Fix errors when deleting a forked project

The problem would occur when the `ForkedProjectLink` was deleted, but
the `ForkNetworkMember` was not. The delete would be rolled back and
retried.

But the error would not be saved because `Project#forked?`
would still be true, because the `ForkNetworkMember` exists. But the
`Project#forked_project_link` would be `nil`. So the validation for
the visibility level would fail.

app/models/project.rb

@@ -1272,7 +1272,7 @@ class Project < ActiveRecord::Base
 
     # self.forked_from_project will be nil before the project is saved, so
     # we need to go through the relation
-    original_project = forked_project_link.forked_from_project
+    original_project = forked_project_link&.forked_from_project
     return true unless original_project
 
     level <= original_project.visibility_level

app/services/projects/unlink_fork_service.rb

@@ -15,8 +15,8 @@ module Projects
 
       refresh_forks_count(@project.forked_from_project)
 
-      @project.forked_project_link.destroy
       @project.fork_network_member.destroy
+      @project.forked_project_link.destroy
     end
 
     def refresh_forks_count(project)

changelogs/unreleased/bvl-fix-deleting-forked-projects.yml

@@ -0,0 +1,5 @@
+---
+title: Fix error when updating a forked project with deleted `ForkedProjectLink`
+merge_request: 14916
+author:
+type: fixed

spec/services/projects/destroy_service_spec.rb

@@ -1,6 +1,8 @@
 require 'spec_helper'
 
 describe Projects::DestroyService do
+  include ProjectForksHelper
+
   let!(:user) { create(:user) }
   let!(:project) { create(:project, :repository, namespace: user.namespace) }
   let!(:path) { project.repository.path_to_repo }
@@ -212,6 +214,21 @@ describe Projects::DestroyService do
     end
   end
 
+  context 'for a forked project with LFS objects' do
+    let(:forked_project) { fork_project(project, user) }
+
+    before do
+      project.lfs_objects << create(:lfs_object)
+      forked_project.forked_project_link.destroy
+      forked_project.reload
+    end
+
+    it 'destroys the fork' do
+      expect { destroy_project(forked_project, user) }
+        .not_to raise_error
+    end
+  end
+
   context 'as the root of a fork network' do
     let!(:fork_network) { create(:fork_network, root_project: project) }