bug#39884: 27.0.50; Emacs may destroy outgoing email messages during sending

[Noam Postavsky <npostavs@gmail.com>]

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On Wed, 15 Apr 2020 at 03:55, Rainer Gemulla <rgemulla@gmx.de> wrote:
>
> Replacing in function mml-parse-1 (mml.el, line 284):
>
> (setq tag (list 'part '(type . "text/plain"))
>        no-markup-p t
>        warn t)
>
> with
>
> (setq tag `(part (type . ,"text/plain"))
>       no-markup-p t
>       warn t)
>
> seems to fix this bug. It's not clear to me why though.

mml-parse-1 calls nconc on tag, so it's likely some destructive
modification of code literals is happening. However, I'm not able to
reproduce the bug following your instructions on the latest emacs-27
checkout. I'm attaching a lisp file which automates the process (I
also tried doing it manually, in case that makes a difference).

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(defconst bug-39884-orig-message "\
From: a <at> b.ce
To: c <at> d.de
Subject: Test
--text follows this line--
Test

<#mml type=message/rfc822 disposition=inline>
<#multipart type=alternative>
<#part type=text/plain charset=\"UTF-8\" disposition=inline nofile=yes>
Some text.
<#part type=text/html charset=\"UTF-8\" nofile=yes>
Some more text.
<#/multipart>
<#/mml>
")

(defconst bug-39884-correct-result "\
From: a <at> b.ce
To: c <at> d.de
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=\"=-=-=\"
--text follows this line--
--=-=-=
Content-Type: text/plain

Test


--=-=-=
Content-Type: message/rfc822
Content-Disposition: inline

--====-=-=
Content-Disposition: inline
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=\"====-=-=\"

Some text.

--====-=-=
Content-Type: text/html; charset=utf-8

Some more text.

--====-=-=--

--=-=-=--
")

(defconst 39884-correct-results-of-step-9
  "\
From: a <at> b.ce
To: c <at> d.de
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=\"=-=-=\"
--text follows this line--
--=-=-=
Content-Type: multipart/alternative; boundary=\"====-=-=\"
Content-Transfer-Encoding: base64

VGV4dAoK
--=-=-=
Content-Type: message/rfc822
Content-Disposition: inline

--====-=-=
Content-Disposition: inline
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=\"====-=-=\"

Some text.

--====-=-=
Content-Type: text/html; charset=utf-8

Some more text.

--====-=-=--

--=-=-=--
")

;; 2. M-x message-mode
(message-mode)
;; 3. Clear scratch buffer, paste original message
(erase-buffer)
(insert bug-39884-orig-message)

;; 4. M-: (mml-to-mime) --> gives CORRECT result
(goto-char (point-min))
(mml-to-mime)
(cl-assert (equal (buffer-string) bug-39884-correct-result))

;; 5. Clear scratch buffer, paste original message
(erase-buffer)
(insert bug-39884-orig-message)

;; 6. Insert a new line "<#secure method=pgpmime mode=sign>" at start of 
;; message (after line "--text follows this line--"). This makes Emacs try 
;; to sign the mail.
(search-backward "--text follows this line--")
(forward-line)
(insert "<#secure method=pgpmime mode=sign>\n")

;; 7. M-: (mml-to-mime) --> throws (expected) signer name error
(cl-assert (condition-case ()
               (progn (goto-char (point-min))
                      (mml-to-mime)
                      nil)
             (error t)))

;; 8. Clear scratch buffer, paste original message
(erase-buffer)
(insert bug-39884-orig-message)

;; 9. M-: (mml-to-mime)
;;   --> broken result (first Content-Type after "text follows..." is 
;; wrong)
(goto-char (point-min))
(mml-to-mime)