When slave support some operations even read-only statement may hold some locks (read will hold locks when isolation level is serializable). So sql thread and worker threads may get deadlock with user threads and have chances to be the victim which would lead to a break between master and slave.

Fix :
We assume each thread have a priority, the priority will decide who is the victim rather than thread's weight and the system threads have high priority(slave threads for now).

In the attach file, a testcase script can reproduce the problem easily and a diff file to show the code to fix it.