subprocess.Popen uses _winapi.CreateProcess() to spawn a child process. It stores the process handle using a Handle class. Popen never explicitly releases the handle, it rely on Handle.__del__ when the Popen object is detroyed (when Popen._handle attribute is cleared).

As discussed in bpo-37380, we could call explicitly CloseHandle(handle) as soon as the child process completes, to release resources in the Windows kernel.

Attached PR implements this fix.

IMHO it's safe to backport PR 14391 to Python 3.7 and 3.8.

See also bpo-37380: subprocess.Popen._cleanup() "The handle is invalid" error when some old process is gone.

We may backport the change to Python 2.7 as well, but in this case terminate() must be changed to test returncode:

        def terminate(self):
            """Terminates the process."""
            # Don't terminate a process that we know has already died.
            if self.returncode is not None:
                return
            ...

terminate() was changed by:

commit a0c9caad66f01328155177180df1c46fe7c62e57
Author: Gregory P. Smith <greg@krypto.org>
Date:   Sun Nov 15 18:19:10 2015 -0800

    Fix issue #6973: When we know a subprocess.Popen process has died, do
    not allow the send_signal(), terminate(), or kill() methods to do
    anything as they could potentially signal a different process.