The test has race condition, which is addressed by the patch.
The race is next:
Master thread is calling execute (effectively blocked, waiting for
callback to be called)
"Async" thread picks up the callback
Call the callback
Then sets the variables in test
After call back is called, master thread is unblocked and may check
the variables (set in point 4 by the "async" thread) earlier then they
actually changed
Changes:
callback should be called as the last step (after flag variables are
set), as it effectively unblock the master thread
fixes#18974
eb6d8e6af2