cmd/mbf: optional emulated target architecture

This enables transparent cross-compilation without breaking purity.

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

cmd/mbf/cache.go

@@ -4,10 +4,13 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"hakurei.app/check"
+	"hakurei.app/container"
 	"hakurei.app/internal/pkg"
+	"hakurei.app/internal/rosa"
 	"hakurei.app/message"
 )
 
@@ -22,6 +25,7 @@ type cache struct {
 	cures, jobs        int
 	hostAbstract, idle bool
 	verboseInit        bool
+	arch               string
 
 	base string
 }
@@ -74,6 +78,50 @@ func (cache *cache) open() (err error) {
 		cache.jobs,
 		base,
 	)
+	if err != nil {
+		return
+	}
+	done <- struct{}{}
+
+	if cache.arch != "" && cache.arch != runtime.GOARCH {
+		var (
+			name   string
+			offset byte
+			magic  string
+			mask   string
+		)
+		switch cache.arch {
+		case "riscv64":
+			name = "riscv64"
+			offset = 0
+			magic = "\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xf3\x00"
+			mask = "\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff"
+
+		default:
+			cache.c.Close()
+			err = pkg.UnsupportedArchError(cache.arch)
+			return
+		}
+
+		var pathname *check.Absolute
+		pathname, _, err = cache.c.Cure(rosa.Std.Load(rosa.QEMU))
+		if err != nil {
+			cache.c.Close()
+			return
+		}
+
+		pkg.RegisterArch(cache.arch, container.BinfmtEntry{
+			Offset: offset,
+			Magic:  magic,
+			Mask:   mask,
+			Interpreter: pathname.Append(
+				"system/bin",
+				"qemu-"+name,
+			),
+		})
+		rosa.DropCaches(cache.arch, rosa.Flags())
+	}
+
 	return
 }