Conditional build dependencies seem to be resolved on host

[hroncok]

Short description of the problem

Conditional BRs (with if in them) seem to be evaluated on host.

Output of rpm -q mock

mock-1.4.18-1.fc30.noarch

Steps to reproduce issue

Run on Fedora 29, 30 or 31. Get this spec:

Name:           reproducer_if
Version:        0.0
Release:        1%{?dist}
Summary:        ...
License:        MIT
BuildRequires:  python3
BuildRequires:  (python3dist(importlib-metadata) if python3 < 3.8)

%description
...

Create a SRPM:

$ rpmbuild -bs reproducer_if.spec

See the dependencies:

$ rpm -qp --requires ../SRPMS/reproducer_if-0.0-1.fc30.src.rpm 
(python3dist(importlib-metadata) if python3 < 3.8)
python3
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(RichDependencies) <= 4.12.0-1

Attempt a rawhide mockbuild:

$ mock -r fedora-rawhide-x86_64 ../SRPMS/reproducer_if-0.0-1.fc30.src.rpm 
...
No matching package to install: '(python3dist(importlib-metadata) if python3 < 3.8)'
Package python3-3.8.0~b4-1.fc32.x86_64 is already installed.
Not all dependencies satisfied
Error: Some packages could not be found.

Any additional notes

The conditional works good on Fedora 32. However mock running on Fedora 30 seems to choke on it. The package providing python3dist(importlib-metadata) was retired on Fedora 32, because it is not needed with Python 3.8.

cc @encukou

See also fedora-infra/koschei#295

[hroncok]

This also affects dynamic buildrequires. When a rawhide build prints (python3dist(importlib-metadata) if python3 < 3.8) in %generate_buildrequires, Fedora 30 mock exists with No matching package to install: '(python3dist(importlib-metadata) if python3 < 3.8)'

[xsuchy]

After quick debugging session with Jarda M. we found that this is DNF bug. https://bugzilla.redhat.com/show_bug.cgi?id=1756902