The cause of the problem is that after query cancel, OlapScanNode::transfer_thread still continues to schedule
OlapScanNode::scanner_thread until all tasks are scheduled.
Although each task does not scan data and exits quickly, it still consumes a lot of resources.
(Guess)This may be the cause of the BUG (#5767) causing the I/O to be full.
So after query cancel, immediately exit the scheduling loop in transfer_thread, and after waiting for
the end of all scanner_threads, transfer_thread will also exit.