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Subject: bug#23963: 25.0.95; Feature request: setup-unwind-protect, complementing unwind-protect
Resent-From: Markus Triska <triska@HIDDEN>
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From: Markus Triska <triska@HIDDEN>
Date: Wed, 13 Jul 2016 08:28:45 +0200
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The manual entry of `unwind-protect' shows an example whose essence is:

    (unwind-protect
        (progn
          (setq process (ftp-...))
          ;; use process
      (delete-process process))

It adds the following important explanation:

    This example has a small [sic] bug: if the user types `C-g' to quit,
    and the quit happens immediately after the function `ftp-...'
    returns but before the variable `process' is set, *the process will
    not be killed*. There is no easy way to fix this bug, but at least
    it is very unlikely.

Would it be good to make such resource leak *impossible* instead? This
seems quite desirable and could be done with a complementing primitive.

Please consider a new primitive `setup-unwind-protect' with 3 arguments:

    1. SETUPFORM, called in such a way that it *cannot be interrupted*
    2. BODYFORM, like in unwind-protect, called after SETUPFORM
    3. UNWINDFORMS, like in unwind-protect

With this primitive, we could write the example above as:

    (setup-unwind-protect
        (setq process (ftp-...))
     (progn
       ;; use process ...
     (delete-process process))

Please consider adding such a construct to Emacs.

A precedence for such a construct is setup_call_cleanup/3 which appears
in the Prolog ISO draft standard and is already included in many systems:

    https://www.complang.tuwien.ac.at/ulrich/iso-prolog/N215

Thank you and all the best!
Markus


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To: Markus Triska <triska@HIDDEN>
Subject: bug#23963: Acknowledgement (25.0.95; Feature request:
 setup-unwind-protect, complementing unwind-protect)
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Date: Wed, 13 Jul 2016 06:29:01 +0000

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Subject: bug#23963: 25.0.95; Feature request: setup-unwind-protect, complementing unwind-protect
Resent-From: Stefan Monnier <monnier@HIDDEN>
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From: Stefan Monnier <monnier@HIDDEN>
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Date: Sat, 27 Jan 2018 16:17:19 -0500
In-Reply-To: <m2d1mif3vm.fsf@HIDDEN> (Markus Triska's message of "Wed,
 13 Jul 2016 08:28:45 +0200")
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Markus Triska <triska@HIDDEN> writes:

> With this primitive, we could write the example above as:
>
>     (setup-unwind-protect
>         (setq process (ftp-...))
>      (progn
>        ;; use process ...
>      (delete-process process))

Given my dislike of `setq` I'd rather use something else.

This said, we also have another problem: if the user hits C-g twice
in a row, the second may interrupt the `delete-process` itself.

So I think what we "really" want is more like

    (let ((oi inhibit-quit)
          (inhibit-quit t))
      (let ((process (ftp-...)))
         (unwind-protect
             (let ((inhibit-quit oi))
               ..use process..)
           (delete-process process))))

At the same time, any function whose name starts with "ftp-" is likely
a bad candidate for running it in a context where it can't be
interrupted (it's likely that in various circumstances it could hang, in
which case the user should be able to hit C-g).


        Stefan

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