bug#3634: emacsclient: unwind-protect prevents printing final form

[Jonas Bernoulli <jonas@bernoul.li>]

Hi

I am trying to write a wrapper around emacsclient that would allow
emacs to be used by external applications to complete some string.

However it does not seem to be possible to
1. return the last form and
2. ensure that emacsclient always returns
at the same time.

(The following examples assumes default-minibuffer-frame exists. Just
remove the respective line if it doesn't.)

This works as long as the user does not abort:

#!/bin/bash
STRING=$(emacsclient -e "\
(select-frame-set-input-focus default-minibuffer-frame)
(read-string \"> \")"
echo ${STRING:1:$((${#STRING}-2))

But when he does, emacsclient never returns. So I tried using
unwind-protect to make sure that emacsclient always returns:

#!/bin/bash
STRING=$(emacsclient -e "\
(let ((emonad-client (car server-clients)))
  (select-frame-set-input-focus default-minibuffer-frame)
  (unwind-protect (read-string \"> \")
    (when (memq emonad-client server-clients)
      (server-delete-client emonad-client t))))")
echo ${STRING:1:$((${#STRING}-2))

Now my script always returns but fails to print the final form to
standard output even when input was NOT aborted.

I tried several things to work around this:

1. Store the return value of read-string in some variable, and using
the variable as last form of let. When I use (message "veni vedi %s"
tmp) as last form I can see that forms after unwind-protect are
actually evaluated, the last form's value is just never printed to
stout.
2. condition-case
3. catch

However in all cases nothing was printed to standard output. It seems
that as soon as some non-local exit is used emacsclient refuses to
print anything to stout. And since exit-minibuffer does (throw 'exit
nil) to abort minibuffer output there is now way to prevent such an
non-local exit.

As a workaround it might be possible to explicitly print to stout, but
I could not find any information on how to do that.

thx

Jonas