Write a spec covering the race condition during group deletion.

- Use multiple threads / database connections to:

  1. Escape the transaction the spec seems to be running
     in (`config.use_transactional_fixtures` is off, but
     `ActiveRecord::Base.connection.open_transactions` is not empty
     at the beginning of the spec.

  2. Simulate a Sidekiq worker performing the hard delete outside of the
     soft-delete transaction.

- The spec is a little clunky, but it was the smallest thing I could get
  working - and even this took a couple of hours. Let me know if you
  have any suggestions to improve it!

spec/services/destroy_group_service_spec.rb

 require 'spec_helper'
 
 describe DestroyGroupService, services: true do
+  include DatabaseConnectionHelpers
+
   let!(:user) { create(:user) }
   let!(:group) { create(:group) }
   let!(:project) { create(:project, namespace: group) }
@@ -50,6 +52,44 @@ describe DestroyGroupService, services: true do
 
   describe 'asynchronous delete' do
     it_behaves_like 'group destruction', true
+
+    context 'potential race conditions' do
+      context "when the `GroupDestroyWorker` task runs immediately" do
+        it "deletes the group" do
+          # Commit the contents of this spec's transaction so far
+          # so subsequent db connections can see it.
+          #
+          # DO NOT REMOVE THIS LINE, even if you see a WARNING with "No
+          # transaction is currently in progress". Without this, this
+          # spec will always be green, since the group created in setup
+          # cannot be seen by any other connections / threads in this spec.
+          Group.connection.commit_db_transaction
+
+          group_record = run_with_new_database_connection do |conn|
+            conn.execute("SELECT * FROM namespaces WHERE id = #{group.id}").first
+          end
+
+          expect(group_record).not_to be_nil
+
+          # Execute the contents of `GroupDestroyWorker` in a separate thread, to
+          # simulate data manipulation by the Sidekiq worker (different database
+          # connection / transaction).
+          expect(GroupDestroyWorker).to receive(:perform_async).and_wrap_original do |m, group_id, user_id|
+            Thread.new { m[group_id, user_id] }.join(5)
+          end
+
+          # Kick off the initial group destroy in a new thread, so that
+          # it doesn't share this spec's database transaction.
+          Thread.new { DestroyGroupService.new(group, user).async_execute }.join(5)
+
+          group_record = run_with_new_database_connection do |conn|
+            conn.execute("SELECT * FROM namespaces WHERE id = #{group.id}").first
+          end
+
+          expect(group_record).to be_nil
+        end
+      end
+    end
   end
 
   describe 'synchronous delete' do

spec/support/database_connection_helpers.rb

+module DatabaseConnectionHelpers
+  def run_with_new_database_connection
+    pool = ActiveRecord::Base.connection_pool
+    conn = pool.checkout
+    yield conn
+  ensure
+    pool.checkin(conn)
+  end
+end