Use default_btrfs_subvolume to find out the prefix on new filesystems

src/lib/y2storage/filesystems/btrfs.rb

@@ -360,8 +360,12 @@ def restore_unshadowed_subvolumes(devicegraph)
       # it lives under the #top_level_btrfs_subvolume. Otherwise, an empty
       # string will be taken as the default subvolume name.
       #
+      # If the filesystem does not exists yet, trust the default_btrfs_subvolume
+      # path as the default one.
+      #
       # @return [String] Default subvolume name
       def subvolumes_prefix
+        return default_btrfs_subvolume.path unless exists_in_raw_probed?
         children = top_level_btrfs_subvolume.children.reject { |s| snapper_path?(s.path) }
         children.size == 1 ? children.first.path : ""
       end

test/y2storage/filesystems/btrfs_test.rb

@@ -774,6 +774,12 @@
   end
 
   describe "#subvolumes_prefix" do
+    let(:exists?) { true }
+
+    before do
+      allow(filesystem).to receive(:exists_in_raw_probed?).and_return(exists?)
+    end
+
     context "when there are no subvolumes" do
       let(:dev_name) { "/dev/sda2" }
 
@@ -813,6 +819,21 @@
           end
         end
       end
+
+      context "when the filesystem does not exist yet" do
+        let(:exists?) { false }
+        let(:default_subvolume) do
+          instance_double(Y2Storage::BtrfsSubvolume, path: "@@")
+        end
+
+        before do
+          allow(filesystem).to receive(:default_btrfs_subvolume).and_return(default_subvolume)
+        end
+
+        it "returns the default subvolume path" do
+          expect(filesystem.subvolumes_prefix).to eq("@@")
+        end
+      end
     end
 
     context "when there are several subvolumes at first level" do