Ensure mergeability check runs on specific cases

We call `MergeRequest#check_mergeability` on MR page load to trigger
mergeability check. Other than that, we rely on calls for
`MergeRequest#mergeable?` to eventually call it as well. This can
cause stale `MergeRequest#merge_status` when:

1. Race condition occurred between the MR page load and the time
   `NewMergeRequestWorker` runs.
2. The `MergeRequest#merge_status` gets updated after the MR page
   has already loaded (e.g. new changes were pushed to the MR's
   target branch).

As a fix, we are calling `MergeRequest#check_mergeability` in more
places to ensure we check for it:

1. We now call it on `MergeRequests::AfterCreateService` to prevent
   race condition issue. This service is called by the
   `NewMergeRequestWorker`. This way, we ensure that we check for
   MR's mergeability after it was created.
2. `MergeRequestPollCachedWidgetEntity#merge_status` has been
   updated so whenever the `MergeRequest#merge_status` gets
   updated and the MR widget polls for updated status, we will
   recheck for mergeability as well. On next poll, it should show
   the updated `merge_status`.
3. The `check_mergeability_async_in_widget` FF has also been
   removed since it doesn't seem to be used at all. We also need
   to keep the call for `MergeRequest#check_mergeability` on the
   show action so we won't need to wait for poll in case the merge
   status gets updated and the user decides to refresh the page.

Changelog: fixed

app/controllers/projects/merge_requests_controller.rb

@@ -100,10 +100,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   # rubocop:disable Metrics/AbcSize
   def show
     close_merge_request_if_no_source_project
-
-    if Feature.disabled?(:check_mergeability_async_in_widget, @project, default_enabled: :yaml)
-      @merge_request.check_mergeability(async: true)
-    end
+    @merge_request.check_mergeability(async: true)
 
     respond_to do |format|
       format.html do

app/serializers/merge_request_poll_cached_widget_entity.rb

@@ -8,7 +8,6 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
   expose :merged_commit_sha
   expose :short_merged_commit_sha
   expose :merge_error
-  expose :public_merge_status, as: :merge_status
   expose :merge_user_id
   expose :source_branch
   expose :source_project_id
@@ -26,6 +25,11 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
   expose :source_branch_exists?, as: :source_branch_exists
   expose :branch_missing?, as: :branch_missing
 
+  expose :merge_status do |merge_request|
+    merge_request.check_mergeability(async: true)
+    merge_request.public_merge_status
+  end
+
   expose :default_squash_commit_message do |merge_request|
     merge_request.default_squash_commit_message(user: request.current_user)
   end

app/serializers/merge_request_poll_widget_entity.rb

@@ -24,8 +24,6 @@ class MergeRequestPollWidgetEntity < Grape::Entity
   end
 
   expose :mergeable do |merge_request, options|
-    next merge_request.mergeable? if Feature.disabled?(:check_mergeability_async_in_widget, merge_request.project, default_enabled: :yaml)
-
     merge_request.mergeable?
   end
 

app/services/merge_requests/after_create_service.rb

@@ -7,7 +7,7 @@ module MergeRequests
     def execute(merge_request)
       prepare_for_mergeability(merge_request) if early_prepare_for_mergeability?(merge_request)
       prepare_merge_request(merge_request)
-      mark_as_unchecked(merge_request) unless early_prepare_for_mergeability?(merge_request)
+      check_mergeability(merge_request) unless early_prepare_for_mergeability?(merge_request)
     end
 
     private
@@ -15,7 +15,7 @@ module MergeRequests
     def prepare_for_mergeability(merge_request)
       create_pipeline_for(merge_request, current_user)
       merge_request.update_head_pipeline
-      mark_as_unchecked(merge_request)
+      check_mergeability(merge_request)
     end
 
     def prepare_merge_request(merge_request)
@@ -55,8 +55,13 @@ module MergeRequests
       end
     end
 
-    def mark_as_unchecked(merge_request)
-      merge_request.mark_as_unchecked if merge_request.preparing?
+    def check_mergeability(merge_request)
+      return unless merge_request.preparing?
+
+      # Need to set to unchecked to be able to check for mergeability or else
+      # it'll be a no-op.
+      merge_request.mark_as_unchecked
+      merge_request.check_mergeability(async: true)
     end
   end
 end

config/feature_flags/development/check_mergeability_async_in_widget.yml

----
-name: check_mergeability_async_in_widget
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58178
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326567
-milestone: '13.11'
-type: development
-group: group::source code
-default_enabled: false

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -57,19 +57,13 @@ RSpec.describe Projects::MergeRequestsController do
         merge_request.mark_as_unchecked!
       end
 
-      context 'check_mergeability_async_in_widget feature flag is disabled' do
-        before do
-          stub_feature_flags(check_mergeability_async_in_widget: false)
+      it 'checks mergeability asynchronously' do
+        expect_next_instance_of(MergeRequests::MergeabilityCheckService) do |service|
+          expect(service).not_to receive(:execute)
+          expect(service).to receive(:async_execute)
         end
 
-        it 'checks mergeability asynchronously' do
-          expect_next_instance_of(MergeRequests::MergeabilityCheckService) do |service|
-            expect(service).not_to receive(:execute)
-            expect(service).to receive(:async_execute)
-          end
-
-          go
-        end
+        go
       end
     end
 

spec/serializers/merge_request_poll_cached_widget_entity_spec.rb

@@ -21,12 +21,6 @@ RSpec.describe MergeRequestPollCachedWidgetEntity do
     is_expected.to include(:target_branch_sha)
   end
 
-  it 'has public_merge_status as merge_status' do
-    expect(resource).to receive(:public_merge_status).and_return('checking')
-
-    expect(subject[:merge_status]).to eq 'checking'
-  end
-
   it 'has blob path data' do
     allow(resource).to receive_messages(
       base_pipeline: pipeline,
@@ -38,6 +32,20 @@ RSpec.describe MergeRequestPollCachedWidgetEntity do
     expect(subject[:blob_path]).to include(:head_path)
   end
 
+  describe 'merge_status' do
+    it 'calls for MergeRequest#check_mergeability' do
+      expect(resource).to receive(:check_mergeability).with(async: true)
+
+      subject[:merge_status]
+    end
+
+    it 'has public_merge_status as merge_status' do
+      expect(resource).to receive(:public_merge_status).and_return('checking')
+
+      expect(subject[:merge_status]).to eq 'checking'
+    end
+  end
+
   describe 'diverged_commits_count' do
     context 'when MR open and its diverging' do
       it 'returns diverged commits count' do

spec/serializers/merge_request_poll_widget_entity_spec.rb

@@ -180,16 +180,6 @@ RSpec.describe MergeRequestPollWidgetEntity do
       it 'calculates mergeability and returns true' do
         expect(subject[:mergeable]).to eq(true)
       end
-
-      context 'when check_mergeability_async_in_widget is disabled' do
-        before do
-          stub_feature_flags(check_mergeability_async_in_widget: false)
-        end
-
-        it 'calculates mergeability and returns true' do
-          expect(subject[:mergeable]).to eq(true)
-        end
-      end
     end
   end
 end

spec/services/merge_requests/after_create_service_spec.rb

@@ -89,10 +89,10 @@ RSpec.describe MergeRequests::AfterCreateService do
         merge_request.mark_as_preparing!
       end
 
-      it 'marks the merge request as unchecked' do
-        execute_service
+      it 'checks for mergeability' do
+        expect(merge_request).to receive(:check_mergeability).with(async: true)
 
-        expect(merge_request.reload).to be_unchecked
+        execute_service
       end
 
       context 'when preparing for mergeability fails' do
@@ -130,9 +130,9 @@ RSpec.describe MergeRequests::AfterCreateService do
             .and_raise(StandardError)
         end
 
-        it 'still marks the merge request as unchecked' do
+        it 'still checks for mergeability' do
+          expect(merge_request).to receive(:check_mergeability).with(async: true)
           expect { execute_service }.to raise_error(StandardError)
-          expect(merge_request.reload).to be_unchecked
         end
 
         context 'when early_prepare_for_mergeability feature flag is disabled' do