Stabilise `nix nar` and subcommands

[fricklerhandwerk]

This implements RFC 136. This issue is agreed-upon by the @nixos/nix-team

• Rename nix nar dump-path to nix nar pack (reserving unpack for the reverse operation)
• Possible extensions for follow-ups:
  • Make sure the command uses all global flags (currently it doesn't)
  • add unpack to create a local directory
  • Read files to cat and ls from pipe, take arbitrary many on the command line (might get tricky due to sequetiality of reads though)
  • Add direct conversion from and to tar

[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

[vicky1999]

Hello, I want to work on this task. can I start working on it?

[fricklerhandwerk]

Thanks @vicky1999, this is very much appreciated!

[vicky1999]

@fricklerhandwerk one doubt regarding dump-path command rename task. I saw that we have dump-path subcommand under store as well. should I rename that also to pack?

[fricklerhandwerk]

We haven't looked at that one, but from what I can tell they're supposed to do the same. We may want to make an alias. Will check back with the team on Friday.

[fricklerhandwerk]

We will reserve nix store import/export for serialising store objects (i.e. with references) while nix nar is for plain directory trees. So let's continue as described in the issue.

Discussed in the Nix team meeting

• @thufschmitt: nix store operates on the store directly, while nix nar can take any path
  • the problem is that nix store takes installables and will expect flake references if you give it a path
• @fricklerhandwerk: should call it import/export for the nix store to dump nar+narinfo (=store objects)
  • then nix nar is a strict subset of that that only deals with file system objects, i.e. without references
• decision: keep nix nar pack, later deal with nix store imprort/export