Extract MergeRequest#open? check to mergeability framework

As part of the effort of improving the checks we have to determine
if a MR is in a mergeable state, this moves the `open?` check to the
mergeability checks framework.

This is behind the `improved_mergeability_checks` feature flag.

app/models/merge_request.rb

@@ -1152,8 +1152,6 @@ class MergeRequest < ApplicationRecord
 
   # rubocop: disable CodeReuse/ServiceClass
   def mergeable_state?(skip_ci_check: false, skip_discussions_check: false)
-    return false unless open?
-
     if Feature.enabled?(:improved_mergeability_checks, self.project, default_enabled: :yaml)
       additional_checks = MergeRequests::Mergeability::RunChecksService.new(
         merge_request: self,
@@ -1164,6 +1162,7 @@ class MergeRequest < ApplicationRecord
       )
       additional_checks.execute.all?(&:success?)
     else
+      return false unless open?
       return false if draft?
       return false if broken?
       return false unless skip_discussions_check || mergeable_discussions_state?

app/services/merge_requests/mergeability/check_open_status_service.rb

+# frozen_string_literal: true
+
+module MergeRequests
+  module Mergeability
+    class CheckOpenStatusService < CheckBaseService
+      def execute
+        if merge_request.open?
+          success
+        else
+          failure
+        end
+      end
+
+      def skip?
+        false
+      end
+
+      def cacheable?
+        false
+      end
+    end
+  end
+end

app/services/merge_requests/mergeability/run_checks_service.rb

@@ -7,6 +7,7 @@ module MergeRequests
       # We want to have the cheapest checks first in the list,
       # that way we can fail fast before running the more expensive ones
       CHECKS = [
+        CheckOpenStatusService,
         CheckDraftStatusService,
         CheckBrokenStatusService,
         CheckDiscussionsStatusService,

spec/services/merge_requests/mergeability/check_open_status_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe MergeRequests::Mergeability::CheckOpenStatusService do
+  subject(:check_open_status) { described_class.new(merge_request: merge_request, params: {}) }
+
+  let(:merge_request) { build(:merge_request) }
+
+  describe '#execute' do
+    before do
+      expect(merge_request).to receive(:open?).and_return(open)
+    end
+
+    context 'when the merge request is open' do
+      let(:open) { true }
+
+      it 'returns a check result with status success' do
+        expect(check_open_status.execute.status).to eq Gitlab::MergeRequests::Mergeability::CheckResult::SUCCESS_STATUS
+      end
+    end
+
+    context 'when the merge request is not open' do
+      let(:open) { false }
+
+      it 'returns a check result with status failed' do
+        expect(check_open_status.execute.status).to eq Gitlab::MergeRequests::Mergeability::CheckResult::FAILED_STATUS
+      end
+    end
+  end
+
+  describe '#skip?' do
+    it 'returns false' do
+      expect(check_open_status.skip?).to eq false
+    end
+  end
+
+  describe '#cacheable?' do
+    it 'returns false' do
+      expect(check_open_status.cacheable?).to eq false
+    end
+  end
+end

spec/services/merge_requests/mergeability/run_checks_service_spec.rb

@@ -47,7 +47,7 @@ RSpec.describe MergeRequests::Mergeability::RunChecksService do
           expect(service).not_to receive(:execute)
         end
 
-        expect(execute).to match_array([success_result, success_result, success_result])
+        expect(execute).to match_array([success_result, success_result, success_result, success_result])
       end
     end