internal/rosa: busybox binary artifact

This installs a statically linked busybox binary distribution for decompressing the gentoo stage3 tarball, since there is no native xz implementation.

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

internal/rosa/rosa.go

@@ -2,10 +2,49 @@
 package rosa
 
 import (
+	"log"
+	"runtime"
+
+	"hakurei.app/container/fhs"
 	"hakurei.app/internal/pkg"
 )
 
 const (
 	// kindEtc is the kind of [pkg.Artifact] of cureEtc.
 	kindEtc = iota + pkg.KindCustomOffset
+
+	// kindBusyboxBin is the kind of [pkg.Artifact] of busyboxBin.
+	kindBusyboxBin
 )
+
+// mustDecode is like [pkg.MustDecode], but replaces the zero value and prints
+// a warning.
+func mustDecode(s string) pkg.Checksum {
+	var fallback = pkg.Checksum{}
+	if s == "" {
+		log.Println(
+			"falling back to",
+			pkg.Encode(fallback),
+			"for unpopulated checksum",
+		)
+		return fallback
+	}
+	return pkg.MustDecode(s)
+}
+
+var (
+	// AbsSystem is the Rosa OS installation prefix.
+	AbsSystem = fhs.AbsRoot.Append("system")
+)
+
+// linuxArch returns the architecture name used by linux corresponding to
+// [runtime.GOARCH].
+func linuxArch() string {
+	switch runtime.GOARCH {
+	case "amd64":
+		return "x86_64"
+
+	default:
+		panic("unsupported target " + runtime.GOARCH)
+	}
+}