internal/rosa: populate mirror snapshot

This produces the same structure served by the dynamic server.

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

cmd/mbf/main.go

@@ -568,6 +568,47 @@ func main() {
 		},
 	)
 
+	c.NewCommand(
+		"snapshot",
+		"Populate an empty directory with a static mirror snapshot",
+		func(args []string) error {
+			if len(args) != 1 {
+				return errors.New("snapshot requires 1 argument")
+			}
+
+			var key ed25519.PrivateKey
+			if p, err := os.ReadFile(filepath.Join(cm.base, "ed25519")); err != nil {
+				return err
+			} else if len(p) != ed25519.PrivateKeySize {
+				return errors.New("invalid private key")
+			} else {
+				key = p
+			}
+
+			var m rosa.Mirror
+			if base, err := os.OpenRoot(cm.base); err != nil {
+				return err
+			} else {
+				m = rosa.NewMirror(msg, base.FS(), key)
+			}
+
+		retry:
+			root, err := os.OpenRoot(args[0])
+			if err != nil {
+				if os.IsNotExist(err) {
+					if err = os.MkdirAll(args[0], 0755); err != nil {
+						return err
+					}
+					goto retry
+				}
+				return err
+			}
+
+			go func() { <-ctx.Done(); _ = root.Close() }()
+			return m.Populate(root)
+		},
+	)
+
 	{
 		var (
 			flagGentoo   string