subprocess triggers a debug assertion in the CRT on failure to execute the process due to closing the pipe *handles* in the except clause using os.close rather than .Close() (os.close closes CRT file descriptors and not handles).

In addition to that once this is fixed there is also a double free/close since we need to set `self._closed_child_pipe_fds = True` once we closed the handles in _execute_child so they won't also be closed in __init__.

To reproduce, do this in a debug build of Python:

    import subprocess
    subprocess.Popen('exe_that_doesnt_exist.exe', stdout=subprocess.PIPE)

See: https://github.com/python/cpython/pull/1218#discussion_r112550959

I determined in #31228 (also on Windows, closed as a duplicate) that a debug build, bad file, and subprocess.PIPE are all required.  Has this been tried on non-Windows?  I confirmed the crash on 3.6.  I do not have a 2.7 repository build.

The two lines above constitute a unittest that currently fails  on a Windows debug build.  Even if there is no such buildbot, I think a test should be added that will fail on developer machines with such builds.  It should be skipped on non-debug and perhaps non-Windows machines.

See https://github.com/python/cpython/pull/1224 and http://bugs.python.org/issue30121

New changeset 4d3851727fb82195e4995c6064b0b2d6cbc031c4 by Victor Stinner (Segev Finer) in branch 'master':
bpo-30121: Fix debug assert in subprocess on Windows (#1224)
https://github.com/python/cpython/commit/4d3851727fb82195e4995c6064b0b2d6cbc031c4

New changeset 9a83f651f31b47b3f6c8b210f7807b26e8c373a5 by Victor Stinner in branch 'master':
Add test_subprocess.test_nonexisting_with_pipes() (#3133)
https://github.com/python/cpython/commit/9a83f651f31b47b3f6c8b210f7807b26e8c373a5

New changeset e76cb435563cd14bb085942dfefbb469b8f40aa9 by Victor Stinner in branch '3.6':
[3.6] bpo-30121: Fix debug assert in subprocess on Windows (#1224) (#3173)
https://github.com/python/cpython/commit/e76cb435563cd14bb085942dfefbb469b8f40aa9

Python 2.7 doesn't seem to be affected by this issue. Extract of Popen.__init__:

        try:
            self._execute_child(args, executable, preexec_fn, close_fds,
                                cwd, env, universal_newlines,
                                startupinfo, creationflags, shell, to_close,
                                p2cread, p2cwrite,
                                c2pread, c2pwrite,
                                errread, errwrite)
        except Exception:
            # Preserve original exception in case os.close raises.
            exc_type, exc_value, exc_trace = sys.exc_info()

            for fd in to_close:
                try:
                    if mswindows:
                        fd.Close()
                    else:
                        os.close(fd)
                except EnvironmentError:
                    pass

On Windows, fd.Close() is always used.

Oops, I merged the pull requests, but I forgot to close the issue.

> Oops, I merged the pull requests, but I forgot to close the issue.

Oops, I forgot to close the issue, again... Thanks Segev for closing it, finally :-)

New changeset 5537646bfacec463b450871dde31cb06c44a0556 by Zachary Ware in branch 'master':
bpo-30121: Fix test_subprocess for Windows Debug builds (GH-5758)
https://github.com/python/cpython/commit/5537646bfacec463b450871dde31cb06c44a0556

New changeset ef0bb5c7b76a49a5f3c5b85b5f9112cfefe54328 by Miss Islington (bot) in branch '3.6':
bpo-30121: Fix test_subprocess for Windows Debug builds (GH-5758)
https://github.com/python/cpython/commit/ef0bb5c7b76a49a5f3c5b85b5f9112cfefe54328

New changeset 622a824802771fc5aa133ae92101bc8303360294 by Miss Islington (bot) in branch '3.7':
bpo-30121: Fix test_subprocess for Windows Debug builds (GH-5758)
https://github.com/python/cpython/commit/622a824802771fc5aa133ae92101bc8303360294