Expose merge request diff variables

Introduce the following variables:
 - CI_MERGE_REQUEST_DIFF_ID
 - CI_MERGE_REQUEST_DIFF_BASE_SHA
behind a feature flag, ci_mr_diff_variables.

For testing, remove the context 'when source project does not exist'
because the factory creates the pipeline in the source project. Also
differentiate between detached and merged results pipelines.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/273387

app/models/ci/pipeline.rb

@@ -774,6 +774,15 @@ module Ci
           variables.append(key: 'CI_MERGE_REQUEST_EVENT_TYPE', value: merge_request_event_type.to_s)
           variables.append(key: 'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA', value: source_sha.to_s)
           variables.append(key: 'CI_MERGE_REQUEST_TARGET_BRANCH_SHA', value: target_sha.to_s)
+
+          if Feature.enabled?(:ci_mr_diff_variables, project)
+            diff = self.merge_request_diff
+            if diff.present?
+              variables.append(key: 'CI_MERGE_REQUEST_DIFF_ID', value: diff.id.to_s)
+              variables.append(key: 'CI_MERGE_REQUEST_DIFF_BASE_SHA', value: diff.base_commit_sha)
+            end
+          end
+
           variables.concat(merge_request.predefined_variables)
         end
 
@@ -1131,6 +1140,22 @@ module Ci
       Gitlab::DataBuilder::Pipeline.build(self)
     end
 
+    def merge_request_diff_sha
+      return unless merge_request?
+
+      if merge_request_pipeline?
+        source_sha
+      else
+        sha
+      end
+    end
+
+    def merge_request_diff
+      return unless merge_request?
+
+      merge_request.merge_request_diff_for(merge_request_diff_sha)
+    end
+
     def push_details
       strong_memoize(:push_details) do
         Gitlab::Git::Push.new(project, before_sha, sha, git_ref)

config/feature_flags/development/ci_mr_diff_variables.yml

+---
+name: ci_mr_diff_variables
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46621
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/285468
+milestone: '13.7'
+type: development
+group: group::pipeline authoring
+default_enabled: false

spec/models/ci/pipeline_spec.rb

@@ -745,8 +745,12 @@ RSpec.describe Ci::Pipeline, :mailer, factory_default: :keep do
     end
 
     context 'when pipeline is merge request' do
+      before do
+        stub_feature_flags(ci_mr_diff_variables: false)
+      end
+
       let(:pipeline) do
-        create(:ci_pipeline, merge_request: merge_request)
+        create(:ci_pipeline, :detached_merge_request_pipeline, merge_request: merge_request)
       end
 
       let(:merge_request) do
@@ -774,30 +778,32 @@ RSpec.describe Ci::Pipeline, :mailer, factory_default: :keep do
             '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_TARGET_BRANCH_SHA' => pipeline.target_sha.to_s,
+            'CI_MERGE_REQUEST_TARGET_BRANCH_SHA' => '',
             '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,
-            'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA' => pipeline.source_sha.to_s,
+            'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA' => '',
             'CI_MERGE_REQUEST_TITLE' => merge_request.title,
             'CI_MERGE_REQUEST_ASSIGNEES' => merge_request.assignee_username_list,
             'CI_MERGE_REQUEST_MILESTONE' => milestone.title,
             'CI_MERGE_REQUEST_LABELS' => labels.map(&:title).sort.join(','),
-            'CI_MERGE_REQUEST_EVENT_TYPE' => pipeline.merge_request_event_type.to_s)
+            'CI_MERGE_REQUEST_EVENT_TYPE' => 'detached')
+        expect(subject.to_hash.keys).not_to include(
+          %w[CI_MERGE_REQUEST_DIFF_ID
+             CI_MERGE_REQUEST_DIFF_BASE_SHA])
       end
 
-      context 'when source project does not exist' do
+      context 'when feature flag ci_mr_diff_variables is enabled' do
         before do
-          merge_request.update_column(:source_project_id, nil)
+          stub_feature_flags(ci_mr_diff_variables: true)
         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])
+        it 'exposes diff variables' do
+          expect(subject.to_hash)
+            .to include(
+              'CI_MERGE_REQUEST_DIFF_ID' => merge_request.merge_request_diff.id.to_s,
+              'CI_MERGE_REQUEST_DIFF_BASE_SHA' => merge_request.merge_request_diff.base_commit_sha)
         end
       end
 
@@ -824,6 +830,51 @@ RSpec.describe Ci::Pipeline, :mailer, factory_default: :keep do
           expect(subject.to_hash.keys).not_to include('CI_MERGE_REQUEST_LABELS')
         end
       end
+
+      context 'with merged results' do
+        let(:pipeline) do
+          create(:ci_pipeline, :merged_result_pipeline, merge_request: merge_request)
+        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_TARGET_BRANCH_SHA' => merge_request.target_branch_sha,
+              '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,
+              'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA' => merge_request.source_branch_sha,
+              'CI_MERGE_REQUEST_TITLE' => merge_request.title,
+              'CI_MERGE_REQUEST_ASSIGNEES' => merge_request.assignee_username_list,
+              'CI_MERGE_REQUEST_MILESTONE' => milestone.title,
+              'CI_MERGE_REQUEST_LABELS' => labels.map(&:title).sort.join(','),
+              'CI_MERGE_REQUEST_EVENT_TYPE' => 'merged_result')
+          expect(subject.to_hash.keys).not_to include(
+            %w[CI_MERGE_REQUEST_DIFF_ID
+               CI_MERGE_REQUEST_DIFF_BASE_SHA])
+        end
+
+        context 'when feature flag ci_mr_diff_variables is enabled' do
+          before do
+            stub_feature_flags(ci_mr_diff_variables: true)
+          end
+
+          it 'exposes diff variables' do
+            expect(subject.to_hash)
+              .to include(
+                'CI_MERGE_REQUEST_DIFF_ID' => merge_request.merge_request_diff.id.to_s,
+                'CI_MERGE_REQUEST_DIFF_BASE_SHA' => merge_request.merge_request_diff.base_commit_sha)
+          end
+        end
+      end
     end
 
     context 'when source is external pull request' do