Fix rebasing when upstream project has protected branches

We pass the service's `project` as an env var when pushing, so that we can do
access checks. As we are pushing to the source project, that should be our
context for rebasing.

app/workers/rebase_worker.rb

@@ -7,7 +7,8 @@ class RebaseWorker
     current_user = User.find(current_user_id)
     merge_request = MergeRequest.find(merge_request_id)
 
-    MergeRequests::RebaseService.new(merge_request.target_project, current_user)
+    MergeRequests::RebaseService
+      .new(merge_request.source_project, current_user)
       .execute(merge_request)
   end
 end

changelogs/unreleased-ee/3071-rebase-not-working-with-forked-projects-in-9-4-2.yml

+---
+title: Fix rebase from fork when upstream has protected branches
+merge_request:
+author:

spec/workers/rebase_worker_spec.rb

+require 'spec_helper'
+
+describe RebaseWorker, '#perform' do
+  context 'when rebasing an MR from a fork where upstream has protected branches' do
+    let(:upstream_project) { create(:project, :repository) }
+    let(:fork_project) { create(:project, :repository) }
+
+    let(:merge_request) do
+      create(:merge_request,
+             source_project: fork_project,
+             source_branch: 'feature_conflict',
+             target_project: upstream_project,
+             target_branch: 'master')
+    end
+
+    before do
+      create(:forked_project_link, forked_to_project: fork_project, forked_from_project: upstream_project)
+    end
+
+    it 'sets the correct project for running hooks' do
+      expect(MergeRequests::RebaseService)
+        .to receive(:new).with(fork_project, merge_request.author).and_call_original
+
+      subject.perform(merge_request, merge_request.author)
+    end
+  end
+end