Stabilise `nix hash` and subcommands

[fricklerhandwerk]

This implements NixOS/rfcs#136. This issue is agreed-upon by the https://github.com/orgs/NixOS/teams/nix-team.

Only leave these two commands, collapsing the existing ones into them:

• nix hash convert

  
  nix hash convert [--hash-algo (sha256|md5|...)] --from base16|...|sri --to (base64|...) <strings>...
  

  • the current implementation makes --from optional, but we are not quite sure how that works. If it doesn't seem robust, we can make it mandatory instead.

  • --algo is optional but will error if it can't infer from SRI or guess

• nix hash path:

  nix hash path --hash-algo (sha256|md5|...) --to (base16|...|sri) --mode nar|flat|git <path>
  

  • algo is mandatory, any default would be arbitrary
  • collapse modes to nar (serialise the directory as a NAR and hash that), flat (single file, error on directory), git (directory, according to RFC 133)

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-08-28-nix-team-meeting-minutes-83/32290/1

[roberth]

This should also align with a builtin. Such a builtin has been proposed, but does not have a satisfactory design as of yet (?)

• primops: add builtins.convertHash #7708

[Ericson2314]

A comment in #4354 suggested we rename our base32 to avoid confusion. I missed that before but now support it — docs aren't good enough.

The new CLI is the perfect time to introduce the name change, since the choice of base is really CLI thing (the language and various file and wire formats do not contain the names of base to my knowledge).

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-09-25-nix-team-meeting-minutes-89/33489/1

[kolloch]

A comment in #4354 suggested we rename our base32 to avoid confusion. I missed that before but now support it — docs aren't good enough.

The new CLI is the perfect time to introduce the name change, since the choice of base is really CLI thing (the language and various file and wire formats do not contain the names of base to my knowledge).

I am not passionate either way except that we should not break existing code and continue accepting "base32" where we currently accept it, maybe with the addition of a deprecation notice.

Has this been decided on by https://github.com/orgs/NixOS/teams/nix-team?

[kolloch]

Currently, this is how the internal hash conversion support looks like:

struct Hash { // ...
     static Hash parseAny(std::string_view s, std::optional<HashType> type);
     // ...
}

In your proposal, you suggest making --type optional but --from obligatory (even though you allow the possibility of auto-detecting).

The current code does the following:

1. If the input is an SRI hash, the hash type is contained in the string and accordingly used.
2. If the input is not an SRI hash, the hash type is required and the hash format is deduced from the length of the input.

I think that this is reasonable. It disallows auto-detection of different formats that result in the same length. But even if we introduce new formats resulting in the same length (e.g. alternative base32 encodings), we could still default to the current formats for compatibility.

Therefore, I'd allow skipping --type and --from for SRI input hashes.

For non-SRI hashes, I'd make --type obligatory (as in the internal code currently) and --from optional. If --from is not given, the default format based on the length is assumed (as now).

It would be nice to get formal approval by the https://github.com/orgs/NixOS/teams/nix-team for this as well.

[kolloch]

I would also make --to optional and default to --to sri.

Some examples with my suggestions implemented for nix hash convert:

bash-5.2$ nix hash convert "sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="
sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc=
bash-5.2$ nix hash convert --to base32 "sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="
09ymwqf5i9q7d4dm7x4pjjcqqj0qrcp5lnznbh42gfsci5hcbqqm
bash-5.2$ nix hash convert --from base32 --to base32 "sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="
error: input hash 'sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc=' does not have the expected format '--from base32'
bash-5.2$ nix hash convert --to base32 "sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc="
09ymwqf5i9q7d4dm7x4pjjcqqj0qrcp5lnznbh42gfsci5hcbqqm
bash-5.2$ nix hash convert 09ymwqf5i9q7d4dm7x4pjjcqqj0qrcp5lnznbh42gfsci5hcbqqm
error: hash '09ymwqf5i9q7d4dm7x4pjjcqqj0qrcp5lnznbh42gfsci5hcbqqm' does not include a type, nor is the type otherwise known from context
bash-5.2$ nix hash convert --type sha256 09ymwqf5i9q7d4dm7x4pjjcqqj0qrcp5lnznbh42gfsci5hcbqqm
sha256-FePFYIlMuycIXPZbWi7LGEiMmZSX9FMbaQenWBzm1Sc=

Tell me what you think!