Delete conflicting orphaned routes

app/models/route.rb

 class Route < ActiveRecord::Base
+  include CaseSensitivity
+
   belongs_to :source, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
   validates :source, presence: true
@@ -10,6 +12,7 @@ class Route < ActiveRecord::Base
   validate :ensure_permanent_paths, if: :path_changed?
+  before_validation :delete_conflicting_orphaned_routes
   after_create :delete_conflicting_redirects
   after_update :delete_conflicting_redirects, if: :path_changed?
   after_update :create_redirect_for_old_path
@@ -78,4 +81,13 @@ class Route < ActiveRecord::Base
   def conflicting_redirect_exists?
     RedirectRoute.permanent.matching_path_and_descendants(path).exists?
   end
+
+  def delete_conflicting_orphaned_routes
+    conflicting = self.class.iwhere(path: path)
+    conflicting_orphaned_routes = conflicting.select do |route|
+      route.source.nil?
+    end
+
+    conflicting_orphaned_routes.each(&:destroy)
+  end
 end

changelogs/unreleased/mk-delete-orphaned-routes-before-validation.yml

+---
+title: Ensure that users can reclaim a namespace or project path that is blocked by
+  an orphaned route
+merge_request: 16242
+author:
+type: fixed

spec/models/route_spec.rb

@@ -79,6 +79,13 @@ describe Route do
   end
   describe 'callbacks' do
+    context 'before validation' do
+      it 'calls #delete_conflicting_orphaned_routes' do
+        expect(route).to receive(:delete_conflicting_orphaned_routes)
+        route.valid?
+      end
+    end
+
     context 'after update' do
       it 'calls #create_redirect_for_old_path' do
         expect(route).to receive(:create_redirect_for_old_path)
@@ -378,4 +385,58 @@ describe Route do
       end
     end
   end
+
+  describe '#delete_conflicting_orphaned_routes' do
+    context 'when there is a conflicting route' do
+      let!(:conflicting_group) { create(:group, path: 'foo') }
+
+      before do
+        route.path = conflicting_group.route.path
+      end
+
+      context 'when the route is orphaned' do
+        let!(:offending_route) { conflicting_group.route }
+
+        before do
+          Group.delete(conflicting_group) # Orphan the route
+        end
+
+        it 'deletes the orphaned route' do
+          expect do
+            route.valid?
+          end.to change { described_class.count }.from(2).to(1)
+        end
+
+        it 'passes validation, as usual' do
+          expect(route.valid?).to be_truthy
+        end
+      end
+
+      context 'when the route is not orphaned' do
+        it 'does not delete the conflicting route' do
+          expect do
+            route.valid?
+          end.not_to change { described_class.count }
+        end
+
+        it 'fails validation, as usual' do
+          expect(route.valid?).to be_falsey
+        end
+      end
+    end
+
+    context 'when there are no conflicting routes' do
+      it 'does not delete any routes' do
+        route
+
+        expect do
+          route.valid?
+        end.not_to change { described_class.count }
+      end
+
+      it 'passes validation, as usual' do
+        expect(route.valid?).to be_truthy
+      end
+    end
+  end
 end