bug#55599: save-buffers-kill-emacs doesn't give a visible prompt when called from command line

[Eli Zaretskii <eliz@gnu.org>]

> From: Peter Ludemann <peter.ludemann@gmail.com>
> Date: Wed, 1 Jun 2022 10:54:02 -0700
> Cc: Michael Albinus <michael.albinus@gmx.de>, 55599@debbugs.gnu.org
> 
> I start the emacs daemon from a terminal by:
>     emacs --daemon
> 
> During startup, there might be some prompts (e.g., I get a pagination prompt from parsing a tar file, when
> emacs is loading my last session's files), and I can deal with them at the terminal.
> 
> When I shutdown the emacs daemon, I would like to also do this at the terminal:
>     emacsclient -e '(save-buffers-kill-emacs)'
> but that doesn't work if there are any prompts (e.g., saving a file or killing a shell process).
> A work-around is to do:
>     emacsclient -c -e '(save-buffers-kill-emacs)'
> and that lets me see the prompts in the minibuffer.
> 
> However, it seems that "emacsclient -e" has access to the terminal's stderr.
> Evidence: I get an error message at the terminal (not in a minibuffer) if I do something with an error, such as
>     emacsclient -e '(man fprintf)'
> 
> If stderr is being passed in to the evaluation code in daemon mode, then it would be nice to also have stdin
> passed in, which would allow interaction at the terminal.

stderr is not passed.  What happens is that any messages sent by the
server are echoed by the client on its own stderr.

However, the reverse direction is not supported.  For it to be
supported, we need a way to tell the server to wait for the client to
return the user's response.  Such a feature doesn't exist, AFAIK, so
to support it we need to extend the client-server protocol.