glibc: fix darwin -> linux cross compilation

[Gaelan]

Motivation for this change

darwin -> linux cross-compilation of glibc was broken for two reasons. This fixes both of them:

• A recent glibc update removed some code we were patching. I fixed this by removing the relevant part of the patch.
• When NIX_COREFOUNDATION_RPATH is set, we create a ld.so binary with an RPATH of [], but ld.so expects not to have an RPATH set at all. I fix this by unsetting the variable in the glibc derivation, as suggested by @parthy, but I'm not sure this is the best solution (in particular, I'm not convinced we should be setting NIX_COREFOUNDATION_RPATH at all when host != darwin). However, this does get cross-compilation working for now.

Tested by doing nix-build . --arg crossSystem '(import ./. {}).lib.systems.examples.gnu64' -A hello on Darwin, copying the resulting closure onto a Linux box, and running hello.

This should only change code on the cross-compilation path, so it shouldn't cause any mass rebuilds.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[LnL7]

This is going to cause problems in multiple places, we should not set this at all if the target platform is linux. This might also be wrong for darwin->darwin cross.

[Gaelan]

Yeah, that's fair. It looks like it's getting set here:

nixpkgs/pkgs/stdenv/darwin/default.nix

Lines 464 to 467 in dbedb52

	preHook = commonPreHook + ''
	export NIX_COREFOUNDATION_RPATH=${pkgs.darwin.CF}/Library/Frameworks
	export PATH_LOCALE=${pkgs.darwin.locale}/share/locale
	'';

But that gets deep enough into the weeds of stdenv and cross infrastructure that I'm not sure how best to fix it. If possible, it would probably be better to do the target check in nix (as opposed to in bash) so we don't cause rebuilds for non-cross packages.

[LnL7]

The way that's currently added isn't great and was also done before most of the cross work. Most things, including the counterpart that overrides this when frameworks are included, use addEnvHooks which I think would improve the situation.

Ideally this would also just be another setup-hook, just for the pure CF. The reason I added it here was to ensure that the hooks from frameworks run last. There's probably a better way to do that.

For testing it might be possible to use an if (stdenv.hostPlatform != stdenv.buildPlatform) then addEnvHook else export in the preHook but that might be too late. Either way the proper thing should probably go to staging.

[siraben]

I was able to cross-compile from darwin to gnu64 using this PR.

$ helloCross="$(nix-build . --arg crossSystem '(import ./. {}).lib.systems.examples.gnu64' -A hello)"
$ nix-copy-closure --to nix-linuxkit "$helloCross"
$ ssh nix-linuxkit "$helloCross/bin/hello"
Hello, world!

[stale]

I marked this as stale due to inactivity. → More info

[siraben]

Still important.

[SuperSandro2000]

Still important.

and still stale. Maybe just open a new PR if you want it to be shipped.

[stale]

I marked this as stale due to inactivity. → More info

[SuperSandro2000]

Closing because author did not react in almost 2 years.

[tartavull]

This seems important to better support Darwin. How can someone help?