fix(backup): privately stage restore archive

tools/tht/internal/backup/preflight_unix_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
+	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
 )
 
 func TestStageArchiveRejectsSymlinkedInstallationAncestor(t *testing.T) {
@@ -46,3 +47,35 @@ func TestStageArchiveRejectsSymlinkedInstallationAncestor(t *testing.T) {
 		t.Fatalf("StageArchive() error = %v, want unsafe symlinked staging-root rejection", err)
 	}
 }
+
+func TestStageArchiveCreatesUnixPrivateRegularFile(t *testing.T) {
+	installation := preflightTestInstallation(t)
+	if err := os.MkdirAll(installation.ControlDirectory(), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	archive := filepath.Join(t.TempDir(), "valid.zip")
+	writePreflightArchive(t, archive, preflightArchiveSpec{
+		entries: []preflightArchiveEntry{{path: "configuration/operator.env", body: []byte("safe")}},
+	})
+	result, err := Preflight(context.Background(), installation, PreflightRequest{Archive: archive, Confirm: true}, permissivePreflightDependencies())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer result.CloseArchive()
+
+	staged, err := result.StageArchive(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer staged.Close()
+	if err := safeio.ValidatePrivateRegular(staged.path); err != nil {
+		t.Fatalf("staged archive privacy = %v, want owner-private regular file", err)
+	}
+	info, err := os.Lstat(staged.path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := info.Mode().Perm(); got != 0o600 {
+		t.Fatalf("staged archive mode = %#o, want 0600", got)
+	}
+}