util: fix TempDir and TempFile on non-root filesystems

util/util.go

@@ -146,9 +146,8 @@ func nextSuffix() string {
 // to remove the file when no longer needed.
 func TempFile(fs billy.Basic, dir, prefix string) (f billy.File, err error) {
 	// This implementation is based on stdlib ioutil.TempFile.
-
 	if dir == "" {
-		dir = os.TempDir()
+		dir = getTempDir(fs)
 	}
 
 	nconflict := 0
@@ -179,7 +178,7 @@ func TempDir(fs billy.Dir, dir, prefix string) (name string, err error) {
 	// This implementation is based on stdlib ioutil.TempDir
 
 	if dir == "" {
-		dir = os.TempDir()
+		dir = getTempDir(fs.(billy.Basic))
 	}
 
 	nconflict := 0
@@ -207,6 +206,15 @@ func TempDir(fs billy.Dir, dir, prefix string) (name string, err error) {
 	return
 }
 
+func getTempDir(fs billy.Basic) string {
+	ch, ok := fs.(billy.Chroot)
+	if !ok || ch.Root() == "" || ch.Root() == "/" || ch.Root() == string(filepath.Separator) {
+		return os.TempDir()
+	}
+
+	return ".tmp"
+}
+
 type underlying interface {
 	Underlying() billy.Basic
 }

util/util_test.go

@@ -25,7 +25,7 @@ func TestTempFile(t *testing.T) {
 	}
 }
 
-func TestTempDir(t *testing.T) {
+func TestTempDir_WithDir(t *testing.T) {
 	fs := memfs.New()
 
 	dir := os.TempDir()
@@ -62,3 +62,30 @@ func TestReadFile(t *testing.T) {
 	}
 
 }
+
+func TestTempDir(t *testing.T) {
+	fs := memfs.New()
+	f, err := util.TempDir(fs, "", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = filepath.Rel(os.TempDir(), f)
+	if err != nil {
+		t.Errorf(`TempDir(fs, "", "") = %s, should be relative to os.TempDir if root filesystem`, f)
+	}
+}
+
+func TestTempDir_WithNonRoot(t *testing.T) {
+	fs := memfs.New()
+	fs, _ = fs.Chroot("foo")
+	f, err := util.TempDir(fs, "", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = filepath.Rel(os.TempDir(), f)
+	if err == nil {
+		t.Errorf(`TempDir(fs, "", "") = %s, should not be relative to os.TempDir on not root filesystem`, f)
+	}
+}