subprocess's Popen.wait() is now thread safe so that multiple threads
may be calling wait() or poll() on a Popen instance at the same time
without losing the Popen.returncode value.  Fixes issue #21291.
subprocess's Popen.wait() is now thread safe so that multiple threads
may be calling wait() or poll() on a Popen instance at the same time
without losing the Popen.returncode value.  Fixes issue #21291.
Add implementation notes