class ActiveRecord::ConnectionAdapters::ConnectionPool

def checkin(conn)

calling #checkout on this pool.
+conn+: an AbstractAdapter object, which was obtained by earlier by

no longer need this connection.
Check-in a database connection back into the pool, indicating that you 
def checkin(conn)
  conn.lock.synchronize do
    synchronize do
      remove_connection_from_thread_cache conn
      conn._run_checkin_callbacks do
        conn.expire
      end
      @available.add conn
    end
  end
end