initrd: implement panic-on-fail interpreterless

[britter]

In order to have initrd not depend on an interpreter like bash being present.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[ElvishJerricco]

In order to have initrd not depend on an interpreter like bash being present.

For the record, there are other script-based services in systemd initrd. But yea this is a step in that direction :)

I think the dd output summary will be a bit of log spam. It'll probably output something like this to the journal.

0+1 records in
0+1 records out
1 byte copied, 2.017e-05 s, 49.6 kB/s

We could instead do something like this maybe:

        serviceConfig = {
          Type = "oneshot";
          ExecStart = "${pkgs.coreutils}/bin/echo c";
          StandardOutput = "file:/proc/sysrq-trigger";
        };

Haven't tested it, but I assume it would work.

EDIT: I just realized how silly it is to worry about log spam in a service that crashes the kernel...

[britter]

For the record, there are other script-based services in systemd initrd.

Yes that's true. This one was the easiest to fix because it does not require writing a new program. I think @WilliButz is working on replacing the other script uses with a Rust program.

[britter]

@ElvishJerricco I think you're still raising a good point. Also your echo solution is more straight forward. I'm going add this change.

[WilliButz]

Thank you!