bug#13522: 24.2; save-buffer removes edited file under some conditions

[Glenn Morris <rgm@gnu.org>]

Eli Zaretskii wrote:

> I can't.  Moreover, the recipe says "Type C-c in the terminal to kill
> Emacs", but C-c does not kill Emacs, only C-x C-c does. 

C-c *in the shell* from which Emacs was started in the foreground, not
from in Emacs; ie interrupt it from outside.

Or even: do C-x C-s, and leave the coding prompt unanswered. You will
find the original file missing until you answer, or quit, the coding
question!

Looks like it has been this way since the unicode merge.
basic-save-buffer-2 calls backup-buffer, which may rename the original
file. It then calls write-region. This may call
select-safe-coding-system, so there can be an arbitrarily long interval
between the original file being renamed to the backup, and the new file
being written.

If you interrupt the coding prompt with C-g, the unwind-protect in
basic-save-buffer-2 puts back the original file. I suppose the problem
could maybe be papered over by adding something equivalent to
kill-emacs-hook, but it's still very far from ideal.

Maybe the right solution is to have the select-safe-coding-system check
in basic-save-buffer-2 before backup-buffer, then pass the resulting
coding system to write-region somehow so it does not need to query
again.