Don't create merge request pipeline without commits

app/models/ci/pipeline.rb

@@ -54,6 +54,7 @@ module Ci
     validates :ref, presence: { unless: :importing? }
     validates :merge_request, presence: { if: :merge_request? }
     validates :merge_request, absence: { unless: :merge_request? }
+    validate :presence_of_commits_in_merge_request, if: :merge_request?
     validates :tag, inclusion: { in: [false], if: :merge_request? }
     validates :status, presence: { unless: :importing? }
     validate :valid_commit_sha, unless: :importing?
@@ -345,6 +346,12 @@ module Ci
       end
     end
 
+    def presence_of_commits_in_merge_request
+      if merge_request&.has_no_commits?
+        errors.add(:merge_request, "must have commits")
+      end
+    end
+
     def git_author_name
       strong_memoize(:git_author_name) do
         commit.try(:author_name)

changelogs/unreleased/not-run-pipeline-on-empty-merge-request.yml

+---
+title: Don't create new merge request pipeline without commits
+merge_request: 24503
+author: Hiroyuki Sato
+type: added

spec/models/ci/pipeline_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe Ci::Pipeline, :mailer do
   let(:user) { create(:user) }
-  set(:project) { create(:project) }
+  set(:project) { create(:project, :repository) }
 
   let(:pipeline) do
     create(:ci_empty_pipeline, status: :created, project: project)
@@ -131,12 +131,18 @@ describe Ci::Pipeline, :mailer do
     context 'when source is merge request' do
       let(:source) { :merge_request }
 
-      context 'when merge request is specified' do
+      context 'when merge request with commits is specified' do
         let(:merge_request) { create(:merge_request, source_project: project, source_branch: 'feature', target_project: project, target_branch: 'master') }
 
         it { expect(pipeline).to be_valid }
       end
 
+      context 'when merge request without commits is specified' do
+        let(:merge_request) { create(:merge_request, source_project: project, source_branch: 'not-merged-branch', target_project: project, target_branch: 'master') }
+
+        it { expect(pipeline).not_to be_valid }
+      end
+
       context 'when merge request is empty' do
         let(:merge_request) { nil }
 
@@ -1036,6 +1042,8 @@ describe Ci::Pipeline, :mailer do
     end
 
     context 'when repository does not exist' do
+      let(:project) { create(:project) }
+
       let(:pipeline) do
         create(:ci_empty_pipeline, project: project, ref: 'master')
       end
@@ -2071,7 +2079,6 @@ describe Ci::Pipeline, :mailer do
   end
 
   describe "#all_merge_requests" do
-    let(:project) { create(:project) }
     let(:pipeline) { create(:ci_empty_pipeline, status: 'created', project: project, ref: 'master') }
 
     it "returns all merge requests having the same source branch" do

spec/models/merge_request_spec.rb

@@ -1378,7 +1378,7 @@ describe MergeRequest do
                  source_project: project,
                  source_branch: source_ref,
                  target_project: project,
-                 target_branch: 'stable')
+                 target_branch: 'merge-test')
         end
 
         before do

spec/services/ci/create_pipeline_service_spec.rb

@@ -736,6 +736,15 @@ describe Ci::CreatePipelineService do
               end
             end
 
+            context 'when there are no commits between source branch and target branch' do
+              let(:ref_name) { 'refs/heads/not-merged-branch' }
+
+              it 'does not create a merge request pipeline' do
+                expect(pipeline).not_to be_persisted
+                expect(pipeline.errors[:merge_request]).to eq(["must have commits"])
+              end
+            end
+
             context 'when merge request is created from a forked project' do
               let(:merge_request) do
                 create(:merge_request,

spec/services/merge_requests/refresh_service_spec.rb

@@ -150,11 +150,15 @@ describe MergeRequests::RefreshService do
           }
         end
 
-        it 'create merge request pipeline' do
+        it 'create merge request pipeline with commits' do
           expect { subject }
             .to change { @merge_request.merge_request_pipelines.count }.by(1)
             .and change { @fork_merge_request.merge_request_pipelines.count }.by(1)
-            .and change { @another_merge_request.merge_request_pipelines.count }.by(1)
+            .and change { @another_merge_request.merge_request_pipelines.count }.by(0)
+
+          expect(@merge_request.has_commits?).to be true
+          expect(@fork_merge_request.has_commits?).to be true
+          expect(@another_merge_request.has_commits?).to be false
         end
 
         context "when branch pipeline was created before a merge request pipline has been created" do