backend/local: Ignore ENOTTY for fsync on Mac

Fixes #4016.

changelog/unreleased/issue-4016

@@ -0,0 +1,8 @@
+Bugfix: Support ExFAT-formatted local backends on macOS Ventura
+
+ExFAT-formatted disks could not be used as local backends starting from macOS
+Ventura. Restic commands would fail with "inappropriate ioctl for device". This
+has been fixed.
+
+https://github.com/restic/restic/issues/4016
+https://github.com/restic/restic/pull/4021

internal/backend/local/local.go

@@ -177,7 +177,7 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 
 	// Ignore error if filesystem does not support fsync.
 	err = f.Sync()
-	syncNotSup := errors.Is(err, syscall.ENOTSUP)
+	syncNotSup := err != nil && (errors.Is(err, syscall.ENOTSUP) || isMacENOTTY(err))
 	if err != nil && !syncNotSup {
 		return errors.WithStack(err)
 	}

internal/backend/local/local_unix.go

@@ -6,6 +6,7 @@ package local
 import (
 	"errors"
 	"os"
+	"runtime"
 	"syscall"
 
 	"github.com/restic/restic/internal/fs"
@@ -19,7 +20,9 @@ func fsyncDir(dir string) error {
 	}
 
 	err = d.Sync()
-	if errors.Is(err, syscall.ENOTSUP) || errors.Is(err, syscall.ENOENT) || errors.Is(err, syscall.EINVAL) {
+	if err != nil &&
+		(errors.Is(err, syscall.ENOTSUP) || errors.Is(err, syscall.ENOENT) ||
+			errors.Is(err, syscall.EINVAL) || isMacENOTTY(err)) {
 		err = nil
 	}
 
@@ -31,6 +34,15 @@ func fsyncDir(dir string) error {
 	return err
 }
 
+// The ExFAT driver on some versions of macOS can return ENOTTY,
+// "inappropriate ioctl for device", for fsync.
+//
+// https://github.com/restic/restic/issues/4016
+// https://github.com/realm/realm-core/issues/5789
+func isMacENOTTY(err error) bool {
+	return runtime.GOOS == "darwin" && errors.Is(err, syscall.ENOTTY)
+}
+
 // set file to readonly
 func setFileReadonly(f string, mode os.FileMode) error {
 	return fs.Chmod(f, mode&^0222)

internal/backend/local/local_windows.go

@@ -7,6 +7,9 @@ import (
 // Can't explicitly flush directory changes on Windows.
 func fsyncDir(dir string) error { return nil }
 
+// Windows is not macOS.
+func isMacENOTTY(err error) bool { return false }
+
 // We don't modify read-only on windows,
 // since it will make us unable to delete the file,
 // and this isn't common practice on this platform.