internal/restic: Handle EINVAL for xattr on Solaris

Also make the errors a bit less verbose by not prepending the operation,
since pkg/xattr already does that. Old errors looked like

    Listxattr: xattr.list /myfiles/.zfs/snapshot: invalid argument

internal/restic/node_xattr.go

@@ -13,31 +13,39 @@ import (
 
 // Getxattr retrieves extended attribute data associated with path.
 func Getxattr(path, name string) ([]byte, error) {
-	b, e := xattr.Get(path, name)
-	if err, ok := e.(*xattr.Error); ok &&
-		(err.Err == syscall.ENOTSUP || err.Err == xattr.ENOATTR) {
-		return nil, nil
-	}
-	return b, errors.Wrap(e, "Getxattr")
+	b, err := xattr.Get(path, name)
+	return b, handleXattrErr(err)
 }
 
 // Listxattr retrieves a list of names of extended attributes associated with the
 // given path in the file system.
 func Listxattr(path string) ([]string, error) {
-	s, e := xattr.List(path)
-	if err, ok := e.(*xattr.Error); ok &&
-		(err.Err == syscall.ENOTSUP || err.Err == xattr.ENOATTR) {
-		return nil, nil
-	}
-	return s, errors.Wrap(e, "Listxattr")
+	l, err := xattr.List(path)
+	return l, handleXattrErr(err)
 }
 
 // Setxattr associates name and data together as an attribute of path.
 func Setxattr(path, name string, data []byte) error {
-	e := xattr.Set(path, name, data)
-	if err, ok := e.(*xattr.Error); ok &&
-		(err.Err == syscall.ENOTSUP || err.Err == xattr.ENOATTR) {
-		return nil
-	}
-	return errors.Wrap(e, "Setxattr")
+	return handleXattrErr(xattr.Set(path, name, data))
+}
+
+func handleXattrErr(err error) error {
+	switch e := err.(type) {
+	case nil:
+		return nil
+
+	case *xattr.Error:
+		// On Solaris, xattr not being supported on a file is signaled
+		// by EINVAL (https://github.com/pkg/xattr/issues/67).
+		// On Linux, xattr calls on files in an SMB/CIFS mount can return
+		// ENOATTR instead of ENOTSUP.
+		switch e.Err {
+		case syscall.EINVAL, syscall.ENOTSUP, xattr.ENOATTR:
+			return nil
+		}
+		return errors.WithStack(e)
+
+	default:
+		return errors.WithStack(e)
+	}
 }