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Re: unwind-protect and inhibit-quit
From: |
Felix Dietrich |
Subject: |
Re: unwind-protect and inhibit-quit |
Date: |
Fri, 16 Jul 2021 23:30:31 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
Stefan Monnier via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:
>> Now I'm curious too... Would something like this work?
>>
>> (let ((inhibit-quit t))
>> (setq process
>> (let ((inhibit-quit nil))
>> (ftp-setup-buffer host file))))
>
> No. The problem is not in the `setq` itself but in the fact that
> a non-local exit from `ftp-setup-buffer` (e.g. because of `C-g`) will
> cause `ftp-setup-buffer` not to return the process.
But at which point is the ‘quit-flag’ actually handled? When unwinding
the function, just before returning a value? The macro
‘with-local-quit’ states that the “quit-flag” “will not be handled until
the next function call”[1]. Could, therefore, a careful and cooperative
“ftp-setup-buffer” ensure that the process is either cleaned-up or
returned? I am thinking of something like the following:
(defun ftp-setup-buffer (host file)
(let (process)
(condition-case nil
(progn
(let ((inhibit-quit t))
;; I am assuming that ‘start-process’ does not block for a
;; long time. Does it? Maybe with a defective drive?
;; How about ‘start-file-process’?
(setq process (start-process …)))
;; do more stuff
process)
(quit
;; I guess another quit could happen here; nest more
;; ‘condition-case’? Itʼs ‘condition-case’ all the way down.
(and (processp process)
(kill-process process))
(setq quit-flag t)
;; Propagate quit and make sure an outside handler receives it.
;; I do not have a good understanding of this. I simply copied
;; it from the ‘with-local-quit’ macro.
(eval '(ignore nil))))))
(let (process)
(unwind-protect
(let ((inhibit-quit t))
(setq process
(with-local-quit
(ftp-setup-buffer host file)))
;; do stuff
)
(and (processp process) (kill-process process))))
> We should devise a more reliable API, tho I'm not completely sure what
> it should look like. Maybe
>
> (let ((list-of-created-processes nil))
> (unwind-protect ...
> (mapc #'delete-process list-of-created-processes)))
>
> Where the low-level primitives which create processes add them to
> `list-of-created-processes`.
How do the low-level C functions to create processes handle quit? If it
happens inside of them, they should be responsible for killing the
process. But I am understanding that there is a step in between the
creation of the process object, the return from a process creation
primitive, and the assignment of the process object to a variable.
> But then this gets into trouble when some unrelated code is run during
> `...` (e.g. via timers or whatnot) which creates unrelated processes,
> so we'd need some extra care to make sure those processes don't get
> added to "the same" `list-of-created-processes`.
If I am allowed to spitball here: create another function call to
“claim” a process, and subject unclaimed processes to a similar clean-up
routine as buffers (with ‘clean-buffer-list’):
(let (process)
(unwind-protect
(progn
(setq process (ftp-setup-buffer host file))
(claim-process process)
;; do stuff
)
(and (processp process)
(kill-process process))))
Add an optional CLAIMED parameter to process creation primitives that
defaults to t in order to not disturb existing code. Naturally, I have
no idea how to implement that nor the skill to figure this out.
Footnotes:
[1] Although there is also this information in (info "(elisp)
Quitting"):
“Eventually, ‘inhibit-quit’ will become ‘nil’ again, such as when
its binding is unwound at the end of a ‘let’ form. At that time,
if ‘quit-flag’ is still non-‘nil’, the requested quit happens
immediately.”
--
Felix Dietrich
- unwind-protect and inhibit-quit, Felix Dietrich, 2021/07/15
- Re: unwind-protect and inhibit-quit,
Felix Dietrich <=
- Re: unwind-protect and inhibit-quit, Stefan Monnier, 2021/07/16
- Re: unwind-protect and inhibit-quit, Felix Dietrich, 2021/07/17
- Re: unwind-protect and inhibit-quit, Eli Zaretskii, 2021/07/17
- Re: unwind-protect and inhibit-quit, Felix Dietrich, 2021/07/17
- Re: unwind-protect and inhibit-quit, Eli Zaretskii, 2021/07/17