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If you build with networking disabled, config_opts['use_host_resolv'] = False is placed into the respective mock chroot config. If you enable networking, it does not do anything with chroot configs because it assumes networking is enabled by default. That is not true though if the networking has been disabled before for a chroot config. In other words, if you build on a builder with networking disabled, that builder will have networking disabled for all consecutive builds (in that build's chroots) and this state cannot be reverted.
Sorry, I forgot a reproducer with a package that needs networking enabled: https://copr.fedorainfracloud.org/coprs/clime/spaceshiftx/build/176049/ -> networking disabled, build fails https://copr.fedorainfracloud.org/coprs/clime/spaceshiftx/build/176050/ -> networking enabled, the same builder allocated, builds fails https://copr.fedorainfracloud.org/coprs/clime/spaceshiftx/build/176054/ -> networking enabled, a different builder allocated, build succceeds
Fixed by github.com/fedora-copr/copr/commit/7e4f941a
Package including this fix has been deployed on production instance of Copr.