xar: fix build on GCC14

[amarshall]

Things done

Verified no-change on aarch64-darwin.

• 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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[toonn]

LGTM

Looks like you'll have to format the file to get the check to pass though.

[amarshall]

Reformatted and amended.

[emilazy]

We already patch the source quite extensively – might it be possible to fix these problems at the root instead? (Upstreaming is hopeless, of course, due to Apple, but we’re already managing a substantial patch set.)

[Sciencentistguy]

The errors are all due to discarding the return code of asprintf. If we want to handle these in the code, we'd need to decide what we want to do in the case asprintf fails. I assume we'd just exit(1) as asprintf can only really fail if memory allocation fails?