loopback: fix Utimens on symlinks

The problem was that os.Chtimes() operates on the target of the
symlink and not on the symlink itself.
When tar extracts an archive containing a symlink, it actually wants
to set the times on the symlink. This can also be tested using
"touch -h".

This patch adds a wrapper for the Linux utimensat(2) syscall
that allows passing flags along with a unit test.

It uses the flag AT_SYMLINK_NOFOLLOW to implement
loopbackFileSystem.Utimens() properly.

It also add handling for UTIME_NOW and UTIME_OMIT that was already
present in loopbackFile.Utimens() but missing in loopbackFileSystem.

Fixes issue 81 ( https://github.com/hanwen/go-fuse/issues/81 ).

fuse/nodefs/files.go

@@ -204,6 +204,7 @@ func (f *loopbackFile) GetAttr(a *fuse.Attr) fuse.Status {
 const _UTIME_NOW = ((1 << 30) - 1)
 const _UTIME_OMIT = ((1 << 30) - 2)
 
+// Utimens - file handle based version of loopbackFileSystem.Utimens()
 func (f *loopbackFile) Utimens(a *time.Time, m *time.Time) fuse.Status {
 	var ts [2]syscall.Timespec
 

fuse/pathfs/loopback.go

@@ -118,16 +118,27 @@ func (fs *loopbackFileSystem) Truncate(path string, offset uint64, context *fuse
 	return fuse.ToStatus(os.Truncate(fs.GetPath(path), int64(offset)))
 }
 
-func (fs *loopbackFileSystem) Utimens(path string, Atime *time.Time, Mtime *time.Time, context *fuse.Context) (code fuse.Status) {
-	var a time.Time
-	if Atime != nil {
-		a = *Atime
+const _UTIME_NOW = ((1 << 30) - 1)
+const _UTIME_OMIT = ((1 << 30) - 2)
+
+// Utimens - path based version of loopbackFile.Utimens()
+func (fs *loopbackFileSystem) Utimens(path string, a *time.Time, m *time.Time, context *fuse.Context) (code fuse.Status) {
+	var ts [2]syscall.Timespec
+
+	if a == nil {
+		ts[0].Nsec = _UTIME_OMIT
+	} else {
+		ts[0].Sec = a.Unix()
 	}
-	var m time.Time
-	if Mtime != nil {
-		m = *Mtime
+
+	if m == nil {
+		ts[1].Nsec = _UTIME_OMIT
+	} else {
+		ts[1].Sec = m.Unix()
 	}
-	return fuse.ToStatus(os.Chtimes(fs.GetPath(path), a, m))
+
+	err := sysUtimensat(0, fs.GetPath(path), &ts, _AT_SYMLINK_NOFOLLOW)
+	return fuse.ToStatus(err)
 }
 
 func (fs *loopbackFileSystem) Readlink(name string, context *fuse.Context) (out string, code fuse.Status) {

fuse/pathfs/syscall_linux.go

@@ -116,3 +116,26 @@ func sysListxattr(path string, dest []byte) (sz int, err error) {
 func sysSetxattr(path string, attr string, val []byte, flag int) error {
 	return syscall.Setxattr(path, attr, val, flag)
 }
+
+
+const _AT_SYMLINK_NOFOLLOW = 0x100
+
+// Linux kernel syscall utimensat(2)
+//
+// Needed to implement SetAttr on symlinks correctly as only utimensat provides
+// AT_SYMLINK_NOFOLLOW.
+func sysUtimensat(dirfd int, pathname string, times *[2]syscall.Timespec, flags int) (err error) {
+
+	// Null-terminated version of pathname
+	p0, err := syscall.BytePtrFromString(pathname)
+	if err != nil {
+		return err
+	}
+
+	_, _, e1 := syscall.Syscall6(syscall.SYS_UTIMENSAT,
+		uintptr(dirfd), uintptr(unsafe.Pointer(p0)), uintptr(unsafe.Pointer(times)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = syscall.Errno(e1)
+	}
+	return
+}

fuse/pathfs/syscall_test.go

+package pathfs
+
+import (
+	"reflect"
+	"os"
+	"testing"
+	"syscall"
+)
+
+func TestSysUtimensat(t *testing.T) {
+
+	symlink := "/tmp/TestSysUtimensat"
+	os.Remove(symlink)
+	err := os.Symlink("/nonexisting/file", symlink)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var ts [2]syscall.Timespec
+	// Atime
+	ts[0].Nsec = 1111
+	ts[0].Sec = 2222
+	// Mtime
+	ts[1].Nsec = 3333
+	ts[1].Sec = 4444
+
+	err = sysUtimensat(0, symlink, &ts, _AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var st syscall.Stat_t
+	err = syscall.Lstat(symlink, &st)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(st.Atim, ts[0]) {
+		t.Errorf("Wrong atime: %v", st.Atim)
+	}
+	if !reflect.DeepEqual(st.Mtim, ts[1]) {
+		t.Errorf("Wrong mtime: %v", st.Mtim)
+	}
+}