bug#25658: 26.0.50; ELisp part in a mail isn't encoded properly

[Katsumi Yamaoka <yamaoka@jpl.org>]

On Thu, 09 Feb 2017 11:35:50 +0900, Katsumi Yamaoka wrote:
> In a message draft, an ELisp part containing non-ASCII letters,
> like the following, is not encoded properly.

> <#part type="application/emacs-lisp" disposition=inline>
> (defun mm-shr (handle)
>   ...
> 	 ;; Remove "soft hyphens".
> 	 (goto-char (point-min))
> 	 (while (search-forward "­" nil t)
> 	   (replace-match "" t t))
> <#/part>

;; Note that "­" is a soft hyphen.

What Gnus wants to do is:

(quoted-printable-encode-string
 (encode-coding-string "­" 'iso-8859-1))
 => "=AD"

However what is actually done is:

(with-temp-buffer
  ;; `mml-generate-mime-1' does:
  (set-buffer-multibyte t)
  (insert "­")
  ;; `mm-encode-body' does:
  (encode-coding-region (point-min) (point-max) 'iso-8859-1)
  ;; `mm-encode-buffer' does:
  (quoted-printable-encode-region (point-min) (point-max))
  (buffer-string))
 => "=3FFFAD"

Hmm.

(with-temp-buffer
  (set-buffer-multibyte t)
  (insert "­")
  (encode-coding-region (point-min) (point-max) 'iso-8859-1)
  (append (buffer-string) nil))
 => (4194221)

This would probably be the multibyte version of:

(append (encode-coding-string "­" 'iso-8859-1) nil)
 => (173)

Doesn't it mean we ought not to use `encode-coding-region'?
Anyway, I think what we should do here would be one of the
following two ways:

(with-temp-buffer
  (set-buffer-multibyte t)
  (insert "­")
  (encode-coding-region (point-min) (point-max) 'iso-8859-1)
  (set-buffer-multibyte nil)
  (quoted-printable-encode-region (point-min) (point-max))
  (buffer-string))
 => "=AD"

I'm not sure whether (set-buffer-multibyte nil) above does not do
anything other than converting characters to the unibyte version
one by one.  OTOH, this is what I often do:

(with-temp-buffer
  (set-buffer-multibyte t)
  (insert "­")
  (insert (prog1
	      (encode-coding-string (buffer-string) 'iso-8859-1)
	    (erase-buffer)
	    (set-buffer-multibyte nil)))
  (quoted-printable-encode-region (point-min) (point-max))
  (buffer-string))
 => "=AD"

Regards,