Make ValidPath (nar) hash optional if ca field is present

[Ericson2314]

Currently, the NAR hash is mandatory. This would mean requiring just one of ca and hash. Having both is still fine.

I figure as long as we have one way to verify a path's contents, we should be fine. Conferred with @LnL7, who thought not doing this was just a holdover from before the ca field existed.

This would be good to give other NAR alternatives slightly more equal footing, which generally sounds nice, but which I'd specifically be interested in because of the git object work for IPFS. It's by no means a blocker though, as we can always just continue to compute the nar has in addition to anything else.

[matthewbauer]

note that one is base16, while the other is base32:

1|/nix/store/9krlzvny65gdc8s7kpb6lkx8cd02c25b-default-builder.sh|sha256:51858393c5698aba2e7c4e2832054a6985ab3eeda5db7373db5627d0acceefa8|1547485019||152|||fixed:r:sha256:1a7grsnd09snvdrp7nx5xlzap1b9982k4a2fghpbm2k9qn9q71ai

but they are equivalent hashes:

$ nix-hash --to-base32 --type sha256 51858393c5698aba2e7c4e2832054a6985ab3eeda5db7373db5627d0acceefa8
1a7grsnd09snvdrp7nx5xlzap1b9982k4a2fghpbm2k9qn9q71ai

$ nix-hash --to-base16 --type sha256 1a7grsnd09snvdrp7nx5xlzap1b9982k4a2fghpbm2k9qn9q71ai
51858393c5698aba2e7c4e2832054a6985ab3eeda5db7373db5627d0acceefa8

[Ericson2314]

CC @zimbatm as you were interested in multiple different hashing schemes.

[stale]

I marked this as stale due to inactivity. → More info