Network failure can cause "impossible" unavailable components

[brson]

trafficstars

See https://github.com/rust-lang-nursery/rustup.rs/issues/708 where cargo didn't exist.

The most likely reason for this to happen is that the network request in live_package_url failed. It should probably have a retry loop and not be allowed to fail for any package in the host_list.

[brson]

Actually, the exact criteria for not being allowed to fail is probably complex. Tier 1 targets can't fail. Other targets can fail, but then every single package for that target also needs to fail. It's not valid for required components to be listed as unavailable if the package they belong to is available.