While testing the dist-repo feature on rpmfusion, I was using the following command:
koji-rpmfusion dist-repo f27-free --arch=x86_64 --arch=i686 --arch=ppc64 --arch=ppc64le --arch=aarch64 --arch=armhfp --allow-missing-signatures --multilib=/home/rpmfusion/kwizart/rpmfusion-tools/rpmfusion-free-multilibs --comps=/home/rpmfusion/kwizart/rpmfusion-free-comps/comps-f27.xml --nowait
Unfortunately, this doesn't work for ppc64 which is a multilib arch, but where ppc is not build anymore. I have the following error with the ppc64 childTask failing with: GenericError: multilib packages missing. See missing_multilib.log http://koji.rpmfusion.org/koji/taskinfo?taskID=165792 (I even tried to cheat by asking to generate a ppc dist-repo).
My suggestion would be to avoid to generate a multilib repo if both related arches aren't explicitly specified (and only issue a warning if one arch is missing).
After some discussion with @tkopecek, I think probably the simplest thing here is to warn but not error. This command doesn't need to strictly enforce multilib closure.
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/580
Please continue any further discussion there.