BlockingFlag needs to provide an interface to synchronize mutex

[frsyuki]

This code has race condition:

while true
  do_something
  if blocking_flag.set?
    # here
    blocking_flag.wait_for_set
  end
end

because "here" does not acquire mutex. It should be like this:

while true
  do_something
  blocking_flag.synchronize do
    if blocking_flag.set?
      # here
      blocking_flag.wait_for_set
    end
  end
end

Thus BlockingFlag#synchronize(&block) or similar interface is necessary.