Evaluate the @root volume name also for btrfs

In a volume setup the special volume declaration
<volume name="@root=identifier"/> was only evaluated for the
LVM volume manager. In case of btrfs a hardcoded root volume
name '@' was used. This commit allows to specify a custom
name for the root volume for btrfs as well and also allows
to specify that there should be no such root volume.
Example:

    <volume name="@root=@"/>

Name the root volume '@'. If not specified this stays as
the default to stay compatible

    <volume name="@root=/"/>

Indicate no root volume is wanted. All subvolumes resides
below root (/)

    <volume name="@root=foo"/>

Name the root volume 'foo'

This is related to Issue #2316 and a first patch to
address the requested changes

kiwi/volume_manager/btrfs.py

@@ -90,6 +90,12 @@ def setup(self, name=None):
 
         :param string name: unused
         """
+        root_volume_name = '@'
+        canonical_volume_list = self.get_canonical_volume_list()
+        for volume in canonical_volume_list.volumes:
+            if volume.is_root_volume and volume.name:
+                root_volume_name = volume.name
+
         self.setup_mountpoint()
 
         filesystem = FileSystem.new(
@@ -112,31 +118,38 @@ def setup(self, name=None):
             Command.run(
                 ['btrfs', 'quota', 'enable', self.mountpoint]
             )
-        root_volume = self.mountpoint + '/@'
-        Command.run(
-            ['btrfs', 'subvolume', 'create', root_volume]
-        )
+        if root_volume_name != '/':
+            root_volume = self.mountpoint + f'/{root_volume_name}'
+            Command.run(
+                ['btrfs', 'subvolume', 'create', root_volume]
+            )
         if self.custom_args['root_is_snapshot']:
-            snapshot_volume = self.mountpoint + '/@/.snapshots'
+            snapshot_volume = self.mountpoint + \
+                f'/{root_volume_name}/.snapshots'.replace('//', '/')
             Command.run(
                 ['btrfs', 'subvolume', 'create', snapshot_volume]
             )
             os.chmod(snapshot_volume, 0o700)
             Path.create(snapshot_volume + '/1')
-            snapshot = self.mountpoint + '/@/.snapshots/1/snapshot'
+            snapshot = self.mountpoint + \
+                f'/{root_volume_name}/.snapshots/1/snapshot'.replace('//', '/')
             Command.run(
                 ['btrfs', 'subvolume', 'create', snapshot]
             )
-            self._set_default_volume('@/.snapshots/1/snapshot')
-            snapshot = self.mountpoint + '/@/.snapshots/1/snapshot'
-            # Mount /@/.snapshots as /.snapshots inside the root
-            snapshots_mount = MountManager(
-                device=self.device,
-                mountpoint=snapshot + '/.snapshots'
+            self._set_default_volume(
+                f'{root_volume_name}/.snapshots/1/snapshot'.replace('//', '/')
             )
-            self.subvol_mount_list.append(snapshots_mount)
+            if root_volume_name != '/':
+                snapshot = self.mountpoint + \
+                    f'/{root_volume_name}/.snapshots/1/snapshot'
+                # Mount /{root_volume_name}/.snapshots as /.snapshots inside root
+                snapshots_mount = MountManager(
+                    device=self.device,
+                    mountpoint=snapshot + '/.snapshots'
+                )
+                self.subvol_mount_list.append(snapshots_mount)
         else:
-            self._set_default_volume('@')
+            self._set_default_volume(root_volume_name)
 
     def create_volumes(self, filesystem_name):
         """

kiwi/xml_state.py

@@ -1680,6 +1680,9 @@ def get_volumes(self) -> List[volume_type]:
         if not have_root_volume_setup:
             # There must always be a root volume setup. It will be the
             # full size volume if no other volume has this setup
+            volume_management = self.get_volume_management()
+            root_volume_name = \
+                defaults.ROOT_VOLUME_NAME if volume_management == 'lvm' else ''
             if have_full_size_volume:
                 size = 'freespace:' + format(
                     Defaults.get_min_volume_mbytes()
@@ -1690,7 +1693,7 @@ def get_volumes(self) -> List[volume_type]:
                 fullsize = True
             volume_type_list.append(
                 volume_type(
-                    name=defaults.ROOT_VOLUME_NAME,
+                    name=root_volume_name,
                     size=size,
                     fullsize=fullsize,
                     mountpoint=None,

test/unit/volume_manager/btrfs_test.py

@@ -27,12 +27,12 @@ def inject_fixtures(self, caplog):
     def setup(self, mock_path):
         self.volumes = [
             volume_type(
-                name='LVRoot', size='freespace:100', realpath='/',
+                name='@', size='freespace:100', realpath='/',
                 mountpoint=None, fullsize=False, label=None,
                 attributes=[], is_root_volume=True
             ),
             volume_type(
-                name='LVetc', size='freespace:200', realpath='/etc',
+                name='etc', size='freespace:200', realpath='/etc',
                 mountpoint='/etc', fullsize=False, label=None,
                 attributes=[], is_root_volume=False
             ),
@@ -42,7 +42,7 @@ def setup(self, mock_path):
                 attributes=[], is_root_volume=False
             ),
             volume_type(
-                name='LVhome', size=None, realpath='/home',
+                name='home', size=None, realpath='/home',
                 mountpoint='/home', fullsize=True, label=None,
                 attributes=[], is_root_volume=False
             )
@@ -186,15 +186,15 @@ def test_create_volumes(
             ),
             call(
                 'tmpdir/@/', volume_type(
-                    name='LVetc', size='freespace:200', realpath='/etc',
+                    name='etc', size='freespace:200', realpath='/etc',
                     mountpoint='/etc', fullsize=False, label=None,
                     attributes=[],
                     is_root_volume=False
                 )
             ),
             call(
                 'tmpdir/@/', volume_type(
-                    name='LVhome', size=None, realpath='/home',
+                    name='home', size=None, realpath='/home',
                     mountpoint='/home', fullsize=True, label=None,
                     attributes=[],
                     is_root_volume=False