GNU bug report logs - #56799
(gnu services configuration) usage of *unspecified* is problematic

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Message #113 received at 56799 <at> debbugs.gnu.org (full text, mbox):

From: Attila Lendvai <attila <at> lendvai.name>
To: Maxim Cournoyer <maxim.cournoyer <at> gmail.com>
Cc: 56799 <at> debbugs.gnu.org, Ludovic Courtès <ludo <at> gnu.org>
Subject: Re: bug#56799: (gnu services configuration) usage of *unspecified* is
 problematic
Date: Thu, 11 Aug 2022 10:15:43 +0000

> OK, I've reread this, and it is indeed a risk, that 'unset could leak in
> the case of a serializable configuration making use of a maybe-value
> field of type maybe-symbol. I've added the unit test suggested as
> 97cb43e732a38758c95b7caf3963507188d011cf (currently marked as 'expected
> to fail'). Luckily no current service uses that.

thank you for that Maxim!

and sorry for my initial, somewhat reactive, and emotionally driven response earlier! maintaining a channel with complex services, and finally getting the changes i needed merged into Guix proper was a source of frustration for me.

i've looked at the current state of the code, and it looks good to me. the only issues i have left are the following:

1) the (eq 'unset ...) scattered around the code; it should be hidden behind an explicit abstraction, but you yourself mentioned this already in an earlier mail. i'd call it CONFIGURATION-FIELD-SET? (instead of MAYBE-SET?). it's longer, but we have completion in emacs, and it won't be used a gazillion times all around the code either.

2) the lack of an abstraction for the unset/unspecified value. whatever we use as the marker should be hidden behind either an exported global variable, or a function called UNSET-CONFIGURATION-FIELD! (or something alike). i should have introduced these myself, and then your fix would have been as simple as replacing *UNSPECIFIED* with 'UNSET in the abstraction.

3) the SYMBOL? corner case that your test captures, but it's not a burning issue for me (it doesn't affect the user facing API, once the above leakages are fixed).

do you agree? if yes, will you implement it, or shall i prepare a patch?

one more note: sometimes it's useful to have a field with a maybe type that also has a default, together with the ability to explicitly unset this field.

an example would be a port specification for a torrent client: it has some default port, but it's possible to explicitly unset the port value to request the allocation of a random port at startup.

to better accommodate for this use case, 2) should probably be implemented not as an UNSET-FOO! function, but as a global variable holding the unset value marker. or maybe both?

--
• attila lendvai
• PGP: 963F 5D5F 45C7 DFCD 0A39
--
“There is only one thing more harmful to society than an elected official forgetting the promises he made in order to get elected; that's when he doesn't forget them.”
	— John McCarthy (1927–2011), father of Lisp

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