internal/rosa: remove xz utils hack

This replaces all xz invocations with the native decompressor. This also significantly cleans up the Gentoo stage3 bootstrap path.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/rosa.go

@@ -26,8 +26,8 @@ const (
 	// kindEtc is the kind of [pkg.Artifact] of cureEtc.
 	kindEtc = iota + pkg.KindCustomOffset
 
-	// kindBusyboxBin is the kind of [pkg.Artifact] of busyboxBin.
-	kindBusyboxBin
+	// kindGentooOverlay is the kind of [pkg.Artifact] of gentooOverlay.
+	kindGentooOverlay
 )
 
 // mustDecode is like [pkg.MustDecode], but replaces the zero value and prints
@@ -97,15 +97,9 @@ func (s *S) earlyLDFLAGS(static bool) string {
 }
 
 const (
-	// _stageBusybox denotes a busybox installation from the busyboxBin
-	// binary distribution. This is defined as a [Stage] to make use of the
-	// toolchain abstractions to preprocess stageGentoo and is not a real,
-	// functioning toolchain. It does not contain any compilers.
-	_stageBusybox Stage = iota
-
 	// stageGentoo denotes the toolchain in a Gentoo stage3 tarball. Special
 	// care must be taken to compile correctly against this stage.
-	stageGentoo
+	stageGentoo Stage = iota
 
 	// stageIntermediateGentoo is like stageIntermediate, but compiled against
 	// stageGentoo.
@@ -239,11 +233,6 @@ func (t Toolchain) New(
 
 	var support []pkg.Artifact
 	switch t.stage {
-	case _stageBusybox:
-		name += "-early"
-		support = slices.Concat([]pkg.Artifact{t.newBusyboxBin()}, extra)
-		env = fixupEnviron(env, nil, "/system/bin")
-
 	case stageGentoo, stageEarly:
 		name += "-boot"
 		support = append(support, extra...)
@@ -251,22 +240,17 @@ func (t Toolchain) New(
 		if t.stage == stageEarly {
 			_, a := t.MustLoad(_stage0Dist)
 			support = append(support, a)
+		} else if t.gentooStage3 == nil {
+			panic(os.ErrInvalid)
 		} else {
-			support = append(support, t.S.New(_stageBusybox).New("gentoo", 0, nil, nil, nil, `
-tar -C /work -xf /usr/src/stage3.tar.xz
-rm -rf /work/dev/ /work/proc/
-ln -vs ../usr/bin /work/bin
-mkdir -vp /work/system/bin
-(cd /work/system/bin && ln -vs \
-	../../bin/sh \
-	../../usr/lib/llvm/*/bin/* \
-	.)
-`, pkg.Path(AbsUsrSrc.Append("stage3.tar.xz"), false,
-				pkg.NewHTTPGet(
-					nil, t.gentooStage3,
-					t.gentooStage3Checksum,
-				),
-			)))
+			support = append(support,
+				t.gentooStage3,
+				gentooOverlay{t.gentooStage3},
+			)
+			env = append(env,
+				"CC=clang",
+				"CXX=clang++",
+			)
 		}
 		env = fixupEnviron(env, []string{
 			EnvTriple + "=" + t.triple(),
@@ -516,20 +500,6 @@ func (t Toolchain) NewPackage(
 		sourceSuffix string
 	)
 
-	if _, ok := source.(pkg.FileArtifact); ok {
-		if attr.Writable || attr.Chmod ||
-			wantsChmod || wantsWrite ||
-			len(attr.Patches) > 0 {
-			panic("source processing requested on a xz-compressed tarball")
-		}
-
-		sourceSuffix = ".tar.xz"
-		scriptEarly += `
-tar -C /usr/src/ -xf '/usr/src/` + name + `.tar.xz'
-mv '/usr/src/` + rn + `' '/usr/src/` + name + `'
-`
-	}
-
 	dir, create := helper.wantsDir()
 	helperScriptEarly := helper.scriptEarly()
 	if attr.EnterSource ||