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Really strange problem with arc-mode.el

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Hi,

I want to add an archive format to emacs. It would work if the magic
bytes wouldn't start with "M" :-)

Now I tried to track down the problem and made up a fake archive format.

I have a test script that should complete without error:

;; testing a problem with archive-mode where the archive has a M as
;; first magic byte
(require 'arc-mode)

(defun archive-mv-summarize (&optional file)
  (unless file
    (setq file buffer-file-name))
  (let ((copy (file-local-copy file))
        (files ()))
    (with-temp-buffer
      (insert-file (or file copy))
      (when copy (delete-file copy))
      (goto-char (point-min))
      (search-forward-regexp "^(")
      (beginning-of-line)
      (let ((alist (read (current-buffer))))
	(mapc (lambda(x)
		(let ((fname (symbol-name (car x)))
		      (fsize (string-width (cdr x))))
		  (push (archive--file-desc fname fname nil fsize
					    "1984-01-01 00:00")
			files)))
	      alist)))
    (archive--summarize-descs (nreverse files))))

;; unfortunately there is no hook => use advice-add
(defun archive-find-type-mv-around (oldfun)
  (widen)
  (goto-char (point-min))
  ;; NOTE: the problem only occurs if the magic bytes start with M
  (cond ((looking-at "MV") 'mv)
	(t (funcall oldfun))))

(advice-add 'archive-find-type :around #'archive-find-type-mv-around)

(add-to-list 'auto-mode-alist '("\\.mv$" . archive-mode))
(add-to-list 'auto-coding-alist '("\\.mv$" . no-conversion))

(with-temp-buffer
  (insert "MV\n")
  (insert "((a . \"foo\") (b . \"bar\"))\n")
  (write-file "/tmp/test.mv"))

(with-current-buffer (find-file-noselect "/tmp/test.mv" nil nil nil)
  (revert-buffer))

But running it like this:
$ emacs -nw -Q --no-init --script ./arc-mv.el

It fails with:
Buffer format not recognized

NOTE!: if you change the magic bytes from "MV" to something like "EV" it
works as expected.

I would really appreciate some help there, thanks!

Jens Thiele

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Really strange problem with arc-mode.el Jens Thiele <karme@karme.de> - 2025-10-04 09:59 +0200
  Re: Really strange problem with arc-mode.el Jens Thiele <karme@karme.de> - 2025-10-06 09:18 +0200

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