fix: modtimes should be part of the content, not of the file

Otherwise f.Duplicate() copies the modTime. Subsequent modtime mutations are
then on this copy instead of the original file's modtime. Effectively any
mutation to a modtime can never be seen by other callers of f.Stat()

Author: hansbogert

memfs/file.go

@@ -17,7 +17,6 @@ type file struct {
 	position int64
 	flag     int
 	mode     os.FileMode
-	modTime  time.Time
 
 	isClosed bool
 }
@@ -81,7 +80,6 @@ func (f *file) WriteAt(p []byte, off int64) (int, error) {
 		return 0, errors.New("write not supported")
 	}
 
-	f.modTime = time.Now()
 	n, err := f.content.WriteAt(p, off)
 	f.position = off + int64(n)
 
@@ -113,7 +111,6 @@ func (f *file) Duplicate(filename string, mode fs.FileMode, flag int) billy.File
 		content: f.content,
 		mode:    mode,
 		flag:    flag,
-		modTime: f.modTime,
 	}
 
 	if isTruncate(flag) {
@@ -132,7 +129,7 @@ func (f *file) Stat() (os.FileInfo, error) {
 		name:    f.Name(),
 		mode:    f.mode,
 		size:    f.content.Len(),
-		modTime: f.modTime,
+		modTime: f.content.modTime,
 	}, nil
 }
 
@@ -178,8 +175,9 @@ func (*fileInfo) Sys() interface{} {
 }
 
 type content struct {
-	name  string
-	bytes []byte
+	name    string
+	bytes   []byte
+	modTime time.Time
 
 	m sync.RWMutex
 }
@@ -205,6 +203,7 @@ func (c *content) WriteAt(p []byte, off int64) (int, error) {
 	if len(c.bytes) < prev {
 		c.bytes = c.bytes[:prev]
 	}
+	c.modTime = time.Now()
 	c.m.Unlock()
 
 	return len(p), nil

memfs/memory_test.go

@@ -66,9 +66,11 @@ func TestModTime(t *testing.T) {
 	// new calls to ModTime() retain existing mod time.
 	assert.Equal(t, modtime, fi1a.ModTime())
 
-	// Writing to to file should change the mod time
-	util.WriteFile(fs, "/file1", []byte{0}, 0o666)
+	// Writing to file should change the mod time
+	err = util.WriteFile(fs, "/file1", []byte{0}, 0o666)
+	require.NoError(t, err)
 	fi1c, err := fs.Stat("/file1")
+	require.NoError(t, err)
 	assert.NotEqual(t, modtime, fi1c.ModTime())
 }
 

memfs/storage.go

@@ -44,11 +44,13 @@ func (s *storage) New(path string, mode fs.FileMode, flag int) (*file, error) {
 
 	name := filepath.Base(path)
 	f := &file{
-		name:    name,
-		content: &content{name: name},
-		mode:    mode,
-		flag:    flag,
-		modTime: time.Now(),
+		name: name,
+		content: &content{
+			name:    name,
+			modTime: time.Now(),
+		},
+		mode: mode,
+		flag: flag,
 	}
 
 	s.mf.Lock()