os: reduce allocations in Readdir on unix

Include syscall.Stat_t on unix to the
unexported fileStat structure rather than
accessing it though an interface.

Additionally add a benchmark for Readdir
(and Readdirnames).

Tested on linux, freebsd, netbsd, openbsd
darwin, solaris, does not touch windows
stuff. Does not change the API, as
discussed on golang-dev.

E.g. on linux/amd64 with a directory of 65 files:

benchmark              old ns/op     new ns/op     delta
BenchmarkReaddir-4     67774         66225         -2.29%

benchmark              old allocs     new allocs     delta
BenchmarkReaddir-4     334            269            -19.46%

benchmark              old bytes     new bytes     delta
BenchmarkReaddir-4     25208         24168         -4.13%

Change-Id: I44ef72a04ad7055523a980f29aa11122040ae8fe
Reviewed-on: https://go-review.googlesource.com/16423
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

12 files changed, 158 insertions, 166 deletions

diff --git a/src/os/file_unix.go b/src/os/file_unix.go
index 68d0a6e64c..8261b90b49 100644
--- a/src/os/file_unix.go
+++ b/src/os/file_unix.go
@@ -12,6 +12,10 @@ import (
 	"syscall"
 )
 
+func sameFile(fs1, fs2 *fileStat) bool {
+	return fs1.sys.Dev == fs2.sys.Dev && fs1.sys.Ino == fs2.sys.Ino
+}
+
 func rename(oldname, newname string) error {
 	e := syscall.Rename(oldname, newname)
 	if e != nil {
@@ -152,23 +156,25 @@ func (f *File) Stat() (FileInfo, error) {
 	if f == nil {
 		return nil, ErrInvalid
 	}
-	var stat syscall.Stat_t
-	err := syscall.Fstat(f.fd, &stat)
+	var fs fileStat
+	err := syscall.Fstat(f.fd, &fs.sys)
 	if err != nil {
 		return nil, &PathError{"stat", f.name, err}
 	}
-	return fileInfoFromStat(&stat, f.name), nil
+	fillFileStatFromSys(&fs, f.name)
+	return &fs, nil
 }
 
 // Stat returns a FileInfo describing the named file.
 // If there is an error, it will be of type *PathError.
 func Stat(name string) (FileInfo, error) {
-	var stat syscall.Stat_t
-	err := syscall.Stat(name, &stat)
+	var fs fileStat
+	err := syscall.Stat(name, &fs.sys)
 	if err != nil {
 		return nil, &PathError{"stat", name, err}
 	}
-	return fileInfoFromStat(&stat, name), nil
+	fillFileStatFromSys(&fs, name)
+	return &fs, nil
 }
 
 // Lstat returns a FileInfo describing the named file.
@@ -176,12 +182,13 @@ func Stat(name string) (FileInfo, error) {
 // describes the symbolic link.  Lstat makes no attempt to follow the link.
 // If there is an error, it will be of type *PathError.
 func Lstat(name string) (FileInfo, error) {
-	var stat syscall.Stat_t
-	err := syscall.Lstat(name, &stat)
+	var fs fileStat
+	err := syscall.Lstat(name, &fs.sys)
 	if err != nil {
 		return nil, &PathError{"lstat", name, err}
 	}
-	return fileInfoFromStat(&stat, name), nil
+	fillFileStatFromSys(&fs, name)
+	return &fs, nil
 }
 
 func (f *File) readdir(n int) (fi []FileInfo, err error) {
diff --git a/src/os/os_test.go b/src/os/os_test.go
index be9fa91028..ef06ba28d1 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -296,6 +296,48 @@ func TestReaddir(t *testing.T) {
 	testReaddir(sysdir.name, sysdir.files, t)
 }
 
+func benchmarkReaddirname(path string, b *testing.B) {
+	var nentries int
+	for i := 0; i < b.N; i++ {
+		f, err := Open(path)
+		if err != nil {
+			b.Fatalf("open %q failed: %v", path, err)
+		}
+		ns, err := f.Readdirnames(-1)
+		f.Close()
+		if err != nil {
+			b.Fatalf("readdirnames %q failed: %v", path, err)
+		}
+		nentries = len(ns)
+	}
+	b.Logf("benchmarkReaddirname %q: %d entries", path, nentries)
+}
+
+func benchmarkReaddir(path string, b *testing.B) {
+	var nentries int
+	for i := 0; i < b.N; i++ {
+		f, err := Open(path)
+		if err != nil {
+			b.Fatalf("open %q failed: %v", path, err)
+		}
+		fs, err := f.Readdir(-1)
+		f.Close()
+		if err != nil {
+			b.Fatalf("readdir %q failed: %v", path, err)
+		}
+		nentries = len(fs)
+	}
+	b.Logf("benchmarkReaddir %q: %d entries", path, nentries)
+}
+
+func BenchmarkReaddirname(b *testing.B) {
+	benchmarkReaddirname(".", b)
+}
+
+func BenchmarkReaddir(b *testing.B) {
+	benchmarkReaddir(".", b)
+}
+
 // Read the directory one entry at a time.
 func smallReaddirnames(file *File, length int, t *testing.T) []string {
 	names := make([]string, length)
diff --git a/src/os/stat_darwin.go b/src/os/stat_darwin.go
index 0eea522015..9dc7a99fb7 100644
--- a/src/os/stat_darwin.go
+++ b/src/os/stat_darwin.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtimespec),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtimespec)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK, syscall.S_IFWHT:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/stat_dragonfly.go b/src/os/stat_dragonfly.go
index 605c1d9b64..69e63230eb 100644
--- a/src/os/stat_dragonfly.go
+++ b/src/os/stat_dragonfly.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtim),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtim)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/stat_freebsd.go b/src/os/stat_freebsd.go
index 2ffb60fe25..e9d38aa722 100644
--- a/src/os/stat_freebsd.go
+++ b/src/os/stat_freebsd.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtimespec),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtimespec)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/stat_linux.go b/src/os/stat_linux.go
index 605c1d9b64..69e63230eb 100644
--- a/src/os/stat_linux.go
+++ b/src/os/stat_linux.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtim),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtim)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/stat_nacl.go b/src/os/stat_nacl.go
index a503b59fa3..d3bed14e43 100644
--- a/src/os/stat_nacl.go
+++ b/src/os/stat_nacl.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtime, st.MtimeNsec),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtime, fs.sys.MtimeNsec)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(sec, nsec int64) time.Time {
diff --git a/src/os/stat_netbsd.go b/src/os/stat_netbsd.go
index 2ffb60fe25..e9d38aa722 100644
--- a/src/os/stat_netbsd.go
+++ b/src/os/stat_netbsd.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtimespec),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtimespec)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/stat_openbsd.go b/src/os/stat_openbsd.go
index 605c1d9b64..69e63230eb 100644
--- a/src/os/stat_openbsd.go
+++ b/src/os/stat_openbsd.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtim),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtim)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/stat_solaris.go b/src/os/stat_solaris.go
index 605c1d9b64..69e63230eb 100644
--- a/src/os/stat_solaris.go
+++ b/src/os/stat_solaris.go
@@ -9,21 +9,12 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *fileStat) bool {
-	stat1 := fs1.sys.(*syscall.Stat_t)
-	stat2 := fs2.sys.(*syscall.Stat_t)
-	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
-}
-
-func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &fileStat{
-		name:    basename(name),
-		size:    int64(st.Size),
-		modTime: timespecToTime(st.Mtim),
-		sys:     st,
-	}
-	fs.mode = FileMode(st.Mode & 0777)
-	switch st.Mode & syscall.S_IFMT {
+func fillFileStatFromSys(fs *fileStat, name string) {
+	fs.name = basename(name)
+	fs.size = int64(fs.sys.Size)
+	fs.modTime = timespecToTime(fs.sys.Mtim)
+	fs.mode = FileMode(fs.sys.Mode & 0777)
+	switch fs.sys.Mode & syscall.S_IFMT {
 	case syscall.S_IFBLK:
 		fs.mode |= ModeDevice
 	case syscall.S_IFCHR:
@@ -39,16 +30,15 @@ func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
 	case syscall.S_IFSOCK:
 		fs.mode |= ModeSocket
 	}
-	if st.Mode&syscall.S_ISGID != 0 {
+	if fs.sys.Mode&syscall.S_ISGID != 0 {
 		fs.mode |= ModeSetgid
 	}
-	if st.Mode&syscall.S_ISUID != 0 {
+	if fs.sys.Mode&syscall.S_ISUID != 0 {
 		fs.mode |= ModeSetuid
 	}
-	if st.Mode&syscall.S_ISVTX != 0 {
+	if fs.sys.Mode&syscall.S_ISVTX != 0 {
 		fs.mode |= ModeSticky
 	}
-	return fs
 }
 
 func timespecToTime(ts syscall.Timespec) time.Time {
diff --git a/src/os/types_notwin.go b/src/os/types_plan9.go
index ea1a073930..6d46ca9dd3 100644
--- a/src/os/types_notwin.go
+++ b/src/os/types_plan9.go
@@ -2,13 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !windows
-
 package os
 
-import (
-	"time"
-)
+import "time"
 
 // A fileStat is the implementation of FileInfo returned by Stat and Lstat.
 type fileStat struct {
diff --git a/src/os/types_unix.go b/src/os/types_unix.go
new file mode 100644
index 0000000000..056220c09b
--- /dev/null
+++ b/src/os/types_unix.go
@@ -0,0 +1,27 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows
+// +build !plan9
+
+package os
+
+import (
+	"syscall"
+	"time"
+)
+
+// A fileStat is the implementation of FileInfo returned by Stat and Lstat.
+type fileStat struct {
+	name    string
+	size    int64
+	mode    FileMode
+	modTime time.Time
+	sys     syscall.Stat_t
+}
+
+func (fs *fileStat) Size() int64        { return fs.size }
+func (fs *fileStat) Mode() FileMode     { return fs.mode }
+func (fs *fileStat) ModTime() time.Time { return fs.modTime }
+func (fs *fileStat) Sys() interface{}   { return &fs.sys }