bug#56799: (gnu services configuration) usage of *unspecified* is problematic

[Attila Lendvai]

> 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?

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• attila lendvai
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