Make sure the settings page renders when root of a fork is deleted

app/helpers/projects_helper.rb

@@ -110,7 +110,15 @@ module ProjectsHelper
   def remove_fork_project_message(project)
     _("You are going to remove the fork relationship to source project %{forked_from_project}. Are you ABSOLUTELY sure?") %
-      { forked_from_project: @project.forked_from_project.name_with_namespace }
+      { forked_from_project: fork_source_name(project) }
+  end
+
+  def fork_source_name(project)
+    if @project.fork_source
+      @project.fork_source.full_name
+    else
+      @project.fork_network&.deleted_root_project_name
+    end
   end
   def project_nav_tabs
@@ -140,8 +148,8 @@ module ProjectsHelper
   def can_change_visibility_level?(project, current_user)
     return false unless can?(current_user, :change_visibility_level, project)
-    if project.forked?
-      project.forked_from_project.visibility_level > Gitlab::VisibilityLevel::PRIVATE
+    if project.fork_source
+      project.fork_source.visibility_level > Gitlab::VisibilityLevel::PRIVATE
     else
       true
     end

app/models/project.rb

@@ -1040,6 +1040,10 @@ class Project < ActiveRecord::Base
     !(forked_project_link.nil? || forked_project_link.forked_from_project.nil?)
   end
+  def fork_source
+    forked_from_project || fork_network&.root_project
+  end
+
   def personal?
     !group
   end

app/views/projects/_home_panel.html.haml

 - empty_repo = @project.empty_repo?
 - fork_network = @project.fork_network
-- forked_from_project = @project.forked_from_project || fork_network&.root_project
 .project-home-panel.text-center{ class: ("empty-project" if empty_repo) }
   .limit-container-width{ class: container_class }
     .avatar-container.s70.project-avatar
@@ -16,13 +15,13 @@
       - if @project.forked?
         %p
-          - if forked_from_project
+          - if @project.fork_source
             #{ s_('ForkedFromProjectPath|Forked from') }
-            = link_to project_path(forked_from_project) do
-              = forked_from_project.full_name
+            = link_to project_path(@project.fork_source) do
+              = fork_source_name(@project)
           - else
             - deleted_message = s_('ForkedFromProjectPath|Forked from %{project_name} (deleted)')
-            = deleted_message % { project_name: fork_network.deleted_root_project_name }
+            = deleted_message % { project_name: fork_source_name(@project) }
     .project-repo-buttons
       .count-buttons

app/views/projects/edit.html.haml

@@ -173,7 +173,10 @@
           %p
             This will remove the fork relationship to source project
             = succeed "." do
-              = link_to @project.forked_from_project.name_with_namespace, project_path(@project.forked_from_project)
+              - if @project.fork_source
+                = link_to(fork_source_name(@project), project_path(@project.fork_source))
+              - else
+                = fork_source_name(@project)
           = form_for([@project.namespace.becomes(Namespace), @project], url: remove_fork_project_path(@project), method: :delete, remote: true, html: { class: 'transfer-project' }) do |f|
             %p
               %strong Once removed, the fork relationship cannot be restored and you will no longer be able to send merge requests to the source.

changelogs/unreleased/bvl-unlink-fixes.yml

+---
+title: Fix issues with forked projects of which the source was deleted
+merge_request: 15150
+author:
+type: fixed

spec/features/projects/settings/forked_project_settings_spec.rb

+require 'spec_helper'
+
+feature 'Settings for a forked project', :js do
+  include ProjectForksHelper
+  let(:user) { create(:user) }
+  let(:original_project) { create(:project) }
+  let(:forked_project) { fork_project(original_project, user) }
+
+  before do
+    original_project.add_master(user)
+    forked_project.add_master(user)
+    sign_in(user)
+  end
+
+  shared_examples 'project settings for a forked projects' do
+    it 'allows deleting the link to the forked project' do
+      visit edit_project_path(forked_project)
+
+      click_button 'Remove fork relationship'
+
+      wait_for_requests
+
+      fill_in('confirm_name_input', with: forked_project.name)
+      click_button('Confirm')
+
+      expect(page).to have_content('The fork relationship has been removed.')
+      expect(forked_project.reload.forked?).to be_falsy
+    end
+  end
+
+  it_behaves_like 'project settings for a forked projects'
+
+  context 'when the original project is deleted' do
+    before do
+      original_project.destroy!
+    end
+
+    it_behaves_like 'project settings for a forked projects'
+  end
+end

spec/models/project_spec.rb

@@ -1923,6 +1923,20 @@ describe Project do
         expect(forked_project.in_fork_network_of?(other_project)).to be_falsy
       end
     end
+
+    describe '#fork_source' do
+      let!(:second_fork) { fork_project(forked_project) }
+
+      it 'returns the direct source if it exists' do
+        expect(second_fork.fork_source).to eq(forked_project)
+      end
+
+      it 'returns the root of the fork network when the directs source was deleted' do
+        forked_project.destroy
+
+        expect(second_fork.fork_source).to eq(project)
+      end
+    end
   end
   describe '#pushes_since_gc' do