Our condition variable emulation under Windows is imperfect, which seems to be the cause of the buildbot hangs. Try to fix it, and add some comments.
Our condition variable emulation under Windows is imperfect, which
seems to be the cause of the buildbot hangs. Try to fix it, and add
some comments.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator