fs: Refactor UtimesNano replacements

Previously, nodeRestoreTimestamps would do something like

	if node.Type == restic.NodeTypeSymlink {
	    return nodeRestoreSymlinkTimestamps(...)
	}
	return syscall.UtimesNano(...)

where nodeRestoreSymlinkTimestamps was either a no-op or a
reimplementation of syscall.UtimesNano that handles symlinks, with some
repeated converting between timestamp types. The Linux implementation
was a bit clumsy, requiring three syscalls to set the timestamps.

In this new setup, there is a function utimesNano that has three
implementations:

* on Linux, it's a modified syscall.UtimesNano that uses
  AT_SYMLINK_NOFOLLOW and AT_FDCWD so it can handle any type in a single
  call;
* on other Unix platforms, it just calls the syscall function after
  skipping symlinks;
* on Windows, it's the modified UtimesNano that was previously called
  nodeRestoreSymlinkTimestamps, except with different arguments.

internal/fs/node.go

@@ -292,18 +292,11 @@ func nodeRestoreMetadata(node *restic.Node, path string, warn func(msg string))
 }
 
 func nodeRestoreTimestamps(node *restic.Node, path string) error {
-	var utimes = [...]syscall.Timespec{
-		syscall.NsecToTimespec(node.AccessTime.UnixNano()),
-		syscall.NsecToTimespec(node.ModTime.UnixNano()),
-	}
+	atime := node.AccessTime.UnixNano()
+	mtime := node.ModTime.UnixNano()
 
-	if node.Type == restic.NodeTypeSymlink {
-		return nodeRestoreSymlinkTimestamps(path, utimes)
+	if err := utimesNano(fixpath(path), atime, mtime, node.Type); err != nil {
+		return &os.PathError{Op: "UtimesNano", Path: path, Err: err}
 	}
-
-	if err := syscall.UtimesNano(fixpath(path), utimes[:]); err != nil {
-		return errors.Wrap(err, "UtimesNano")
-	}
-
 	return nil
 }

internal/fs/node_aix.go

@@ -3,15 +3,7 @@
 
 package fs
 
-import (
-	"syscall"
-
-	"github.com/restic/restic/internal/restic"
-)
-
-func nodeRestoreSymlinkTimestamps(_ string, _ [2]syscall.Timespec) error {
-	return nil
-}
+import "github.com/restic/restic/internal/restic"
 
 // nodeRestoreExtendedAttributes is a no-op on AIX.
 func nodeRestoreExtendedAttributes(_ *restic.Node, _ string) error {

internal/fs/node_darwin.go

@@ -1,7 +0,0 @@
-package fs
-
-import "syscall"
-
-func nodeRestoreSymlinkTimestamps(path string, utimes [2]syscall.Timespec) error {
-	return nil
-}

internal/fs/node_linux.go

@@ -1,18 +1,15 @@
 package fs
 
 import (
-	"syscall"
-
-	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/restic"
 	"golang.org/x/sys/unix"
 )
 
-func nodeRestoreSymlinkTimestamps(path string, utimes [2]syscall.Timespec) error {
+// utimesNano is like syscall.UtimesNano, except that it does not follow symlinks.
+func utimesNano(path string, atime, mtime int64, _ restic.NodeType) error {
 	times := []unix.Timespec{
-		{Sec: utimes[0].Sec, Nsec: utimes[0].Nsec},
-		{Sec: utimes[1].Sec, Nsec: utimes[1].Nsec},
+		unix.NsecToTimespec(atime),
+		unix.NsecToTimespec(mtime),
 	}
-
-	err := unix.UtimesNanoAt(unix.AT_FDCWD, path, times, unix.AT_SYMLINK_NOFOLLOW)
-	return errors.Wrap(err, "UtimesNanoAt")
+	return unix.UtimesNanoAt(unix.AT_FDCWD, path, times, unix.AT_SYMLINK_NOFOLLOW)
 }

internal/fs/node_linux_test.go

@@ -0,0 +1,19 @@
+package fs
+
+import (
+	"io/fs"
+	"strings"
+	"testing"
+
+	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/restic"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestRestoreSymlinkTimestampsError(t *testing.T) {
+	d := t.TempDir()
+	node := restic.Node{Type: restic.NodeTypeSymlink}
+	err := nodeRestoreTimestamps(&node, d+"/nosuchfile")
+	rtest.Assert(t, errors.Is(err, fs.ErrNotExist), "want ErrNotExist, got %q", err)
+	rtest.Assert(t, strings.Contains(err.Error(), d), "filename not in %q", err)
+}

internal/fs/node_netbsd.go

@@ -1,14 +1,6 @@
 package fs
 
-import (
-	"syscall"
-
-	"github.com/restic/restic/internal/restic"
-)
-
-func nodeRestoreSymlinkTimestamps(_ string, _ [2]syscall.Timespec) error {
-	return nil
-}
+import "github.com/restic/restic/internal/restic"
 
 // nodeRestoreExtendedAttributes is a no-op on netbsd.
 func nodeRestoreExtendedAttributes(_ *restic.Node, _ string) error {

internal/fs/node_openbsd.go

@@ -1,14 +1,6 @@
 package fs
 
-import (
-	"syscall"
-
-	"github.com/restic/restic/internal/restic"
-)
-
-func nodeRestoreSymlinkTimestamps(_ string, _ [2]syscall.Timespec) error {
-	return nil
-}
+import "github.com/restic/restic/internal/restic"
 
 // nodeRestoreExtendedAttributes is a no-op on openbsd.
 func nodeRestoreExtendedAttributes(_ *restic.Node, _ string) error {

internal/fs/node_solaris.go

@@ -1,7 +0,0 @@
-package fs
-
-import "syscall"
-
-func nodeRestoreSymlinkTimestamps(path string, utimes [2]syscall.Timespec) error {
-	return nil
-}

internal/fs/node_unix_notlinux.go

@@ -0,0 +1,21 @@
+//go:build !linux && unix
+
+package fs
+
+import (
+	"syscall"
+
+	"github.com/restic/restic/internal/restic"
+)
+
+// utimesNano is like syscall.UtimesNano, except that it skips symlinks.
+func utimesNano(path string, atime, mtime int64, typ restic.NodeType) error {
+	if typ == restic.NodeTypeSymlink {
+		return nil
+	}
+
+	return syscall.UtimesNano(path, []syscall.Timespec{
+		syscall.NsecToTimespec(atime),
+		syscall.NsecToTimespec(mtime),
+	})
+}

internal/fs/node_windows.go

@@ -42,8 +42,8 @@ func lchown(_ string, _ int, _ int) (err error) {
 	return nil
 }
 
-// restoreSymlinkTimestamps restores timestamps for symlinks
-func nodeRestoreSymlinkTimestamps(path string, utimes [2]syscall.Timespec) error {
+// utimesNano is like syscall.UtimesNano, except that it sets FILE_FLAG_OPEN_REPARSE_POINT.
+func utimesNano(path string, atime, mtime int64, _ restic.NodeType) error {
 	// tweaked version of UtimesNano from go/src/syscall/syscall_windows.go
 	pathp, e := syscall.UTF16PtrFromString(fixpath(path))
 	if e != nil {
@@ -63,8 +63,8 @@ func nodeRestoreSymlinkTimestamps(path string, utimes [2]syscall.Timespec) error
 		}
 	}()
 
-	a := syscall.NsecToFiletime(syscall.TimespecToNsec(utimes[0]))
-	w := syscall.NsecToFiletime(syscall.TimespecToNsec(utimes[1]))
+	a := syscall.NsecToFiletime(atime)
+	w := syscall.NsecToFiletime(mtime)
 	return syscall.SetFileTime(h, nil, &a, &w)
 }