morphlib.fsutils: setup_device_mapping simplify

losetup has a --show option that will print the loop device the file
was mapped to, this is better than parsing the output of losetup -j.

1 files changed, 3 insertions, 7 deletions

diff --git a/morphlib/fsutils.py b/morphlib/fsutils.py
index 1a833599..770cafa8 100644
--- a/morphlib/fsutils.py
+++ b/morphlib/fsutils.py
@@ -44,13 +44,9 @@ def setup_device_mapping(runcmd, image_name):
         start = int(match.group(1)) * 512
         if start != 0:
             break
-    
-    runcmd(['losetup', '-o', str(start), '-f', image_name])
-    
-    out = runcmd(['losetup', '-j', image_name])
-    line = out.strip()
-    i = line.find(':')
-    return line[:i]
+
+    device = runcmd(['losetup', '--show', '-o', str(start), '-f', image_name])
+    return device.strip()
 
 def create_fs(runcmd, partition):
     runcmd(['mkfs.btrfs', '-L', 'baserock', partition])