Fix race condition between pipeline creation and MR diff_head_sha update

app/services/ci/create_pipeline_service.rb

@@ -63,13 +63,10 @@ module Ci
     private
 
     def update_merge_requests_head_pipeline
-      merge_requests = MergeRequest.where(source_branch: @pipeline.ref, source_project: @pipeline.project)
+      return unless pipeline.latest?
 
-      merge_requests = merge_requests.select do |mr|
-        mr.diff_head_sha == @pipeline.sha
-      end
-
-      MergeRequest.where(id: merge_requests).update_all(head_pipeline_id: @pipeline.id)
+      MergeRequest.where(source_project: @pipeline.project, source_branch: @pipeline.ref).
+        update_all(head_pipeline_id: @pipeline.id)
     end
 
     def skip_ci?

app/services/merge_requests/create_service.rb

@@ -30,15 +30,12 @@ module MergeRequests
     def head_pipeline_for(merge_request)
       return unless merge_request.source_project
 
-      sha = merge_request.source_branch_head&.id
-
+      sha = merge_request.source_branch_sha
       return unless sha
 
-      pipelines =
-        Ci::Pipeline.where(ref: merge_request.source_branch, project_id: merge_request.source_project.id, sha: sha).
-        order(id: :desc)
+      pipelines = merge_request.source_project.pipelines.where(ref: merge_request.source_branch, sha: sha)
 
-      pipelines.first
+      pipelines.order(id: :desc).first
     end
   end
 end

spec/services/ci/create_pipeline_service_spec.rb

@@ -72,10 +72,11 @@ describe Ci::CreatePipelineService, services: true do
           end
         end
 
-        context 'when merge request head commit sha does not match pipeline sha' do
+        context 'when the pipeline is not the latest for the branch' do
           it 'does not update merge request head pipeline' do
             merge_request = create(:merge_request, source_branch: 'master', target_branch: "branch_1", source_project: project)
-            allow_any_instance_of(MergeRequestDiff).to receive(:head_commit).and_return(double(id: 1234))
+
+            allow_any_instance_of(Ci::Pipeline).to receive(:latest?).and_return(false)
 
             pipeline