Mark merge request as preparing on create

When a merge request is created, we set the `merge_status` to
`preparing` so we don't check its mergeable or mark it as mergeable
until it's fully prepared.

This prevents race condition like asynchronous pipeline creation
which can lead to inconsistent behavior.

app/services/merge_requests/create_service.rb

@@ -14,6 +14,8 @@ module MergeRequests
     end
 
     def after_create(issuable)
+      issuable.mark_as_preparing
+
       # Add new items to MergeRequests::AfterCreateService if they can
       # be performed in Sidekiq
       NewMergeRequestWorker.perform_async(issuable.id, current_user.id)

changelogs/unreleased/291012-mark-mr-as-preparing.yml

+---
+title: Mark merge request as preparing on create
+merge_request: 56086
+author:
+type: other

spec/services/merge_requests/create_service_spec.rb

@@ -57,6 +57,10 @@ RSpec.describe MergeRequests::CreateService, :clean_gitlab_redis_shared_state do
         expect(Event.where(attributes).count).to eq(1)
       end
 
+      it 'sets the merge_status to preparing' do
+        expect(merge_request.reload).to be_preparing
+      end
+
       describe 'when marked with /wip' do
         context 'in title and in description' do
           let(:opts) do