Confirm existence of head_pipeline if pipeline success required

Pipelines are created by an async worker, so a rapid sequence of API
calls can trigger a state where the pipeline, whose existence is part of
determining if we wait for the pipeline to successfully complete before
merging, can trigger the MR to be immediately merged instead.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55127

changelogs/unreleased/55127-add-delay-after-mr-creation-for-async-tasks-to-complete.yml

+---
+title: Wait for pipeline creation to complete before accepting a MR via API
+merge_request: 27978
+author: kerrizor
+type: fixed

lib/api/merge_requests.rb

@@ -367,6 +367,10 @@ module API
         merge_request = find_project_merge_request(params[:merge_request_iid])
         merge_when_pipeline_succeeds = to_boolean(params[:merge_when_pipeline_succeeds])
 
+        if merge_when_pipeline_succeeds || merge_request.merge_when_pipeline_succeeds
+          render_api_error!('Not allowed: pipeline does not exist', 405) unless merge_request.head_pipeline
+        end
+
         # Merge request can not be merged
         # because user dont have permissions to push into target branch
         unauthorized! unless merge_request.can_be_merged_by?(current_user)

spec/requests/api/merge_requests_spec.rb

@@ -1495,6 +1495,33 @@ describe API::MergeRequests do
       expect(json_response['merge_when_pipeline_succeeds']).to eq(true)
     end
 
+    context 'when the MR requires pipeline success' do
+      it 'returns 405 if the pipeline is missing' do
+        allow_any_instance_of(MergeRequest)
+          .to receive(:merge_when_pipeline_succeeds).and_return(true)
+        allow_any_instance_of(MergeRequest).to receive(:head_pipeline).and_return(nil)
+
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user)
+
+        expect(response).to have_gitlab_http_status(405)
+        expect(json_response['message']).to eq('Not allowed: pipeline does not exist')
+      end
+    end
+
+    context 'when the request requires pipeline success' do
+      it 'returns 405 if the pipeline is missing' do
+        allow_any_instance_of(MergeRequest)
+          .to receive(:merge_when_pipeline_succeeds).and_return(true)
+        allow_any_instance_of(MergeRequest).to receive(:head_pipeline).and_return(nil)
+
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user),
+              params: { merge_when_pipeline_succeeds: true }
+
+        expect(response).to have_gitlab_http_status(405)
+        expect(json_response['message']).to eq('Not allowed: pipeline does not exist')
+      end
+    end
+
     it "returns 404 for an invalid merge request IID" do
       put api("/projects/#{project.id}/merge_requests/12345/merge", user)