[Rpm-ecosystem] Fwd: [Rpm-maint] Fixing macro scoping

[Neal Gompa]

On Mon, Feb 6, 2017 at 4:50 AM, Panu Matilainen
<pmatilai at laiskiainen.org> wrote:
> On 02/06/2017 10:56 AM, Vít Ondruch wrote:
>>
>>
>>
>> Dne 3.2.2017 v 13:14 Panu Matilainen napsal(a):
>>>
>>>
>>> Stunned silence on rpm-maint, forwarding to rpm-ecosystem in hopes of
>>> a larger and livelier audience...
>>>
>>>     - Panu -
>>>
>>> -------- Forwarded Message --------
>>> Subject: [Rpm-maint] Fixing macro scoping
>>> Date: Mon, 23 Jan 2017 12:30:21 +0200
>>> From: Panu Matilainen <pmatilai at laiskiainen.org>
>>> To: rpm-maint at lists.rpm.org <rpm-maint at lists.rpm.org>
>>>
>>>
>>> Consider the following snippet, originating from
>>> https://bugzilla.redhat.com/show_bug.cgi?id=552944:
>>>
>>> %{!?foo: %define foo bar}
>>> %define dofoo() true
>>> echo 1: %{foo}
>>> %dofoo
>>> echo 2: %{foo}
>>>
>>> I'd assume everybody agrees both %{foo}'s should evaluate to the same
>>> value, but that is not the case currently. Using a cli-variant of the
>>> above:
>>>
>>> [pmatilai at sopuli rpm]$ rpm --define 'dofoo() true' --eval '%{!?foo:
>>> %define foo bar}' --eval '%foo' --eval '%dofoo' --eval '%foo'
>>> warning: Macro %foo defined but not used within scope
>>>
>>> bar
>>> true
>>> %foo
>>>
>>> The flaw here is that rpm supposedly implements block level scoping
>>> for macros (so in the above example, "foo" would only exist in the
>>> {!?foo:...} block), but doesn't actually enforce that, unless a
>>> parametric macro is "called". Current rpm warns about it, but warnings
>>> or not this behavior doesn't make the slightest sense.
>>>
>>> The question is, what do you think %{foo} should evaluate to in this
>>> case?
>>>
>>> Fixing it to honor the strict "block scoping" concept is not hard, now
>>> that the scoping level is honored from Lua too (see
>>>
>>> https://github.com/rpm-software-management/rpm/commit/1767bc4fd82bfacee622e698f9f0ae42c02126fa).
>>> In this case the above reproducer would emit
>>>
>>> %foo
>>> true
>>> %foo
>>
>>
>> As far as I can tell, usage of %define is discouraged and I never really
>> understood why. But this example explains it and this should be the
>> right behavior IMO.
>
>
> Yup, people are told to avoid %define like it was the plague, which is just
> plain bullsh**. You just need to understand the scoping of macros. Which
> almost nobody does, no matter how many times it gets repeated here or there.
> That the rpm implementation of the scoping is very buggy doesn't exactly
> help of course.
>
> So I've really started thinking it's better to make rpm behave the way
> people expect it to instead of banging my head against this for another 10+
> years.
>

For what it's worth, Andreas Scherer's debbuild[1] treats %define just
like %global in terms of scope, which I guess is the expectation of
most people (though that might be because it's not exactly well
documented that there *are* rules for %define scoping). That said, if
the scoping rules are clear (and work as they're supposed to!), I
don't see a reason why not to actually have scoping for %define.

[1]: https://github.com/ascherer/debbuild

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