stdenv: assertion '(final).hasSharedLibraries' failed

[vkryachko]

Describe the bug

Nix fails to build nix on nixpkgs master.

Steps To Reproduce

Steps to reproduce the behavior:

1. on unstable channel
2. nix-channel --update
3. This works: nix-build -E 'with import <nixpkgs> {}; nix'
4. This fails: nix-build -E 'with import <nixpkgs> {}; let unused = {whatever.extensions =[];}; in nix' --show-trace
5. Assertion failure:

error: assertion '(final).hasSharedLibraries' failed

       at /nix/store/v63vjk8lz2fsj52qkfgbbw1mbgplcdmc-nixpkgs/nixpkgs/lib/systems/default.nix:89:25:

           88|       extensions = rec {
           89|         sharedLibrary = assert final.hasSharedLibraries;
             |                         ^
           90|           /**/ if final.isDarwin  then ".dylib"

Expected behavior

Both invocations should succeed as they are isomorphic, i.e. 4. has an unused let binding that causes nix to fail.

Additional context

Looks like this PR may have introduced the assert that causes the problem #238154 , commit

After some investigation a colleague pointed out that the issue may have to do with the instability of attrset comparison due to laziness in nix, comparison happens here

nixpkgs/pkgs/stdenv/booter.nix

Lines 105 to 108 in 5136088

	pkgsBuildTarget =
	if args.stdenv.targetPlatform == args.stdenv.hostPlatform
	then pkgsBuildHost
	else assert args.stdenv.hostPlatform == args.stdenv.buildPlatform; thisStage;

and looks like invocation in 4. somehow cause a different evaluation order of attrset attribute comparison:

• if isStatic is compared first we don't see the assert
• If extensions compares first we stumble on the assert and fail.

nix-repl> pkgs.stdenv.hostPlatform.extensions
{ executable = ""; library = ".so"; sharedLibrary = ".so"; staticLibrary = ".a"; }

nix-repl> pkgsStatic.stdenv.hostPlatform.extensions
{ executable = ""; library = ".a"; sharedLibrary = «error: error: assertion '(final).hasSharedLibraries' failed

       at /nix/store/v63vjk8lz2fsj52qkfgbbw1mbgplcdmc-nixpkgs/nixpkgs/lib/systems/default.nix:89:25:

           88|       extensions = rec {
           89|         sharedLibrary = assert final.hasSharedLibraries;
             |                         ^
           90|           /**/ if final.isDarwin  then ".dylib"»; staticLibrary = ".a"; }

Notify maintainers

@amjoseph-nixpkgs who added the assert that fails
@edolstra @lovesegfault @Artturin nix package maintainers

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.1.25-1rodete1-amd64`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.16.1`
 - channels(root): `"nixpkgs"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixpkgs`

[trofi]

With something like NixOS/nix#8711 we can deterministically crash the eval:

$ NIX_VALIDATE_EVAL_NONDETERMINISM=1 nix-instantiate -E 'with import <nixpkgs> {}; nix'
error:
...
       error: assertion '(final).hasSharedLibraries' failed

[Artturin]

Same issue in NixOS/nix#8701

[nixos-discourse]

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

https://discourse.nixos.org/t/nix-language-evaluation-bug/30582/2

[ghost]

This was my fault.

The fix is: #244118

Unfortunately we can't provide useful error messages when people use sub-attributes of {host,target,build}Platform, because it is expected that you can compare two *Platform attrsets using ==, which will force the throw "error message".

We should probably move anything fallible (like extensions.sharedLibrary) out of these *Platform attrsets, so we can provide useful error messages instead of just returning null.