Update epic issue when an issue is moved

The existing epic issue is updated instead of creating a new epic issue
- there is no reason to keep reference both to old and new issues.

Related gitlab-ce#41949

app/services/issues/move_service.rb

 module Issues
   class MoveService < Issues::BaseService
+    prepend ::EE::Issues::MoveService
+
     MoveError = Class.new(StandardError)
 
     def execute(issue, new_project)

changelogs/unreleased-ee/move-epic-issue.yml

+---
+title: Update epic issue reference when moving an issue
+merge_request:
+author:
+type: fixed

ee/app/services/ee/issues/move_service.rb

+module EE
+  module Issues
+    module MoveService
+      def update_old_issue
+        rewrite_epic_issue
+        super
+      end
+
+      private
+
+      # rubocop:disable Gitlab/ModuleWithInstanceVariables
+      def rewrite_epic_issue
+        return unless epic_issue = @old_issue.epic_issue
+        return unless can?(current_user, :update_epic, epic_issue.epic.group)
+
+        epic_issue.update(issue_id: @new_issue.id)
+        @old_issue.reload
+      end
+      # rubocop:enable Gitlab/ModuleWithInstanceVariables
+    end
+  end
+end

ee/spec/services/ee/issues/move_service_spec.rb

+require 'spec_helper'
+
+describe Issues::MoveService do
+  let(:user) { create(:user) }
+  let(:old_project) { create(:project) }
+  let(:new_project) { create(:project, group: create(:group)) }
+  let(:old_issue) { create(:issue, project: old_project, author: user) }
+  let(:move_service) { described_class.new(old_project, user) }
+
+  describe '#rewrite_epic_issue' do
+    context 'issue assigned to epic' do
+      let!(:epic_issue) { create(:epic_issue, issue: old_issue) }
+
+      before do
+        stub_licensed_features(epics: true)
+        old_project.add_reporter(user)
+        new_project.add_reporter(user)
+      end
+
+      it 'updates epic issue reference' do
+        epic_issue.epic.group.add_reporter(user)
+
+        new_issue = move_service.execute(old_issue, new_project)
+
+        expect(new_issue.epic_issue).to eq(epic_issue)
+      end
+
+      it 'ignores epic issue reference if user can not update the epic' do
+        new_issue = move_service.execute(old_issue, new_project)
+
+        expect(new_issue.epic_issue).to be_nil
+      end
+    end
+  end
+end