Fix MR pipelines run on only: refex

lib/gitlab/ci/build/policy/refs.rb

@@ -32,10 +32,14 @@ module Gitlab
             return true if pipeline.source == pattern
             return true if pipeline.source&.pluralize == pattern
 
-            if pattern.first == "/" && pattern.last == "/"
-              Regexp.new(pattern[1...-1]) =~ pipeline.ref
-            else
-              pattern == pipeline.ref
+            # patterns can be matched only when branch or tag is used
+            # the pattern matching does not work for merge requests pipelines
+            if pipeline.branch? || pipeline.tag?
+              if pattern.first == "/" && pattern.last == "/"
+                Regexp.new(pattern[1...-1]) =~ pipeline.ref
+              else
+                pattern == pipeline.ref
+              end
             end
           end
         end

spec/services/ci/create_pipeline_service_spec.rb

@@ -810,6 +810,64 @@ describe Ci::CreatePipelineService do
             end
           end
         end
+
+        context "when config uses regular expression for only keyword" do
+          let(:config) do
+            {
+              build: {
+                stage: 'build',
+                script: 'echo',
+                only: ["/^#{ref_name}$/"]
+              }
+            }
+          end
+
+          context 'when merge request is specified' do
+            let(:merge_request) do
+              create(:merge_request,
+                source_project: project,
+                source_branch: ref_name,
+                target_project: project,
+                target_branch: 'master')
+            end
+
+            it 'does not create a merge request pipeline' do
+              expect(pipeline).not_to be_persisted
+
+              expect(pipeline.errors[:base])
+                .to eq(['No stages / jobs for this pipeline.'])
+            end
+          end
+        end
+
+        context "when config has 'except: [tags]'" do
+          let(:config) do
+            {
+              build: {
+                stage: 'build',
+                script: 'echo',
+                except: ['tags']
+              }
+            }
+          end
+
+          context 'when merge request is specified' do
+            let(:merge_request) do
+              create(:merge_request,
+                source_project: project,
+                source_branch: ref_name,
+                target_project: project,
+                target_branch: 'master')
+            end
+
+            it 'does not create a merge request pipeline' do
+              expect(pipeline).not_to be_persisted
+
+              expect(pipeline.errors[:base])
+                .to eq(['No stages / jobs for this pipeline.'])
+            end
+          end
+        end
       end
 
       context 'when source is web' do