Expose merge request pipeline variables

Introduce the following variables

- CI_MERGE_REQUEST_ID
- CI_MERGE_REQUEST_IID
- CI_MERGE_REQUEST_REF_PATH
- CI_MERGE_REQUEST_PROJECT_ID
- CI_MERGE_REQUEST_PROJECT_PATH
- CI_MERGE_REQUEST_PROJECT_URL
- CI_MERGE_REQUEST_TARGET_BRANCH_NAME
- CI_MERGE_REQUEST_SOURCE_PROJECT_ID
- CI_MERGE_REQUEST_SOURCE_PROJECT_PATH
- CI_MERGE_REQUEST_SOURCE_PROJECT_URL
- CI_MERGE_REQUEST_SOURCE_BRANCH_NAME

app/models/ci/pipeline.rb

@@ -605,13 +605,18 @@ module Ci
     end
 
     def predefined_variables
-      Gitlab::Ci::Variables::Collection.new
-        .append(key: 'CI_PIPELINE_IID', value: iid.to_s)
-        .append(key: 'CI_CONFIG_PATH', value: ci_yaml_file_path)
-        .append(key: 'CI_PIPELINE_SOURCE', value: source.to_s)
-        .append(key: 'CI_COMMIT_MESSAGE', value: git_commit_message.to_s)
-        .append(key: 'CI_COMMIT_TITLE', value: git_commit_full_title.to_s)
-        .append(key: 'CI_COMMIT_DESCRIPTION', value: git_commit_description.to_s)
+      Gitlab::Ci::Variables::Collection.new.tap do |variables|
+        variables.append(key: 'CI_PIPELINE_IID', value: iid.to_s)
+        variables.append(key: 'CI_CONFIG_PATH', value: ci_yaml_file_path)
+        variables.append(key: 'CI_PIPELINE_SOURCE', value: source.to_s)
+        variables.append(key: 'CI_COMMIT_MESSAGE', value: git_commit_message.to_s)
+        variables.append(key: 'CI_COMMIT_TITLE', value: git_commit_full_title.to_s)
+        variables.append(key: 'CI_COMMIT_DESCRIPTION', value: git_commit_description.to_s)
+
+        if merge_request? && merge_request
+          variables.concat(merge_request.predefined_variables)
+        end
+      end
     end
 
     def queued_duration

app/models/merge_request.rb

@@ -1070,6 +1070,42 @@ class MergeRequest < ActiveRecord::Base
     actual_head_pipeline&.has_test_reports?
   end
 
+  def predefined_variables
+    Gitlab::Ci::Variables::Collection.new.tap do |variables|
+      variables.append(key: 'CI_MERGE_REQUEST_ID', value: id.to_s)
+      variables.append(key: 'CI_MERGE_REQUEST_IID', value: iid.to_s)
+
+      variables.append(key: 'CI_MERGE_REQUEST_REF_PATH',
+                       value: ref_path.to_s)
+
+      variables.append(key: 'CI_MERGE_REQUEST_PROJECT_ID',
+                       value: project.id.to_s)
+
+      variables.append(key: 'CI_MERGE_REQUEST_PROJECT_PATH',
+                       value: project.full_path)
+
+      variables.append(key: 'CI_MERGE_REQUEST_PROJECT_URL',
+                       value: project.web_url)
+
+      variables.append(key: 'CI_MERGE_REQUEST_TARGET_BRANCH_NAME',
+                       value: target_branch.to_s)
+
+      if source_project
+        variables.append(key: 'CI_MERGE_REQUEST_SOURCE_PROJECT_ID',
+                         value: source_project.id.to_s)
+
+        variables.append(key: 'CI_MERGE_REQUEST_SOURCE_PROJECT_PATH',
+                         value: source_project.full_path)
+
+        variables.append(key: 'CI_MERGE_REQUEST_SOURCE_PROJECT_URL',
+                         value: source_project.web_url)
+
+        variables.append(key: 'CI_MERGE_REQUEST_SOURCE_BRANCH_NAME',
+                         value: source_branch.to_s)
+      end
+    end
+  end
+
   # rubocop: disable CodeReuse/ServiceClass
   def compare_test_reports
     unless has_test_reports?

changelogs/unreleased/expose-mr-pipeline-variables.yml

+---
+title: Expose merge request pipeline variables
+merge_request: 23398
+author:
+type: changed

spec/models/ci/pipeline_spec.rb

@@ -350,6 +350,50 @@ describe Ci::Pipeline, :mailer do
                             CI_COMMIT_TITLE
                             CI_COMMIT_DESCRIPTION]
     end
+
+    context 'when source is merge request' do
+      let(:pipeline) do
+        create(:ci_pipeline, source: :merge_request, merge_request: merge_request)
+      end
+
+      let(:merge_request) do
+        create(:merge_request,
+               source_project: project,
+               source_branch: 'feature',
+               target_project: project,
+               target_branch: 'master')
+      end
+
+      it 'exposes merge request pipeline variables' do
+        expect(subject.to_hash)
+          .to include(
+            'CI_MERGE_REQUEST_ID' => merge_request.id.to_s,
+            'CI_MERGE_REQUEST_IID' => merge_request.iid.to_s,
+            'CI_MERGE_REQUEST_REF_PATH' => merge_request.ref_path.to_s,
+            'CI_MERGE_REQUEST_PROJECT_ID' => merge_request.project.id.to_s,
+            'CI_MERGE_REQUEST_PROJECT_PATH' => merge_request.project.full_path,
+            'CI_MERGE_REQUEST_PROJECT_URL' => merge_request.project.web_url,
+            'CI_MERGE_REQUEST_TARGET_BRANCH_NAME' => merge_request.target_branch.to_s,
+            'CI_MERGE_REQUEST_SOURCE_PROJECT_ID' => merge_request.source_project.id.to_s,
+            'CI_MERGE_REQUEST_SOURCE_PROJECT_PATH' => merge_request.source_project.full_path,
+            'CI_MERGE_REQUEST_SOURCE_PROJECT_URL' => merge_request.source_project.web_url,
+            'CI_MERGE_REQUEST_SOURCE_BRANCH_NAME' => merge_request.source_branch.to_s)
+      end
+
+      context 'when source project does not exist' do
+        before do
+          merge_request.update_column(:source_project_id, nil)
+        end
+
+        it 'does not expose source project related variables' do
+          expect(subject.to_hash.keys).not_to include(
+            %w[CI_MERGE_REQUEST_SOURCE_PROJECT_ID
+               CI_MERGE_REQUEST_SOURCE_PROJECT_PATH
+               CI_MERGE_REQUEST_SOURCE_PROJECT_URL
+               CI_MERGE_REQUEST_SOURCE_BRANCH_NAME])
+        end
+      end
+    end
   end
 
   describe '#protected_ref?' do