[mount] Factor out the default btrfs configuration

src/modules/mount/main.py

@@ -29,6 +29,24 @@ _ = gettext.translation("calamares-python",
 def pretty_name():
     return _("Mounting partitions.")
 
+
+def get_btrfs_subvolumes():
+    """
+    Gets the job-configuration for btrfs subvolumes, or if there is
+    none given, returns a default configuration that matches
+    the setup (/ and /home) from before configurability was introduced.
+    """
+    btrfs_subvolumes = libcalamares.job.configuration.get("btrfsSubvolumes", None)
+    # Warn if there's no configuration at all, and empty configurations are
+    # replaced by a simple root-only layout.
+    if btrfs_subvolumes is None:
+        libcalamares.utils.warning("No configuration for btrfsSubvolumes")
+    if not btrfs_subvolumes:
+        btrfs_subvolumes = [ dict(mountPoint="/", subvolume="/@") ]
+
+    return btrfs_subvolumes
+
+
 def mount_partition(root_mount_point, partition, partitions):
     """
     Do a single mount of @p partition inside @p root_mount_point.
@@ -74,13 +92,7 @@ def mount_partition(root_mount_point, partition, partitions):
     # Special handling for btrfs subvolumes. Create the subvolumes listed in mount.conf
     if fstype == "btrfs" and partition["mountPoint"] == '/':
         # Root has been mounted to btrfs volume -> create subvolumes from configuration
-        btrfs_subvolumes = libcalamares.job.configuration.get("btrfsSubvolumes", None)
-        # Warn if there's no configuration at all, and empty configurations are
-        # replaced by a simple root-only layout.
-        if btrfs_subvolumes is None:
-            libcalamares.utils.warning("No configuration for btrfsSubvolumes")
-        if not btrfs_subvolumes:
-            btrfs_subvolumes = [ dict(mountPoint="/", subvolume="/@") ]
+        btrfs_subvolumes = get_btrfs_subvolumes()
 
         subvolume_mountpoints = [d['mountPoint'] for d in btrfs_subvolumes]
         # Check if listed mountpoints besides / are already present and don't create subvolume for those