Fix ProcessCommitWorker when upstream project is deleted

app/workers/process_commit_worker.rb

@@ -79,9 +79,10 @@ class ProcessCommitWorker
   # Avoid reprocessing commits that already exist in the upstream
   # when project is forked. This will also prevent duplicated system notes.
   def commit_exists_in_upstream?(project, commit_hash)
-    return false unless project.forked?
+    upstream_project = project.fork_source
+
+    return false unless upstream_project
 
-    upstream_project = project.forked_from_project
     commit_id = commit_hash.with_indifferent_access[:id]
     upstream_project.commit(commit_id).present?
   end

changelogs/unreleased/45592-nomethoderror-undefined-method-commit-for-nil-nilclass-in-sidekiq-caused-by-a-merge-commit-in-a-forked-project-with-upstream-project-deleted.yml

+---
+title: Process commits as normal in forks when the upstream project is deleted
+merge_request: 20534
+author:
+type: fixed

spec/workers/process_commit_worker_spec.rb

 require 'spec_helper'
 
 describe ProcessCommitWorker do
+  include ProjectForksHelper
+
   let(:worker) { described_class.new }
   let(:user) { create(:user) }
   let(:project) { create(:project, :public, :repository) }
@@ -32,17 +34,43 @@ describe ProcessCommitWorker do
       worker.perform(project.id, user.id, commit.to_hash)
     end
 
-    context 'when commit already exists in upstream project' do
-      let(:forked) { create(:project, :public, :repository) }
-
-      it 'does not process commit message' do
-        create(:forked_project_link, forked_to_project: forked, forked_from_project: project)
+    context 'when the project is forked' do
+      context 'when commit already exists in the upstream project' do
+        it 'does not process the commit message' do
+          forked = fork_project(project, user, repository: true)
 
           expect(worker).not_to receive(:process_commit_message)
 
           worker.perform(forked.id, user.id, forked.commit.to_hash)
         end
       end
+
+      context 'when the commit does not exist in the upstream project' do
+        it 'processes the commit message' do
+          empty_project = create(:project, :public)
+          forked = fork_project(empty_project, user, repository: true)
+
+          TestEnv.copy_repo(forked,
+                            bare_repo: TestEnv.factory_repo_path_bare,
+                            refs: TestEnv::BRANCH_SHA)
+
+          expect(worker).to receive(:process_commit_message)
+
+          worker.perform(forked.id, user.id, forked.commit.to_hash)
+        end
+      end
+
+      context 'when the upstream project no longer exists' do
+        it 'processes the commit message' do
+          forked = fork_project(project, user, repository: true)
+          project.destroy!
+
+          expect(worker).to receive(:process_commit_message)
+
+          worker.perform(forked.id, user.id, forked.commit.to_hash)
+        end
+      end
+    end
   end
 
   describe '#process_commit_message' do