Add assignee tests into new ee test script

The behavior of assignee push options is not the same on GitLab EE
where many assignees are allowed as on GitLab FLOSS where only one
assignee is allowed.

So let's also have tests for GitLab EE.

ee/spec/services/merge_requests/push_options_handler_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe MergeRequests::PushOptionsHandlerService do
+  include ProjectForksHelper
+
+  let_it_be(:project) { create(:project, :public, :repository) }
+  let_it_be(:user1) { create(:user, developer_projects: [project]) }
+  let_it_be(:user2) { create(:user, developer_projects: [project]) }
+  let_it_be(:user3) { create(:user, developer_projects: [project]) }
+  let_it_be(:forked_project) { fork_project(project, user1, repository: true) }
+
+  let(:service) { described_class.new(project, user1, changes, push_options) }
+  let(:source_branch) { 'fix' }
+  let(:new_branch_changes) { "#{Gitlab::Git::BLANK_SHA} 570e7b2abdd848b95f2f578043fc23bd6f6fd24d refs/heads/#{source_branch}" }
+  let(:existing_branch_changes) { "d14d6c0abdd253381df51a723d58691b2ee1ab08 570e7b2abdd848b95f2f578043fc23bd6f6fd24d refs/heads/#{source_branch}" }
+  let(:error_mr_required) { "A merge_request.create push option is required to create a merge request for branch #{source_branch}" }
+
+  before do
+    stub_licensed_features(multiple_merge_request_assignees: true)
+  end
+
+  shared_examples_for 'a service that can create a merge request' do
+    subject(:last_mr) { MergeRequest.last }
+
+    it 'creates a merge request with the correct target branch' do
+      branch = push_options[:target] || project.default_branch
+
+      expect { service.execute }.to change { MergeRequest.count }.by(1)
+      expect(last_mr.target_branch).to eq(branch)
+    end
+
+    context 'when project has been forked', :sidekiq_might_not_need_inline do
+      let(:forked_project) { fork_project(project, user1, repository: true) }
+      let(:service) { described_class.new(forked_project, user1, changes, push_options) }
+
+      before do
+        allow(forked_project).to receive(:empty_repo?).and_return(false)
+      end
+
+      it 'sets the correct source and target project' do
+        service.execute
+
+        expect(last_mr.source_project).to eq(forked_project)
+        expect(last_mr.target_project).to eq(project)
+      end
+    end
+  end
+
+  shared_examples_for 'a service that does not create a merge request' do
+    it do
+      expect { service.execute }.not_to change { MergeRequest.count }
+    end
+  end
+
+  # In the non-foss version of GitLab, there can be many assignees
+  shared_examples_for 'a service that can change assignees of a merge request' do |count|
+    subject(:last_mr) { MergeRequest.last }
+
+    it 'changes assignee count' do
+      service.execute
+
+      expect(last_mr.assignees.count).to eq(count)
+    end
+  end
+
+  describe '`assign` push option' do
+    let(:push_options) { { assign: { user2.id => 1, user3.id => 1 } } }
+
+    context 'with a new branch' do
+      let(:changes) { new_branch_changes }
+
+      it_behaves_like 'a service that does not create a merge request'
+
+      it 'adds an error to the service' do
+        service.execute
+
+        expect(service.errors).to include(error_mr_required)
+      end
+
+      context 'when coupled with the `create` push option in ee' do
+        let(:push_options) { { create: true, assign: { user2.id => 1, user3.id => 1 } } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can change assignees of a merge request', 2
+      end
+    end
+
+    context 'with an existing branch but no open MR' do
+      let(:changes) { existing_branch_changes }
+
+      it_behaves_like 'a service that does not create a merge request'
+
+      it 'adds an error to the service' do
+        service.execute
+
+        expect(service.errors).to include(error_mr_required)
+      end
+
+      context 'when coupled with the `create` push option in ee' do
+        let(:push_options) { { create: true, assign: { user2.id => 1, user3.id => 1 } } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can change assignees of a merge request', 2
+      end
+    end
+
+    context 'with an existing branch that has a merge request open in ee' do
+      let(:changes) { existing_branch_changes }
+      let!(:merge_request) { create(:merge_request, source_project: project, source_branch: source_branch)}
+
+      it_behaves_like 'a service that does not create a merge request'
+      it_behaves_like 'a service that can change assignees of a merge request', 2
+    end
+  end
+
+  describe '`unassign` push option' do
+    let(:push_options) { { assign: { user2.id => 1, user3.id => 1 }, unassign: { user1.id => 1, user3.id => 1 } } }
+
+    context 'with a new branch' do
+      let(:changes) { new_branch_changes }
+
+      it_behaves_like 'a service that does not create a merge request'
+
+      it 'adds an error to the service' do
+        service.execute
+
+        expect(service.errors).to include(error_mr_required)
+      end
+
+      context 'when coupled with the `create` push option' do
+        let(:push_options) { { create: true, assign: { user2.id => 1, user3.id => 1 }, unassign: { user1.id => 1, user3.id => 1 } } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can change assignees of a merge request', 1
+      end
+    end
+
+    context 'with an existing branch but no open MR' do
+      let(:changes) { existing_branch_changes }
+
+      it_behaves_like 'a service that does not create a merge request'
+
+      it 'adds an error to the service' do
+        service.execute
+
+        expect(service.errors).to include(error_mr_required)
+      end
+
+      context 'when coupled with the `create` push option' do
+        let(:push_options) { { create: true, assign: { user2.id => 1, user3.id => 1 }, unassign: { user1.id => 1, user3.id => 1 } } }
+
+        it_behaves_like 'a service that can create a merge request'
+        it_behaves_like 'a service that can change assignees of a merge request', 1
+      end
+    end
+
+    context 'with an existing branch that has a merge request open' do
+      let(:changes) { existing_branch_changes }
+      let!(:merge_request) { create(:merge_request, source_project: project, source_branch: source_branch)}
+
+      it_behaves_like 'a service that does not create a merge request'
+      it_behaves_like 'a service that can change assignees of a merge request', 1
+    end
+  end
+end