bug#15535: Acknowledgement (24.3.50; basic-save-buffer should update buffer-file-coding-system value if the contents were written using different coding system)

[Eli Zaretskii <eliz@gnu.org>]

(I've added Handa-san to this discussion, as I'm not sure I didn't miss
anything in looking into this.)

> Date: Sun, 06 Oct 2013 02:09:01 +0300
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> Sorry, here's a better test:
> 
> (ert-deftest save-buffer-updates-buffer-file-coding-system ()
>    (let ((file (expand-file-name "foo" temporary-file-directory))
>          (default-buffer-file-coding-system 'utf-8-unix))
>      (find-file file)
>      (insert "abcdef\n")
>      (save-buffer)
>      (kill-buffer)
>      (unwind-protect
>          (with-current-buffer (find-file-noselect file)
>            (should (eq 'undecided (coding-system-change-eol-conversion
>                                    buffer-file-coding-system nil)))
>            (insert "водка матрёшка селёдка")
>            (save-buffer)
>            (let ((coding-system buffer-file-coding-system))
>              (kill-buffer)
>              (should (eq 'utf-8-unix coding-system))))
>        (delete-file file))))
> 
> Likewise, succeeds on 24.3, fails on trunk.

Thanks.  For the record, a simpler test case is this:

 emacs -Q
 C-x C-f foo RET
 
Insert some ASCII text, then save the buffer, kill it, and visit the
file again:

 C-x C-s
 C-x k RET
 C-x C-f foo RET

You now have foo with `undecided' as its buffer-file-coding-system.
Then:

 C-u C-\ cyrillic-translit RET
 abvgde
 C-\
 C-x C-s

The file is saved (as UTF-8, as can be seen by examining it on disk),
but without asking for encoding, and without changing
buffer-file-coding-system to reflect the actual encoding.

What happens is that `undecided' silently encodes the buffer in UTF-8,
but never communicates that fact back to its callers.  So write-region
thinks it used `undecided', as does select-safe-coding-system.  The
latter is actually equipped to DTRT when the `prefer-utf-8' variant of
`undecided' is used, but that is not the case here.

Is this what was supposed to happen, or is something misbehaving here?

If the former, we could perhaps add some flag to struct undecided_spec
and set it whenever the encoder used by `undecided' sees a non-ASCII
character, and then use that flag to set last-coding-system-used to
UTF-8.  Does this make sense?