local backend: ignore not supported error on sync()

Closes #2395

changelog/unreleased/issue-2395

@@ -0,0 +1,12 @@
+Enhancement: Ignore sync errors when operation not supported by local filesystem
+
+The local backend has been modified to work with filesystems which doesn't support
+the `sync` operation. This operation is normally used by restic to ensure that data
+files are fully written to disk before continuing.
+
+For these limited filesystems, saving a file in the backend would previously fail with
+an "operation not supported" error. This error is now ignored, which means that e.g.
+an SMB mount on macOS can now be used as storage location for a repository.
+
+https://github.com/restic/restic/issues/2395
+https://forum.restic.net/t/sync-errors-on-mac-over-smb/1859

internal/backend/local/local.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"syscall"
 
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
@@ -115,8 +116,13 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 	}
 
 	if err = f.Sync(); err != nil {
-		_ = f.Close()
+		pathErr, ok := err.(*os.PathError)
-		return errors.Wrap(err, "Sync")
+		isNotSupported := ok && pathErr.Op == "sync" && pathErr.Err == syscall.ENOTSUP
+		// ignore error if filesystem does not support the sync operation
+		if !isNotSupported {
+			_ = f.Close()
+			return errors.Wrap(err, "Sync")
+		}
 	}
 
 	err = f.Close()