When FE send cancel rpc to BE, it does not notify the wait_for_start() thread, so that the fragment will be blocked and occupy the execution thread. Add a max wait time for wait_for_start() thread. So that it will not block forever.
When FE send cancel rpc to BE, it does not notify the wait_for_start() thread, so that the fragment will be blocked and occupy the execution thread. Add a max wait time for wait_for_start() thread. So that it will not block forever.