Restoring a backup created with "--compress-filter 'gzip -3'" fails

[keepiru]

Qubes OS version:

Backup created on R3.2, attempted restore on R4.0

Affected component(s):

qvm-backup-restore

Steps to reproduce the behavior:

On R3.2:
qvm-backup -z -Z 'gzip -3' -d sys-usb /media/user/backups

On R4.0:
qvm-backup-restore -d sys-usb /run/media/user/backups/qubes-backup-2018-04-20T010203 my-vm

Expected behavior:

Backup is restored without errors.

Actual behavior:

app: Checking backup content...
qvm-backup-restore: error: Invalid backup header (value)

General notes:

I was able to work around the issue with this patch:

diff -r -u site-packages-orig/qubesadmin/backup/restore.py site-packages/qubesadmin/backup/restore.py
--- site-packages-orig/qubesadmin/backup/restore.py	2018-03-03 19:33:29.000000000 -0800
+++ site-packages/qubesadmin/backup/restore.py	2018-04-27 23:37:00.206134065 -0700
@@ -143,9 +143,6 @@
             if key not in self.header_keys.keys():
                 # Ignoring unknown option
                 continue
-            if not _re_alphanum.match(value):
-                raise QubesException("Invalid backup header ("
-                                                 "value)")
             if getattr(self, self.header_keys[key]) is not None:
                 raise QubesException(
                     "Duplicated header line: {}".format(key))
@@ -153,6 +150,8 @@
                 value = value.lower() in ["1", "true", "yes"]
             elif key in self.int_options:
                 value = int(value)
+            if value == 'gzip -3':
+                value = 'gzip'
             setattr(self, self.header_keys[key], value)
 
         self.validate()