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

Panu Matilainen pmatilai at laiskiainen.org
Mon Feb 6 09:50:42 UTC 2017

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.

	- Panu -

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