feat: support to FileInfo.Sys() (#19)

Co-authored-by: Liam Galvin <liamgalvin@protonmail.com>

fileinfo.go

@@ -10,6 +10,7 @@ type fileinfo struct {
 	size     int64
 	modified time.Time
 	mode     fs.FileMode
+	sys      interface{}
 }
 
 // Name is the base name of the file (without directory)
@@ -48,7 +49,7 @@ func (f fileinfo) IsDir() bool {
 	return f.Mode().IsDir()
 }
 
-// Sys is the underlying data source of the file (always nil)
+// Sys is the underlying data source of the file (can return nil)
 func (f fileinfo) Sys() interface{} {
-	return nil
+	return f.sys
 }

fs.go

@@ -158,3 +158,17 @@ func (m *FS) SetModified(name string, modified time.Time) error {
 	}
 	return &fs.PathError{Op: "set modified", Path: name, Err: fs.ErrNotExist}
 }
+
+// SetSys set underlying data source to file or directory
+func (m *FS) SetSys(name string, sys interface{}) error {
+	name = cleanse(name)
+	if f, err := m.dir.getFile(name); err == nil {
+		f.info.sys = sys
+		return nil
+	}
+	if f, err := m.dir.getDir(name); err == nil {
+		f.info.sys = sys
+		return nil
+	}
+	return &fs.PathError{Op: "set sys", Path: name, Err: fs.ErrNotExist}
+}

fs_test.go

@@ -263,6 +263,55 @@ func Test_AllOperations(t *testing.T) {
 		_, err = memfs.Stat("not_found.txt")
 		assert.Error(t, err)
 	})
+
+	t.Run("Set sys to directory", func(t *testing.T) {
+		type sysStat struct {
+			Gid int
+			Uid int
+		}
+
+		sys := sysStat{
+			Uid: 123,
+			Gid: 456,
+		}
+
+		stat, err := memfs.Stat("files/a/b/c")
+		assert.NoError(t, err)
+		assert.Nil(t, stat.Sys())
+
+		err = memfs.SetSys("files/a/b/c", sys)
+		assert.NoError(t, err)
+
+		stat, err = memfs.Stat("files/a/b/c")
+		assert.NoError(t, err)
+		assert.Equal(t, sys, stat.Sys())
+	})
+
+	t.Run("Set sys to file", func(t *testing.T) {
+		type sysStat struct {
+			Gid int
+			Uid int
+		}
+
+		sys := sysStat{
+			Uid: 123,
+			Gid: 456,
+		}
+
+		stat, err := memfs.Stat("test.txt")
+		assert.NoError(t, err)
+		assert.Nil(t, stat.Sys())
+
+		err = memfs.SetSys("test.txt", sys)
+		assert.NoError(t, err)
+
+		stat, err = memfs.Stat("test.txt")
+		assert.NoError(t, err)
+		assert.Equal(t, sys, stat.Sys())
+
+		err = memfs.SetSys("not_found.txt", sys)
+		assert.Error(t, err)
+	})
 }
 
 func Test_ConcurrentWritesToDirectory(t *testing.T) {