pkgs/test/top-level: add tests for package set composability

This adds some basic tests to compose package sets. The cases that are
currently broken
are commented out, they include things like:

- pkgsStatic.pkgsMusl losing the isStatic flag
- pkgsCross.ppc64-musl.pkgsMusl losing the gcc.abi setting
- pkgsCross.mingwW64.pkgsStatic losing the config string
- pkgsLLVM.pkgsMusl losing the useLLVM flag
- pkgsLLVM.pkgsStatic losing the useLLVM flag
- pkgsLLVM.pkgsi686Linux losing the useLLVM flag

And probably more.

pkgs/test/top-level/stage.nix

@@ -0,0 +1,127 @@
+# run like this:
+#  nix-build pkgs/test/top-level/stage.nix
+{
+  localSystem ? {
+    system = builtins.currentSystem;
+  },
+}:
+
+with import ../../top-level { inherit localSystem; };
+
+let
+  # To silence platform specific evaluation errors
+  discardEvaluationErrors = e: (builtins.tryEval e).success -> e;
+
+  # Basic test for idempotency of the package set, i.e:
+  # Applying the same package set twice should work and
+  # not change anything.
+  isIdempotent = set: discardEvaluationErrors (pkgs.${set}.stdenv == pkgs.${set}.${set}.stdenv);
+
+  # Some package sets should be noops in certain circumstances.
+  # This is very similar to the idempotency test, but not going
+  # via the super' overlay.
+  isNoop =
+    parent: child:
+    discardEvaluationErrors (
+      (lib.getAttrFromPath parent pkgs).stdenv == (lib.getAttrFromPath parent pkgs).${child}.stdenv
+    );
+
+  allMuslExamples = builtins.attrNames (
+    lib.filterAttrs (_: system: lib.hasSuffix "-musl" system.config) lib.systems.examples
+  );
+
+  allLLVMExamples = builtins.attrNames (
+    lib.filterAttrs (_: system: system.useLLVM or false) lib.systems.examples
+  );
+
+  # A package set should only change specific configuration, but needs
+  # to keep all other configuration from previous layers in place.
+  # Each package set has one or more key characteristics for which we
+  # test here. Those should be kept, even when applying the "set" package
+  # set.
+  isComposable =
+    set:
+    (
+      # Can't compose two different libcs...
+      builtins.elem set [ "pkgsLLVMLibc" ]
+      || discardEvaluationErrors (
+        pkgsCross.mingwW64.${set}.stdenv.hostPlatform.config == "x86_64-w64-mingw32"
+      )
+    )
+    && (
+      # Can't compose two different libcs...
+      builtins.elem set [ "pkgsLLVMLibc" ]
+      || discardEvaluationErrors (pkgsCross.mingwW64.${set}.stdenv.hostPlatform.libc == "msvcrt")
+    )
+    && discardEvaluationErrors (pkgsCross.ppc64-musl.${set}.stdenv.hostPlatform.gcc.abi == "elfv2")
+    && discardEvaluationErrors (
+      builtins.elem "trivialautovarinit" pkgs.pkgsExtraHardening.${set}.stdenv.cc.defaultHardeningFlags
+    )
+    && discardEvaluationErrors (pkgs.pkgsLLVM.${set}.stdenv.hostPlatform.useLLVM)
+    && (
+      # Can't compose two different libcs...
+      builtins.elem set [
+        "pkgsMusl"
+        "pkgsStatic"
+      ]
+      || discardEvaluationErrors (pkgs.pkgsLLVMLibc.${set}.stdenv.hostPlatform.isLLVMLibc)
+    )
+    && discardEvaluationErrors (pkgs.pkgsArocc.${set}.stdenv.hostPlatform.useArocc)
+    && discardEvaluationErrors (pkgs.pkgsZig.${set}.stdenv.hostPlatform.useZig)
+    && discardEvaluationErrors (pkgs.pkgsLinux.${set}.stdenv.buildPlatform.isLinux)
+    && (
+      # Can't compose two different libcs...
+      builtins.elem set [ "pkgsLLVMLibc" ]
+      || discardEvaluationErrors (pkgs.pkgsMusl.${set}.stdenv.hostPlatform.isMusl)
+    )
+    && discardEvaluationErrors (pkgs.pkgsStatic.${set}.stdenv.hostPlatform.isStatic)
+    && discardEvaluationErrors (pkgs.pkgsi686Linux.${set}.stdenv.hostPlatform.isx86_32)
+    && discardEvaluationErrors (pkgs.pkgsx86_64Darwin.${set}.stdenv.hostPlatform.isx86_64);
+in
+
+# Appends same defaultHardeningFlags again on each .pkgsExtraHardening - thus not idempotent.
+# assert isIdempotent "pkgsExtraHardening";
+# TODO: Remove the isDarwin condition, which currently results in infinite recursion.
+# Also see https://github.com/NixOS/nixpkgs/pull/330567#discussion_r1894653309
+assert (stdenv.hostPlatform.isDarwin || isIdempotent "pkgsLLVM");
+# TODO: This currently results in infinite recursion, even on Linux
+# assert isIdempotent "pkgsLLVMLibc";
+assert isIdempotent "pkgsArocc";
+assert isIdempotent "pkgsZig";
+assert isIdempotent "pkgsLinux";
+assert isIdempotent "pkgsMusl";
+assert isIdempotent "pkgsStatic";
+assert isIdempotent "pkgsi686Linux";
+assert isIdempotent "pkgsx86_64Darwin";
+
+# TODO: fails
+# assert isNoop [ "pkgsStatic" ] "pkgsMusl";
+# TODO: fails because of ppc64-musl
+# assert lib.all (sys: isNoop [ "pkgsCross" sys ] "pkgsMusl") allMuslExamples;
+assert lib.all (sys: isNoop [ "pkgsCross" sys ] "pkgsLLVM") allLLVMExamples;
+
+# TODO: fails
+# assert isComposable "pkgsExtraHardening";
+# assert isComposable "pkgsLLVM";
+# assert isComposable "pkgsLLVMLibc";
+# assert isComposable "pkgsArocc";
+# assert isComposable "pkgsZig";
+# TODO: attribute 'abi' missing
+# assert isComposable "pkgsMusl";
+# TODO: fails
+# assert isComposable "pkgsStatic";
+# assert isComposable "pkgsi686Linux";
+
+# Special cases regarding buildPlatform vs hostPlatform
+# TODO: fails
+# assert discardEvaluationErrors (pkgsCross.gnu64.pkgsMusl.stdenv.hostPlatform.isMusl);
+# assert discardEvaluationErrors (pkgsCross.gnu64.pkgsi686Linux.stdenv.hostPlatform.isx86_32);
+# assert discardEvaluationErrors (pkgsCross.mingwW64.pkgsLinux.stdenv.hostPlatform.isLinux);
+# assert discardEvaluationErrors (pkgsCross.aarch64-darwin.pkgsx86_64Darwin.stdenv.hostPlatform.isx86_64);
+
+# pkgsCross should keep upper cross settings
+# TODO: fails
+# assert discardEvaluationErrors (with pkgsStatic.pkgsCross.gnu64.stdenv.hostPlatform; isGnu && isStatic);
+# assert discardEvaluationErrors (with pkgsLLVM.pkgsCross.musl64.stdenv.hostPlatform; isMusl && useLLVM);
+
+emptyFile