darwin, loopback: emulate UTIME_OMIT

All but the newest MacOS versions (xnu-4570.1.46 / High Sierra)
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 MacOS, 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.

To not duplicate the logic in pathfs and nodefs, an
internal "utimens" helper package is created.

fuse/nodefs/files_darwin.go

@@ -10,6 +10,7 @@ import (
 	"unsafe"
 
 	"github.com/hanwen/go-fuse/fuse"
+	"github.com/hanwen/go-fuse/internal/utimens"
 )
 
 func (f *loopbackFile) Allocate(off uint64, sz uint64, mode uint32) fuse.Status {
@@ -65,8 +66,6 @@ func (f *loopbackFile) Allocate(off uint64, sz uint64, mode uint32) fuse.Status
 	return fuse.OK
 }
 
-const _UTIME_OMIT = ((1 << 30) - 2)
-
 // timeToTimeval - Convert time.Time to syscall.Timeval
 //
 // Note: This does not use syscall.NsecToTimespec because
@@ -79,22 +78,18 @@ 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.
+// MacOS before High Sierra lacks utimensat() and UTIME_OMIT.
+// We emulate using utimes() and extra GetAttr() calls.
 func (f *loopbackFile) Utimens(a *time.Time, m *time.Time) fuse.Status {
-	tv := make([]syscall.Timeval, 2)
-	if a == nil {
-		tv[0].Usec = _UTIME_OMIT
-	} else {
-		tv[0] = timeToTimeval(a)
-	}
-
-	if m == nil {
-		tv[1].Usec = _UTIME_OMIT
-	} else {
-		tv[1] = timeToTimeval(m)
+	var attr fuse.Attr
+	if a == nil || m == nil {
+		var status fuse.Status
+		status = f.GetAttr(&attr)
+		if !status.Ok() {
+			return status
+		}
 	}
-
+	tv := utimens.Fill(a, m, &attr)
 	f.lock.Lock()
 	err := syscall.Futimes(int(f.File.Fd()), tv)
 	f.lock.Unlock()

fuse/pathfs/loopback_darwin.go

@@ -9,39 +9,21 @@ import (
 	"time"
 
 	"github.com/hanwen/go-fuse/fuse"
+	"github.com/hanwen/go-fuse/internal/utimens"
 )
 
-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
-// that does not work properly for times before 1970,
-// see https://github.com/golang/go/issues/12777
-func timeToTimeval(t *time.Time) syscall.Timeval {
-	var tv syscall.Timeval
-	tv.Usec = int32(t.Nanosecond() / 1000)
-	tv.Sec = t.Unix()
-	return tv
-}
-
-// OSX does not have the utimensat syscall neded to implement this properly.
-// We do our best to emulate it using futimes.
+// MacOS before High Sierra lacks utimensat() and UTIME_OMIT.
+// We 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)
+	var attr *fuse.Attr
+	if a == nil || m == nil {
+		var status fuse.Status
+		attr, status = fs.GetAttr(path, context)
+		if !status.Ok() {
+			return status
+		}
 	}
-
-	if m == nil {
-		tv[1].Usec = _UTIME_OMIT
-	} else {
-		tv[1] = timeToTimeval(m)
-	}
-
+	tv := utimens.Fill(a, m, attr)
 	err := syscall.Utimes(fs.GetPath(path), tv)
 	return fuse.ToStatus(err)
 }

internal/utimens/utimens_darwin.go

@@ -0,0 +1,42 @@
+// Copyright 2016 the Go-FUSE Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package utimens
+
+import (
+	"syscall"
+	"time"
+
+	"github.com/hanwen/go-fuse/fuse"
+)
+
+// timeToTimeval - convert time.Time to syscall.Timeval
+//
+// Note: This does not use syscall.NsecToTimespec because
+// that does not work properly for times before 1970,
+// see https://github.com/golang/go/issues/12777
+func timeToTimeval(t *time.Time) syscall.Timeval {
+	var tv syscall.Timeval
+	tv.Usec = int32(t.Nanosecond() / 1000)
+	tv.Sec = t.Unix()
+	return tv
+}
+
+// Fill - fill the missing timestamp value (if any) from attr, and pack
+// both into a syscall.Timeval slice that can be passed to syscall.Utimes()
+// or syscall.Futimes().
+func Fill(a *time.Time, m *time.Time, attr *fuse.Attr) []syscall.Timeval {
+	if a == nil {
+		a2 := time.Unix(int64(attr.Atime), int64(attr.Atimensec))
+		a = &a2
+	}
+	if m == nil {
+		m2 := time.Unix(int64(attr.Mtime), int64(attr.Mtimensec))
+		m = &m2
+	}
+	tv := make([]syscall.Timeval, 2)
+	tv[0] = timeToTimeval(a)
+	tv[1] = timeToTimeval(m)
+	return tv
+}