darwin, loopback: emulate UTIME_OMIT

All but the newest OSX versions (xnu-4570.1.46, released 2017)
lack utimensat() and UTIME_OMIT, see:
https://github.com/apple/darwin-xnu/blame/0a798f6738bc1db01281fc08ae024145e84df927/bsd/sys/stat.h#L537

The UTIME_OMIT value is intepreted literally by darwin, resulting
in all files getting a 1970 timestamp
( rfjakob/gocryptfs#229 ).

Emulate UTIME_OMIT by filling in the missing values
using an extra GetAttr call.

fuse/pathfs/loopback_darwin.go

@@ -11,9 +11,6 @@ import (
 	"github.com/hanwen/go-fuse/fuse"
 )
 
-const _UTIME_NOW = ((1 << 30) - 1)
-const _UTIME_OMIT = ((1 << 30) - 2)
-
 // timeToTimeval - Convert time.Time to syscall.Timeval
 //
 // Note: This does not use syscall.NsecToTimespec because
@@ -26,22 +23,28 @@ func timeToTimeval(t *time.Time) syscall.Timeval {
 	return tv
 }
 
-// OSX does not have the utimensat syscall neded to implement this properly.
-// We do our best to emulate it using futimes.
+// All but the newest OSX versions (xnu-4570.1.46, released 2017)
+// lack utimensat() and UTIME_OMIT, see:
+// https://github.com/apple/darwin-xnu/blame/0a798f6738bc1db01281fc08ae024145e84df927/bsd/sys/stat.h#L537
+//
+// We do our best to emulate using utimes() and extra GetAttr() calls.
 func (fs *loopbackFileSystem) Utimens(path string, a *time.Time, m *time.Time, context *fuse.Context) fuse.Status {
-	tv := make([]syscall.Timeval, 2)
-	if a == nil {
-		tv[0].Usec = _UTIME_OMIT
-	} else {
-		tv[0] = timeToTimeval(a)
+	if a == nil || m == nil {
+		// Fill the missing value from the file
+		attr, status := fs.GetAttr(path, context)
+		if !status.Ok() {
+			return status
+		}
+		if a == nil {
+			a = time.Unix(attr.Atime, attr.Atimensec)
+		}
+		if m == nil {
+			m = time.Unix(attr.Mtime, attr.Mtimensec)
+		}
 	}
-
-	if m == nil {
-		tv[1].Usec = _UTIME_OMIT
-	} else {
-		tv[1] = timeToTimeval(m)
-	}
-
+	var tv [2]syscall.Timeval
+	tv[0] = timeToTimeval(a)
+	tv[1] = timeToTimeval(m)
 	err := syscall.Utimes(fs.GetPath(path), tv)
 	return fuse.ToStatus(err)
 }