nixos-rebuild-ng: re-exec in the current version if re-exec to a new one fails

[thiagokokada]

This closes the last gap for issue #166499 (comment).

The idea here is that if we try to re-exec and it fails (for example, in case we are building for a CPU arch that our current system doesn't support), Python raise a OSError like below where I am trying to load nix from aarch64-darwin in x86_64-linux:

$ file /nix/store/c4kw7qpffsq7vd6brwgkfnglvj85ivqk-nix-2.24.11/bin/nix
/nix/store/c4kw7qpffsq7vd6brwgkfnglvj85ivqk-nix-2.24.11/bin/nix: Mach-O 64-bit arm64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|WEAK_DEFINES|BINDS_TO_WEAK|PIE>

$ python
Python 3.12.7 (main, Oct  1 2024, 02:05:46) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.execv("/nix/store/c4kw7qpffsq7vd6brwgkfnglvj85ivqk-nix-2.24.11/bin/nix", ["nix"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 8] Exec format error

So we capture this (or any other exception really) and re-exec to the current version if the new one fails.

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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[thiagokokada]

CC @r-vdp @Scrumplex.

[nixos-discourse]

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

https://discourse.nixos.org/t/prs-ready-for-review/3032/4977

[thiagokokada]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 366271

---

x86_64-linux

✅ 2 packages built:

• nixos-rebuild-ng
• nixos-rebuild-ng.dist

[thiagokokada]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 366271

---

aarch64-darwin

✅ 2 packages built:

• nixos-rebuild-ng
• nixos-rebuild-ng.dist

[thiagokokada]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 366271

---

x86_64-darwin

✅ 2 packages built:

• nixos-rebuild-ng
• nixos-rebuild-ng.dist

[r-vdp]

Diff LGTM!