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From: Stefan Monnier <monnier@HIDDEN>
To: Markus Triska <triska@HIDDEN>
Subject: Re: bug#23963: 25.0.95;
Feature request: setup-unwind-protect, complementing unwind-protect
Message-ID: <jwvlggjnh6u.fsf-monnier+bug#23963@HIDDEN>
References: <m2d1mif3vm.fsf@HIDDEN>
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
bug-gnu-emacs@HIDDEN:bug#23963; Package emacs.
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From: Markus Triska <triska@HIDDEN>
To: bug-gnu-emacs@HIDDEN
Subject: 25.0.95;
Feature request: setup-unwind-protect, complementing unwind-protect
Date: Wed, 13 Jul 2016 08:28:45 +0200
Message-ID: <m2d1mif3vm.fsf@HIDDEN>
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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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Markus Triska <triska@HIDDEN>:bug-gnu-emacs@HIDDEN.
Full text available.bug-gnu-emacs@HIDDEN:bug#23963; Package emacs.
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