The root of the problem is that to interrupt a slave SQL thread
wait, the STOP SLAVE implementation uses thd->awake(THD::NOT_KILLED).
This appears as a spurious wakeup (e.g. from a sleep on a
condition variable) to the code that the slave SQL thread is
executing at the time of the STOP. If the code is not written
to be spurious-wakeup safe, unexpected behavior can occur. For
the reported case, this problem led to an infinite loop around
the interruptible_wait() function in item_func.cc (SLEEP()
function implementation).  The loop was not being properly
restarted and, consequently, would not come to an end. Since the
SLEEP function sleeps on a timed event in order to be killable
and to perform periodic checks until the requested time has
elapsed, the spurious wake up was causing the requested sleep
time to be reset every two seconds.

The solution is to calculate the requested absolute time only
once and to ensure that the thread only sleeps until this
time is elapsed. In case of a spurious wake up, the sleep is
restarted using the previously calculated absolute time. This
restores the behavior present in previous releases. If a slave
thread is executing a SLEEP function, a STOP SLAVE statement
will wait until the time requested in the sleep function
has elapsed.

mysql-test/extra/rpl_tests/rpl_start_stop_slave.test:
  Add test case for Bug#56096.
mysql-test/suite/rpl/r/rpl_stm_start_stop_slave.result:
  Add test case result for Bug#56096.
sql/item_func.cc:
  Reorganize interruptible_wait into a class so that the absolute
  time can be preserved across calls to the wait function. This
  allows the sleep to be properly restarted in the presence of
  spurious wake ups, including those generated by a STOP SLAVE.