Prevent triggering pipelines when target branch is updatedprevent-running-mr-pipelines-when-target-updated

Currently, pipelines for merge requests are triggered when source or
target branch is updated. However, we should create only when source
branch is updated, because it runs unexpected pipelines.

3 files changed, 40 insertions, 9 deletions

diff --git a/app/services/merge_requests/refresh_service.rb b/app/services/merge_requests/refresh_service.rb
index 51d27673787..e0460f7081c 100644
--- a/app/services/merge_requests/refresh_service.rb
+++ b/app/services/merge_requests/refresh_service.rb
@@ -21,6 +21,7 @@ module MergeRequests
       post_merge_manually_merged
       reload_merge_requests
       outdate_suggestions
+      refresh_pipelines_on_merge_requests
       reset_merge_when_pipeline_succeeds
       mark_pending_todos_done
       cache_merge_requests_closing_issues
@@ -107,8 +108,6 @@ module MergeRequests
         end
 
         merge_request.mark_as_unchecked
-        create_pipeline_for(merge_request, current_user)
-        UpdateHeadPipelineForMergeRequestWorker.perform_async(merge_request.id)
       end
 
       # Upcoming method calls need the refreshed version of
@@ -134,6 +133,13 @@ module MergeRequests
       end
     end
 
+    def refresh_pipelines_on_merge_requests
+      merge_requests_for_source_branch.each do |merge_request|
+        create_pipeline_for(merge_request, current_user)
+        UpdateHeadPipelineForMergeRequestWorker.perform_async(merge_request.id)
+      end
+    end
+
     def reset_merge_when_pipeline_succeeds
       merge_requests_for_source_branch.each(&:reset_merge_when_pipeline_succeeds)
     end
diff --git a/changelogs/unreleased/prevent-running-mr-pipelines-when-target-updated.yml b/changelogs/unreleased/prevent-running-mr-pipelines-when-target-updated.yml
new file mode 100644
index 00000000000..d003ca55feb
--- /dev/null
+++ b/changelogs/unreleased/prevent-running-mr-pipelines-when-target-updated.yml
@@ -0,0 +1,5 @@
+---
+title: Create pipelines for merge requests only when source branch is updated
+merge_request: 26921
+author:
+type: fixed
diff --git a/spec/services/merge_requests/refresh_service_spec.rb b/spec/services/merge_requests/refresh_service_spec.rb
index bd10523bc94..5ed06df7072 100644
--- a/spec/services/merge_requests/refresh_service_spec.rb
+++ b/spec/services/merge_requests/refresh_service_spec.rb
@@ -146,7 +146,10 @@ describe MergeRequests::RefreshService do
         stub_ci_pipeline_yaml_file(YAML.dump(config))
       end
 
-      subject { service.new(@project, @user).execute(@oldrev, @newrev, 'refs/heads/master') }
+      subject { service.new(project, @user).execute(@oldrev, @newrev, ref) }
+
+      let(:ref) { 'refs/heads/master' }
+      let(:project) { @project }
 
       context "when .gitlab-ci.yml has merge_requests keywords" do
         let(:config) do
@@ -162,14 +165,17 @@ describe MergeRequests::RefreshService do
         it 'create detached 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(0)
 
           expect(@merge_request.has_commits?).to be_truthy
-          expect(@fork_merge_request.has_commits?).to be_truthy
           expect(@another_merge_request.has_commits?).to be_falsy
         end
 
+        it 'does not create detached merge request pipeline for forked project' do
+          expect { subject }
+            .not_to change { @fork_merge_request.merge_request_pipelines.count }
+        end
+
         it 'create detached merge request pipeline for non-fork merge request' do
           subject
 
@@ -177,11 +183,25 @@ describe MergeRequests::RefreshService do
             .to be_detached_merge_request_pipeline
         end
 
-        it 'create legacy detached merge request pipeline for fork merge request' do
-          subject
+        context 'when service is hooked by target branch' do
+          let(:ref) { 'refs/heads/feature' }
 
-          expect(@fork_merge_request.merge_request_pipelines.first)
-            .to be_legacy_detached_merge_request_pipeline
+          it 'does not create detached merge request pipeline' do
+            expect { subject }
+              .not_to change { @merge_request.merge_request_pipelines.count }
+          end
+        end
+
+        context 'when service runs on forked project' do
+          let(:project) { @fork_project }
+
+          it 'creates legacy detached merge request pipeline for fork merge request' do
+            expect { subject }
+              .to change { @fork_merge_request.merge_request_pipelines.count }.by(1)
+
+            expect(@fork_merge_request.merge_request_pipelines.first)
+              .to be_legacy_detached_merge_request_pipeline
+          end
         end
 
         context 'when ci_use_merge_request_ref feature flag is false' do