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.

This patch fixes the failing utimens tests.

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.
 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)
+	// MacOS before High Sierra lacks utimensat() and UTIME_OMIT.
+	// We emulate using utimes() and extra GetAttr() calls.
+	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 +1,40 @@
+// Copyright 2018 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 converts time.Time to syscall.Timeval
+func timeToTimeval(t *time.Time) 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
+	var tv syscall.Timeval
+	tv.Usec = int32(t.Nanosecond() / 1000)
+	tv.Sec = t.Unix()
+	return tv
+}
+
+// Fill converts a and m to a syscall.Timeval slice that can be passed
+// to syscall.Utimes. Missing values (if any) are taken from attr
+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
+}