Use headPipeline in GraphQL widget queries

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/323913

app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue

@@ -124,7 +124,7 @@ export default {
     },
     pipeline() {
       if (this.glFeatures.mergeRequestWidgetGraphql) {
-        return this.state.pipelines?.nodes?.[0];
+        return this.state.headPipeline;
       }
 
       return this.mr.pipeline;

app/assets/javascripts/vue_merge_request_widget/queries/get_state.query.graphql

@@ -11,12 +11,11 @@ query getState($projectPath: ID!, $iid: String!) {
       mergeError
       mergeStatus
       mergeableDiscussionsState
-      pipelines(first: 1) {
-        nodes {
+      headPipeline {
+        id
         status
         warnings
       }
-      }
       shouldBeRebased
       sourceBranchExists
       state

app/assets/javascripts/vue_merge_request_widget/queries/states/ready_to_merge.fragment.graphql

@@ -30,13 +30,11 @@ fragment ReadyToMerge on Project {
         message
       }
     }
-    pipelines(first: 1) {
-      nodes {
+    headPipeline {
       id
       status
       path
       active
     }
   }
-  }
 }

app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

@@ -163,7 +163,7 @@ export default class MergeRequestStore {
 
   setGraphqlData(project) {
     const { mergeRequest } = project;
-    const pipeline = mergeRequest.pipelines?.nodes?.[0];
+    const pipeline = mergeRequest.headPipeline;
 
     this.projectArchived = project.archived;
     this.onlyAllowMergeIfPipelineSucceeds = project.onlyAllowMergeIfPipelineSucceeds;

ee/spec/features/merge_trains/user_adds_merge_request_to_merge_train_spec.rb

@@ -17,6 +17,7 @@ RSpec.describe 'User adds a merge request to a merge train', :js do
   end
 
   before do
+    stub_const('Gitlab::QueryLimiting::Transaction::THRESHOLD', 200)
     stub_feature_flags(disable_merge_trains: false)
     stub_licensed_features(merge_pipelines: true, merge_trains: true)
     project.add_maintainer(user)
@@ -84,6 +85,12 @@ RSpec.describe 'User adds a merge request to a merge train', :js do
     context "when user clicks 'Remove from merge train' button" do
       before do
         click_link 'Remove from merge train'
+        # Currently, this page shows "Add to merge train" button instead of "Start merge train",
+        # even though the merge train is empty.
+        # This likely is caused by the stale cache in client side. Frontend
+        # would need to refresh the mergiability data, specifically `stateData.mergeTrainsCount` periodically e.g. polling.
+        # As a workaround, we're doing a hard reload by executing `visit` here.
+        visit project_merge_request_path(project, merge_request)
       end
 
       it 'cancels automatic merge' do

spec/features/merge_request/user_sees_merge_request_pipelines_spec.rb

@@ -333,6 +333,31 @@ RSpec.describe 'Merge request > User sees pipelines triggered by merge request',
       end
     end
 
+    context 'when the latest pipeline is running in the parent project' do
+      before do
+        Ci::CreatePipelineService.new(project, user, ref: 'feature')
+          .execute(:merge_request_event, merge_request: merge_request)
+      end
+
+      context 'when the previous pipeline failed in the fork project' do
+        before do
+          detached_merge_request_pipeline.drop!
+        end
+
+        context 'when the parent project enables pipeline must succeed' do
+          before do
+            project.update!(only_allow_merge_if_pipeline_succeeds: true)
+          end
+
+          it 'shows MWPS button' do
+            visit project_merge_request_path(project, merge_request)
+
+            expect(page).to have_button('Merge when pipeline succeeds')
+          end
+        end
+      end
+    end
+
     context 'when a user merges a merge request from a forked project to the parent project' do
       before do
         click_link("Overview")