Fix the problem of pipelines changes with external PRs [RUN ALL RSPEC] [RUN AS-IF-FOSS]

app/models/ci/pipeline.rb

@@ -1093,6 +1093,8 @@ module Ci
           merge_request.modified_paths
         elsif branch_updated?
           push_details.modified_paths
+        elsif external_pull_request? && ::Feature.enabled?(:ci_modified_paths_of_external_prs, project, default_enabled: :yaml)
+          external_pull_request.modified_paths
         end
       end
     end
@@ -1117,6 +1119,10 @@ module Ci
       merge_request_id.present?
     end
 
+    def external_pull_request?
+      external_pull_request_id.present?
+    end
+
     def detached_merge_request_pipeline?
       merge_request? && target_sha.nil?
     end

app/models/external_pull_request.rb

@@ -72,6 +72,10 @@ class ExternalPullRequest < ApplicationRecord
     end
   end
 
+  def modified_paths
+    project.repository.diff_stats(target_sha, source_sha).paths
+  end
+
   private
 
   def actual_source_branch_sha

config/feature_flags/development/ci_modified_paths_of_external_prs.yml

+---
+name: ci_modified_paths_of_external_prs
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60736
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330605
+milestone: '13.12'
+type: development
+group: group::pipeline authoring
+default_enabled: false

spec/models/ci/pipeline_spec.rb

@@ -1939,6 +1939,30 @@ RSpec.describe Ci::Pipeline, :mailer, factory_default: :keep do
         expect(pipeline.modified_paths).to match(merge_request.modified_paths)
       end
     end
+
+    context 'when source is an external pull request' do
+      let(:pipeline) do
+        create(:ci_pipeline, source: :external_pull_request_event, external_pull_request: external_pull_request)
+      end
+
+      let(:external_pull_request) do
+        create(:external_pull_request, project: project, target_sha: '281d3a7', source_sha: '498214d')
+      end
+
+      it 'returns external pull request modified paths' do
+        expect(pipeline.modified_paths).to match(external_pull_request.modified_paths)
+      end
+
+      context 'when the FF ci_modified_paths_of_external_prs is disabled' do
+        before do
+          stub_feature_flags(ci_modified_paths_of_external_prs: false)
+        end
+
+        it 'returns nil' do
+          expect(pipeline.modified_paths).to be_nil
+        end
+      end
+    end
   end
 
   describe '#all_worktree_paths' do

spec/models/external_pull_request_spec.rb

@@ -3,7 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe ExternalPullRequest do
-  let(:project) { create(:project) }
+  let_it_be(:project) { create(:project, :repository) }
+
   let(:source_branch) { 'the-branch' }
   let(:status) { :open }
 
@@ -217,4 +218,18 @@ RSpec.describe ExternalPullRequest do
       expect(pull_request).not_to be_from_fork
     end
   end
+
+  describe '#modified_paths' do
+    let(:pull_request) do
+      build(:external_pull_request, project: project, target_sha: '281d3a7', source_sha: '498214d')
+    end
+
+    subject(:modified_paths) { pull_request.modified_paths }
+
+    it 'returns modified paths' do
+      expect(modified_paths).to eq ['bar/branch-test.txt',
+                                    'files/js/commit.coffee',
+                                    'with space/README.md']
+    end
+  end
 end