Comment 4 for bug 1511783

From what I can tell so far it seems to be an issue with rewriteSourcesList in DistUpgradeController and this part of it.

542 logging.debug("BaseMetaPkg '%s' has no candidate.origins" % pkgname)
 543 try:
 544 distro = get_distro()
 545 distro.get_sources(self.sources)
 546 distro.enable_component("main")

get_distro is from python3-apt and I got as far as figuring out that get_sources ended up creating an empty comps list.