it and re-archiving it.

See also: variables `tar-update-datestamp' and `tar-anal-blocksize'.
\\{tar-mode-map}"
  (and buffer-file-name
       (file-writable-p buffer-file-name)
       (setq buffer-read-only nil))    ; undo what `special-mode' did
  (make-local-variable 'tar-parse-info)
  (setq-local require-final-newline nil) ; binary data, dude...
  (setq-local local-enable-local-variables nil)
  (setq-local next-line-add-newlines nil)
  (setq-local tar-file-name-coding-system
              (or file-name-coding-system
	          default-file-name-coding-system
	          locale-coding-system))
  ;; Prevent loss of data when saving the file.
  (setq-local file-precious-flag t)
  (buffer-disable-undo)
  (widen)
  ;; Now move the Tar data into an auxiliary buffer, so we can use the main
  ;; buffer for the summary.
  (cl-assert (not (tar-data-swapped-p)))
  (setq-local revert-buffer-function #'tar-mode-revert)
  ;; We started using write-contents-functions, but this hook is not
  ;; used during auto-save, so we now use
  ;; write-region-annotate-functions which hooks at a lower-level.
  (add-hook 'write-region-annotate-functions #'tar-write-region-annotate nil t)
  (add-hook 'kill-buffer-hook #'tar-mode-kill-buffer-hook nil t)
  (add-hook 'change-major-mode-hook #'tar-change-major-mode-hook nil t)
  ;; Tar data is made of bytes, not chars.
  (set-buffer-multibyte nil)            ;Hopefully a no-op.
  (setq-local tar-data-buffer (generate-new-buffer
                               (format " *tar-data %s*"
                                       (file-name-nondirectory
                                        (or buffer-file-name (buffer-name))))))
  (condition-case err
      (progn
        (tar-swap-data)
        (tar-summarize-buffer)
        (tar-next-line 0))
    (error
     ;; If summarizing caused an error, then maybe the buffer doesn't contain
     ;; tar data.  Rather than show a mysterious empty buffer, let's
     ;; revert to fundamental-mode.
     (fundamental-mode)
     (signal (car err) (cdr err)))))

(autoload 'woman-tar-extract-file "woman"
  "In tar mode, run the WoMan man-page browser on this file." t)

(define-minor-mode tar-subfile-mode
  "Minor mode for editing an element of a tar-file.

This mode arranges for \"saving\" this buffer to write the data
into the tar-file buffer that it came from.  The changes will
actually appear on disk when you save the tar-file's buffer."
  ;; Don't do this, because it is redundant and wastes mode line space.
  ;; :lighter " TarFile"
  nil nil nil
  (or (and (boundp 'tar-superior-buffer) tar-superior-buffer)
      (error "This buffer is not an element of a tar file"))
  (cond (tar-subfile-mode
	 (add-hook 'write-file-functions #'tar-subfile-save-buffer nil t)
	 ;; turn off auto-save.
	 (auto-save-mode -1)
	 (setq buffer-auto-save-file-name nil))
	(t
	 (remove-hook 'write-file-functions #'tar-subfile-save-buffer t))))


;; Revert the buffer and recompute the dired-like listing.
(defun tar-mode-revert (&optional no-auto-save no-confirm)
  (unwind-protect
      (let ((revert-buffer-function nil))
        (if (tar-data-swapped-p) (tar-swap-data))
        ;; FIXME: If we ask for confirmation, the user will be temporarily
        ;; looking at the raw data.
        (revert-buffer no-auto-save no-confirm 'preserve-modes)
        ;; Recompute the summary.
        (if (buffer-live-p tar-data-buffer) (kill-buffer tar-data-buffer))
        (tar-mode))
    (unless (tar-data-swapped-p) (tar-swap-data))))


(defun tar-next-line (arg)
  "Move cursor vertically down ARG lines and to the start of the filename."
  (interactive "p")
  (forward-line arg)
  (goto-char (or (next-single-property-change (point) 'mouse-face) (point))))

(defun tar-previous-line (arg)
  "Move cursor vertically up ARG lines and to the start of the filename."
  (interactive "p")
  (tar-next-line (- arg)))

(defun tar-current-position ()
  "Return the `tar-parse-info' index for the current line."
  (count-lines (point-min) (line-beginning-position)))

(defun tar-current-descriptor (&optional noerror)
  "Return the tar-descriptor of the current line, or signals an error."
  ;; I wish lines had plists, like in ZMACS...
  (or (nth (tar-current-position)
	   tar-parse-info)
      (if noerror
	  nil
	  (error "This line does not describe a tar-file entry"))))

(defun tar--describe-as-link (descriptor)
  (let ((link-p (tar-header-link-type descriptor)))
    (if link-p
	(cond ((eq link-p 5) "a directory")
              ((eq link-p 20) "a tar directory header")
              ((eq link-p 28) "a next has longname")
              ((eq link-p 29) "a multivolume-continuation")
              ((eq link-p 35) "a sparse entry")
              ((eq link-p 38) "a volume header")
              ((eq link-p 55) "a pax global extended header")
              ((eq link-p 72) "a pax extended header")
              (t "a link")))))

(defun tar--check-descriptor (descriptor)
  (let ((link-desc (tar--describe-as-link descriptor)))
    (when link-desc
      (error "This is %s, not a real file" link-desc))))

(defun tar-get-descriptor ()
  (let* ((descriptor (tar-current-descriptor))
	 (size (tar-header-size descriptor)))
    (tar--check-descriptor descriptor)
    (if (zerop size) (message "This is a zero-length file"))
    descriptor))

(defun tar-get-file-descriptor (file)
  ;; Used by package.el.
  (let ((desc ()))
    (dolist (hdr tar-parse-info)
      (when (equal file (tar-header-name hdr))
        (setq desc hdr)))
    (tar--check-descriptor desc)
    desc))

(defun tar-mouse-extract (event)
  "Extract a file whose tar directory line you click on."
  (interactive "e")
  (with-current-buffer (window-buffer (posn-window (event-end event)))
    (save-excursion
      (goto-char (posn-point (event-end event)))
      ;; Just make sure this doesn't get an error.
      (tar-get-descriptor)))
  (select-window (posn-window (event-end event)))
  (goto-char (posn-point (event-end event)))
  (tar-extract))

(defun tar-file-name-handler (op &rest args)
  "Helper function for `tar-extract'."
  (or (eq op 'file-exists-p)
      (let ((file-name-handler-alist nil))
	(apply op args))))

(defun tar--extract (descriptor)
  "Extract this entry of the tar file into its own buffer."
  (let* ((name (tar-header-name descriptor))
	 (size (tar-header-size descriptor))
	 (start (tar-header-data-start descriptor))
	 (end (+ start size))
         (tarname (buffer-name))
         (bufname (concat (file-name-nondirectory name)
                          " ("
                          tarname
                          ")"))
         (buffer (generate-new-buffer bufname)))
    (with-current-buffer tar-data-buffer
      (let (coding)
        (narrow-to-region start end)
        (goto-char start)
        (setq coding (or coding-system-for-read
                         (and set-auto-coding-function
                              (funcall set-auto-coding-function
                                       name (- end start)))
                         ;; The following binding causes
                         ;; find-buffer-file-type-coding-system
                         ;; (defined on dos-w32.el) to act as if
                         ;; the file being extracted existed, so
                         ;; that the file's contents' encoding and
                         ;; EOL format are auto-detected.
                         (let ((file-name-handler-alist
                                '(("" . tar-file-name-handler))))
                           (car (find-operation-coding-system
                                 'insert-file-contents
                                 (cons name (current-buffer)) t)))))
        (if (or (not coding)
                (eq (coding-system-type coding) 'undecided))
            (setq coding (detect-coding-region start end t)))
        (if (coding-system-get coding :for-unibyte)
            (with-current-buffer buffer
              (set-buffer-multibyte nil)))
        (widen)
        (with-current-buffer buffer
          (setq buffer-undo-list t))
        (decode-coding-region start end coding buffer)
        (with-current-buffer buffer
          (setq buffer-undo-list nil))))
    buffer))

(defun tar-goto-file (file)
  "Go to FILE in the current buffer.
FILE should be a relative file name.  If FILE can't be found,
return nil.  Otherwise point is returned."
  (let ((start (point))
        found)
    (goto-char (point-min))
    (while (and (not found)
                (not (eobp)))
      (forward-line 1)
      (when-let ((descriptor (ignore-errors (tar-get-descriptor))))
        (when (equal (tar-header-name descriptor) file)
          (setq found t))))
    (if (not found)
        (progn
          (goto-char start)
          nil)
      (point))))

(defun tar-next-file-displayer (file regexp n)
  "Return a closure to display the next file after FILE that matches REGEXP."
  (let ((short (replace-regexp-in-string "\\`.*!" "" file))
        next)
    ;; The tar buffer chops off leading "./", so do the same
    ;; here.
    (setq short (replace-regexp-in-string "\\`\\./" "" file))
    (tar-goto-file short)
    (while (and (not next)
                ;; Stop if we reach the end/start of the buffer.
                (if (> n 0)
                    (not (eobp))
                  (not (save-excursion
                         (beginning-of-line)
                         (bobp)))))
      (tar-next-line n)
      (when-let ((descriptor (ignore-errors (tar-get-descriptor))))
        (let ((candidate (tar-header-name descriptor))
              (buffer (current-buffer)))
          (when (and candidate
                     (string-match-p regexp candidate))
            (setq next (lambda ()
                         (kill-buffer (current-buffer))
                         (switch-to-buffer buffer)
                         (tar-extract)))))))
    (unless next
      ;; If we didn't find a next/prev file, then restore
      ;; point.
      (tar-goto-file short))
    next))

(defun tar-extract (&optional other-window-p)
  "In Tar mode, extract this entry of the tar file into its own buffer."
  (interactive)
  (let* ((view-p (eq other-window-p 'view))
	 (descriptor (tar-get-descriptor))
	 (name (tar-header-name descriptor))
         (tar-buffer (current-buffer))
         (tarname (buffer-name))
         (read-only-p (or buffer-read-only view-p))
         (new-buffer-file-name (expand-file-name
                                ;; `:' is not allowed on Windows
                                (concat tarname "!"
                                        (if (string-match "/" name)
                                            name
                                          ;; Make sure `name' contains a /
                                          ;; so set-auto-mode doesn't try
                                          ;; to look at `tarname' for hints.
                                          (concat "./" name)))))
         (buffer (get-file-buffer new-buffer-file-name))
         (just-created nil))
    (unless buffer
      (setq buffer (tar--extract descriptor))
      (setq just-created t)
      (with-current-buffer buffer
        (goto-char (point-min))
        (setq buffer-file-name new-buffer-file-name)
        (setq buffer-file-truename
              (abbreviate-file-name buffer-file-name))
        (archive-try-jka-compr)       ;Pretty ugly hack :-(
        ;; Force buffer-file-coding-system to what
        ;; decode-coding-region actually used.
        (set-buffer-file-coding-system last-coding-system-used t)
        ;; Set the default-directory to the dir of the
        ;; superior buffer.
        (setq default-directory
              (with-current-buffer tar-buffer
                default-directory))
        (set-buffer-modified-p nil)
        (normal-mode)                   ; pick a mode.
        (setq-local tar-superior-buffer tar-buffer)
        (setq-local tar-superior-descriptor descriptor)
        (setq buffer-read-only read-only-p)
        (tar-subfile-mode 1)))
    (cond
     (view-p
      (view-buffer buffer (and just-created 'kill-buffer-if-not-modified)))
     ((eq other-window-p 'display) (display-buffer buffer))
     (other-window-p (switch-to-buffer-other-window buffer))
     (t (switch-to-buffer buffer)))))


(defun tar-extract-other-window ()
  "In Tar mode, find this entry of the tar file in another window."
  (interactive)
  (tar-extract t))

(defun tar-display-other-window ()
  "In Tar mode, display this entry of the tar file in another window."
  (interactive)
  (tar-extract 'display))

(defun tar-view ()
  "In Tar mode, view the tar file entry on this line."
  (interactive)
  (tar-extract 'view))


(defun tar-read-file-name (&optional prompt)
  "Read a file name with this line's entry as the default."
  (or prompt (setq prompt "Copy to: "))
  (let* ((default-file (expand-file-name
			(tar-header-name (tar-current-descriptor))))
	 (target (expand-file-name
		  (read-file-name prompt
				  (file-name-directory default-file)
				  default-file nil))))
    (if (or (string= "" (file-name-nondirectory target))
	    (file-directory-p target))
	(setq target (concat (if (string-match "/$" target)
				 (substring target 0 (1- (match-end 0)))
				 target)
			     "/"
			     (file-name-nondirectory default-file))))
    target))


(defun tar-copy (&optional to-file)
  "In Tar mode, extract this entry of the tar file into a file on disk.
If TO-FILE is not supplied, it is prompted for, defaulting to the name of
the current tar-entry.

If `tar-copy-preserve-time' is non-nil, the original
timestamp (if present in the tar file) will be used on the
extracted file."
  (interactive (list (tar-read-file-name)))
  (let* ((descriptor (tar-get-descriptor))
	 (name (tar-header-name descriptor))
	 (size (tar-header-size descriptor))
	 (date (tar-header-date descriptor))
	 (start (tar-header-data-start descriptor))
	 (end (+ start size))
	 (inhibit-file-name-handlers inhibit-file-name-handlers)
	 (inhibit-file-name-operation inhibit-file-name-operation))
    (with-current-buffer
	(if (tar-data-swapped-p) tar-data-buffer (current-buffer))
      ;; Inhibit compressing a subfile again if *both* name and
      ;; to-file are handled by jka-compr
      (if (and (eq (find-file-name-handler name 'write-region)
		   'jka-compr-handler)
	       (eq (find-file-name-handler to-file 'write-region)
		   'jka-compr-handler))
	  (setq inhibit-file-name-handlers
		(cons 'jka-compr-handler
		      (and (eq inhibit-file-name-operation 'write-region)
			   inhibit-file-name-handlers))
		inhibit-file-name-operation 'write-region))
      (let ((coding-system-for-write 'no-conversion))
	(write-region start end to-file nil nil nil t))
      (when (and tar-copy-preserve-time
                 date)
	(set-file-times to-file date 'nofollow)))
    (message "Copied tar entry %s to %s" name to-file)))

(defun tar-new-entry (filename &optional index)
  "Insert a new empty regular file before point."
  (interactive "*sFile name: ")
  (let* ((index   (or index (tar-current-position)))
	 (d-list  (and (not (zerop index))
		       (nthcdr (+ -1 index) tar-parse-info)))
	 (pos     (if d-list
		      (tar-header-data-end (car d-list))
		    (point-min)))
	 (new-descriptor
	  (tar-new-regular-file-header filename)))
    ;; Update the data buffer; fill the missing descriptor fields.
    (with-current-buffer tar-data-buffer
      (goto-char pos)
      (insert (tar-header-serialize new-descriptor t))
      (setf  (tar-header-data-start new-descriptor)
	     (copy-marker (point) nil)))
    ;; Update tar-parse-info.
    (if d-list
	(setcdr d-list     (cons new-descriptor (cdr d-list)))
      (setq tar-parse-info (cons new-descriptor tar-parse-info)))
    ;; Update the listing buffer.
    (save-excursion
      (goto-char (point-min))
      (forward-line index)
      (let ((inhibit-read-only t))
	(insert (tar-header-block-summarize new-descriptor) ?\n)))
    ;; .
    index))

(defun tar-flag-deleted (p &optional unflag)
  "In Tar mode, mark this sub-file to be deleted from the tar file.
With a prefix argument, mark that many files."
  (interactive "p")
  (beginning-of-line)
  (dotimes (_ (abs p))
    (if (tar-current-descriptor unflag) ; barf if we're not on an entry-line.
	(progn
	  (delete-char 1)
	  (insert (if unflag " " "D"))))
    (forward-line (if (< p 0) -1 1)))
  (if (eobp) nil (forward-char 36)))

(defun tar-unflag (p)
  "In Tar mode, un-mark this sub-file if it is marked to be deleted.
With a prefix argument, un-mark that many files forward."
  (interactive "p")
  (tar-flag-deleted p t))

(defun tar-unflag-backwards (p)
  "In Tar mode, un-mark this sub-file if it is marked to be deleted.
With a prefix argument, un-mark that many files backward."
  (interactive "p")
  (tar-flag-deleted (- p) t))


(defun tar-expunge-internal ()
  "Expunge the tar-entry specified by the current line."
  (let ((descriptor (tar-current-descriptor)))
    ;;
    ;; delete the current line...
    (delete-region (line-beginning-position) (line-beginning-position 2))
    ;;
    ;; delete the data pointer...
    (setq tar-parse-info (delq descriptor tar-parse-info))
    ;;
    ;; delete the data from inside the file...
    (with-current-buffer tar-data-buffer
      (delete-region (or (tar-header-header-start descriptor)
                         (- (tar-header-data-start descriptor) 512))
                     (tar-header-data-end descriptor)))))


(defun tar-expunge (&optional noconfirm)
  "In Tar mode, delete all the archived files flagged for deletion.
This does not modify the disk image; you must save the tar file itself
for this to be permanent."
  (interactive)
  (if (or noconfirm
	  (y-or-n-p "Expunge files marked for deletion? "))
      (let ((n 0))
	(save-excursion
	  (goto-char (point-min))
	  (while (not (eobp))
	    (if (= (following-char) ?D)
		(progn (tar-expunge-internal)
		       (setq n (1+ n)))
		(forward-line 1)))
	  ;; after doing the deletions, add any padding that may be necessary.
	  (tar-pad-to-blocksize))
	(if (zerop n)
	    (message "Nothing to expunge.")
	    (message "%s files expunged.  Be sure to save this buffer." n)))))


(defun tar-clear-modification-flags ()
  "Remove the stars at the beginning of each line."
  (interactive)
  (save-excursion
    (goto-char (point-min))
    (while (not (eobp))
      (if (not (eq (following-char) ?\s))
	  (progn (delete-char 1) (insert " ")))
      (forward-line 1))))


(defun tar-chown-entry (new-uid)
  "Change the user-id associated with this entry in the tar file.
If this tar file was written by GNU tar, then you will be able to edit
the user id as a string; otherwise, you must edit it as a number.
You can force editing as a number by calling this with a prefix arg.
This does not modify the disk image; you must save the tar file itself
for this to be permanent."
  (interactive
   (list
    (let ((descriptor (tar-current-descriptor)))
      (if (or current-prefix-arg
              (not (tar-header-magic descriptor)))
          (read-number
           "New UID number: "
           (format "%s" (tar-header-uid descriptor)))
        (read-string "New UID string: " (tar-header-uname descriptor))))))
  (cond ((stringp new-uid)
	 (setf (tar-header-uname (tar-current-descriptor)) new-uid)
	 (tar-alter-one-field tar-uname-offset
                              (concat (encode-coding-string
                                       new-uid tar-file-name-coding-system)
                                      "\000")))
	(t
	 (setf (tar-header-uid (tar-current-descriptor)) new-uid)
	 (tar-alter-one-field tar-uid-offset
	   (concat (substring (format "%6o" new-uid) 0 6) "\000 ")))))


(defun tar-chgrp-entry (new-gid)
  "Change the group-id associated with this entry in the tar file.
If this tar file was written by GNU tar, then you will be able to edit
the group id as a string; otherwise, you must edit it as a number.
You can force editing as a number by calling this with a prefix arg.
This does not modify the disk image; you must save the tar file itself
for this to be permanent."
  (interactive
   (list
    (let ((descriptor (tar-current-descriptor)))
      (if (or current-prefix-arg
              (not (tar-header-magic descriptor)))
          (read-number
           "New GID number: "
           (format "%s" (tar-header-gid descriptor)))
        (read-string "New GID string: " (tar-header-gname descriptor))))))
  (cond ((stringp new-gid)
	 (setf (tar-header-gname (tar-current-descriptor)) new-gid)
	 (tar-alter-one-field tar-gname-offset
                              (concat (encode-coding-string
                                       new-gid tar-file-name-coding-system)
                                      "\000")))
	(t
	 (setf (tar-header-gid (tar-current-descriptor)) new-gid)
	 (tar-alter-one-field tar-gid-offset
	   (concat (substring (format "%6o" new-gid) 0 6) "\000 ")))))

(defun tar-rename-entry (new-name)
  "Change the name associated with this entry in the tar file.
This does not modify the disk image; you must save the tar file itself
for this to be permanent."
  (interactive
    (list (read-string "New name: "
	    (tar-header-name (tar-current-descriptor)))))
  (if (string= "" new-name) (error "zero length name"))
  (let ((encoded-new-name (encode-coding-string new-name
						tar-file-name-coding-system))
        (descriptor (tar-current-descriptor))
        (prefix nil))
    (when (tar-header-header-start descriptor)
      ;; FIXME: Make it work for ././@LongLink.
      (error "Rename with @LongLink format is not implemented"))

    (when (and (> (length encoded-new-name) 98)
               (string-match "/" encoded-new-name
			     (- (length encoded-new-name) 99))
	       (< (match-beginning 0) 155))
      (unless (equal (tar-header-magic descriptor) "ustar\0")
        (tar-alter-one-field tar-magic-offset (concat "ustar\0" "00")))
      (setq prefix (substring encoded-new-name 0 (match-beginning 0)))
      (setq encoded-new-name (substring encoded-new-name (match-end 0))))

    (if (> (length encoded-new-name) 98) (error "name too long"))
    (setf (tar-header-name descriptor) new-name)
    (tar-alter-one-field 0
     (substring (concat encoded-new-name (make-string 99 0)) 0 99))
    (if prefix
        (tar-alter-one-field tar-prefix-offset
         (substring (concat prefix (make-string 155 0)) 0 155)))))


(defun tar-chmod-entry (new-mode)
  "Change the protection bits associated with this entry in the tar file.
This does not modify the disk image; you must save the tar file itself
for this to be permanent."
  (interactive (list (tar-parse-octal-integer-safe
		       (read-string "New protection (octal): "))))
  (setf (tar-header-mode (tar-current-descriptor)) new-mode)
  (tar-alter-one-field tar-mode-offset
    (concat (substring (format "%6o" new-mode) 0 6) "\000 ")))


(defun tar-alter-one-field (data-position new-data-string &optional descriptor)
  (unless descriptor (setq descriptor (tar-current-descriptor)))
  ;;
  ;; update the header-line.
  (let ((col (current-column)))
    (delete-region (line-beginning-position)
                   (prog2 (forward-line 1)
                       (point)
                     ;; Insert the new text after the old, before deleting,
                     ;; to preserve markers such as the window start.
                     (insert (tar-header-block-summarize descriptor) "\n")))
    (forward-line -1) (move-to-column col))

  (cl-assert (tar-data-swapped-p))
  (with-current-buffer tar-data-buffer
    (let* ((start (- (tar-header-data-start descriptor) 512)))
        ;;
        ;; delete the old field and insert a new one.
        (goto-char (+ start data-position))
        (delete-region (point) (+ (point) (length new-data-string))) ; <--
        (cl-assert (not (or enable-multibyte-characters
                            (multibyte-string-p new-data-string))))
        (insert new-data-string)
        ;;
        ;; compute a new checksum and insert it.
        (let ((chk (tar-header-block-checksum
		  (buffer-substring start (+ start 512)))))
	(goto-char (+ start tar-chk-offset))
	(delete-region (point) (+ (point) 8))
	(insert (format "%6o\0 " chk))
	(setf (tar-header-checksum descriptor) chk)
	;;
	;; ok, make sure we didn't botch it.
	(tar-header-block-check-checksum
	 (buffer-substring start (+ start 512))
	 chk (tar-header-name descriptor))
	))))


(defun tar-octal-time (timeval)
  ;; Format a timestamp as 11 octal digits.
  (format "%011o" (time-convert timeval 'integer)))

(defun tar-subfile-save-buffer ()
  "In tar subfile mode, save this buffer into its parent tar-file buffer.
This doesn't write anything to disk; you must save the parent tar-file buffer
to make your changes permanent."
  (interactive)
  (if (not (and (boundp 'tar-superior-buffer) tar-superior-buffer))
      (error "This buffer has no superior tar file buffer"))
  (if (not (and (boundp 'tar-superior-descriptor) tar-superior-descriptor))
      (error "This buffer doesn't have an index into its superior tar file!"))
  (let ((subfile (current-buffer))
        (coding buffer-file-coding-system)
        (descriptor tar-superior-descriptor)
        subfile-size)
    (with-current-buffer tar-superior-buffer
      (let* ((start (tar-header-data-start descriptor))
             (size (tar-header-size descriptor))
             (head (memq descriptor tar-parse-info)))
        (if (not head)
            (error "Can't find this tar file entry in its parent tar file!"))
        (with-current-buffer tar-data-buffer
          ;; delete the old data...
          (let* ((data-start start)
                 (data-end (+ data-start (tar-roundup-512 size))))
            (narrow-to-region data-start data-end)
            (delete-region (point-min) (point-max))
            ;; insert the new data...
            (goto-char data-start)
            (let ((dest (current-buffer)))
              (with-current-buffer subfile
                (save-restriction
                  (widen)
                  (encode-coding-region (point-min) (point-max) coding dest))))
            (setq subfile-size (- (point-max) (point-min)))
            ;;
            ;; pad the new data out to a multiple of 512...
            (let ((subfile-size-pad (tar-roundup-512 subfile-size)))
              (goto-char (point-max))
              (insert (make-string (- subfile-size-pad subfile-size) 0))
              ;;
              ;; update the data of this files...
              (setf (tar-header-size descriptor) subfile-size)
              ;;
              ;; Update the size field in the header block.
              (widen))))
        ;;
        ;; alter the descriptor-line and header
        ;;
        (let ((position (- (length tar-parse-info) (length head))))
          (goto-char (point-min))
          (forward-line position)
	  (tar-alter-one-field tar-size-offset (format "%11o " subfile-size))
	  ;;
	  ;; Maybe update the datestamp.
	  (when tar-update-datestamp
	    (tar-alter-one-field tar-time-offset
				 (concat (tar-octal-time (current-time)) " "))))
        ;; After doing the insertion, add any necessary final padding.
        (tar-pad-to-blocksize))
      (set-buffer-modified-p t)         ; mark the tar file as modified
      (tar-next-line 0))
    (set-buffer-modified-p nil)       ; mark the tar subfile as unmodified
    (message "Saved into tar-buffer `%s'.  Be sure to save that buffer!"
             (buffer-name tar-superior-buffer))
    ;; Prevent basic-save-buffer from changing our coding-system.
    (setq last-coding-system-used buffer-file-coding-system)
    ;; Prevent ordinary saving from happening.
    t))


;; When this function is called, it is sure that the buffer is unibyte.
(defun tar-pad-to-blocksize ()
  "If we are being anal about tar file blocksizes, fix up the current buffer.
Leaves the region wide."
  (if (null tar-anal-blocksize)
      nil
    (let* ((last-desc (nth (1- (length tar-parse-info)) tar-parse-info))
	   (start (tar-header-data-start last-desc))
	   (link-p (tar-header-link-type last-desc))
	   (size (if link-p 0 (tar-header-size last-desc)))
	   (data-end (+ start size))
	   (bbytes (ash tar-anal-blocksize 9))
	   (pad-to (+ bbytes (* bbytes (/ (- data-end (point-min)) bbytes)))))
      ;; If the padding after the last data is too long, delete some;
      ;; else insert some until we are padded out to the right number of blocks.
      ;;
      (with-current-buffer tar-data-buffer
        (let ((goal-end (+ (point-min) pad-to)))
          (if (> (point-max) goal-end)
              (delete-region goal-end (point-max))
            (goto-char (point-max))
            (insert (make-string (- goal-end (point-max)) ?\0))))))))


;; Used in write-region-annotate-functions to write tar-files out correctly.
(defun tar-write-region-annotate (start _end)
  ;; When called from write-file (and auto-save), `start' is nil.
  ;; When called from M-x write-region, we assume the user wants to save
  ;; (part of) the summary, not the tar data.
  (unless (or start (not (tar-data-swapped-p)))
  (tar-clear-modification-flags)
    (set-buffer tar-data-buffer)
    nil))

(provide 'tar-mode)

;;; tar-mode.el ends here