memfs: Add deterministic modification times

memfs/memory.go

@@ -228,6 +228,7 @@ type file struct {
 	position int64
 	flag     int
 	mode     os.FileMode
+	modTime  time.Time
 
 	isClosed bool
 }
@@ -291,6 +292,7 @@ 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)
 
@@ -322,6 +324,7 @@ func (f *file) Duplicate(filename string, mode os.FileMode, flag int) billy.File
 		content: f.content,
 		mode:    mode,
 		flag:    flag,
+		modTime: f.modTime,
 	}
 
 	if isTruncate(flag) {
@@ -337,9 +340,10 @@ func (f *file) Duplicate(filename string, mode os.FileMode, flag int) billy.File
 
 func (f *file) Stat() (os.FileInfo, error) {
 	return &fileInfo{
-		name: f.Name(),
-		mode: f.mode,
-		size: f.content.Len(),
+		name:    f.Name(),
+		mode:    f.mode,
+		size:    f.content.Len(),
+		modTime: f.modTime,
 	}, nil
 }
 
@@ -354,9 +358,10 @@ func (f *file) Unlock() error {
 }
 
 type fileInfo struct {
-	name string
-	size int
-	mode os.FileMode
+	name    string
+	size    int
+	mode    os.FileMode
+	modTime time.Time
 }
 
 func (fi *fileInfo) Name() string {
@@ -371,8 +376,8 @@ func (fi *fileInfo) Mode() os.FileMode {
 	return fi.mode
 }
 
-func (*fileInfo) ModTime() time.Time {
-	return time.Now()
+func (fi *fileInfo) ModTime() time.Time {
+	return fi.modTime
 }
 
 func (fi *fileInfo) IsDir() bool {

memfs/memory_test.go

@@ -6,10 +6,12 @@ import (
 	"os"
 	"runtime"
 	"testing"
+	"time"
 
 	"github.com/go-git/go-billy/v5"
 	"github.com/go-git/go-billy/v5/util"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestRootExists(t *testing.T) {
@@ -28,6 +30,39 @@ func TestCapabilities(t *testing.T) {
 	assert.Equal(t, billy.DefaultCapabilities&^billy.LockCapability, caps)
 }
 
+func TestModTime(t *testing.T) {
+	fs := New()
+	_, err := fs.Create("/file1")
+	require.NoError(t, err)
+
+	if runtime.GOOS == "windows" {
+		time.Sleep(20 * time.Millisecond)
+	}
+
+	_, err = fs.Create("/file2")
+	require.NoError(t, err)
+
+	fi1a, err := fs.Stat("/file1")
+	require.NoError(t, err)
+
+	fi2, err := fs.Stat("/file2")
+	require.NoError(t, err)
+
+	fi1b, err := fs.Stat("/file1")
+	require.NoError(t, err)
+
+	modtime := fi1a.ModTime()
+
+	// file 1 and file 2 should have different mod times.
+	assert.NotEqual(t, modtime, fi2.ModTime())
+
+	// a new file info for the same unmodified file, should still match mod time.
+	assert.Equal(t, modtime, fi1b.ModTime())
+
+	// new calls to ModTime() retain existing mod time.
+	assert.Equal(t, modtime, fi1a.ModTime())
+}
+
 func TestNegativeOffsets(t *testing.T) {
 	fs := New()
 	f, err := fs.Create("negative")

memfs/storage.go

@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+	"time"
 )
 
 type storage struct {
@@ -46,6 +47,7 @@ func (s *storage) New(path string, mode os.FileMode, flag int) (*file, error) {
 		content: &content{name: name},
 		mode:    mode,
 		flag:    flag,
+		modTime: time.Now(),
 	}
 
 	s.files[path] = f