internal/rosa: mirror service via external cache

This provides an authenticated implementation of the external cache.

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

cmd/mbf/cache.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"net/http"
 	"os"
 	"path/filepath"
 	"testing"
@@ -9,6 +10,7 @@ import (
 	"hakurei.app/check"
 	"hakurei.app/container"
 	"hakurei.app/internal/pkg"
+	"hakurei.app/internal/rosa"
 	"hakurei.app/message"
 )
 
@@ -30,7 +32,7 @@ type cache struct {
 	// Loaded artifact of [rosa.QEMU].
 	qemu pkg.Artifact
 
-	base string
+	base, mirror string
 }
 
 // open opens the underlying [pkg.Cache].
@@ -86,6 +88,21 @@ func (cache *cache) open() (err error) {
 	}
 	done <- struct{}{}
 
+	if cache.mirror != "" {
+		var pub []byte
+		pub, err = os.ReadFile(base.Append("ed25519.pub").String())
+		if err != nil {
+			cache.c.Close()
+			return
+		}
+		var r rosa.Remote
+		if r, err = rosa.NewRemote(cache.mirror, pub, http.DefaultClient); err != nil {
+			cache.c.Close()
+			return err
+		}
+		cache.c.SetExternal(r)
+	}
+
 	if cache.qemu != nil {
 		var pathname *check.Absolute
 		pathname, _, err = cache.c.Cure(cache.qemu)