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bug#46407: 27.1; Hooks with permanent-local-hook are not cleared of lamb
From: |
Lars Ingebrigtsen |
Subject: |
bug#46407: 27.1; Hooks with permanent-local-hook are not cleared of lambdas |
Date: |
Mon, 24 May 2021 23:55:28 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) |
jakanakaevangeli <jakanakaevangeli@chiru.no> writes:
> When kill-all-local-variables encounters a hook variable with its
> 'permanent-local property set to 'permanent-local-hook, it removes from
> its value every element except for t, functions with
> 'permanent-local-hook property and anything that isn't a symbol
> (see the comment at src/buffer.c:1072).
>
> This means that, for the following code
>
> (defvar 'some-hook nil)
> (add-hook 'some-hook #'some-fun nil t)
> (add-hook 'some-hook (lambda () (test)) nil t)
>
> whether some-fun is removed depends on some-fun's permanent-local-hook
> property, which is expected. As for the anonymous lambda function, it is
> not predictable, whether it will be kept or removed. In fact, it depends
> on some-fun's permanent-local-hook property.
Well, it depends on the hook's permanent-local-hook property, but it's
true that add-hook will automatically set that for you if you pass in a
symbol with that property set.
So, yes, that's a strange side effect of this interface, but I'm not
sure anything could be done about it at this stage (it was introduced in
this form almost two decades ago).
jakanakaevangeli <jakanakaevangeli@chiru.no> writes:
> Also one more thing:
>
> (defvar some-hook nil)
> (add-hook 'some-hook #'some-fun nil t)
> (put 'some-fun 'permanent-local-hook t)
>
> If we mark a function as permanent-local-hook only after adding it to a
> hook, the hook symbol will not have its permanent-local property set to
> 'permanent-local-hook, so some-fun will not be kept on removal of local
> variables.
>
> This can be a real non-theoretical problem when adding an autoloaded
> function to a hook.
You mean if the property isn't part of the autoloaded signature? Yes,
that's true.
As far as I can tell, this permanent-local-hook stuff isn't used
anywhere in the Emacs tree, and it seems like a pretty odd and (as this
bug report shows) inconsistent interface. I'm not sure whether it's
worth trying to fix, or we should just document that it's iffy.
--
(domestic pets only, the antidote for overdose, milk.)
bloggy blog: http://lars.ingebrigtsen.no