Okay, dumb question: Is there a reason the Windows code explicitly initializes c2pwrite in the "stdout not passed" case, while the Linux code leaves it as -1? Windows doesn't look like it would have the problem (because c2pwrite is always set to a non-default value), and it seems like the fix for Linux could just mimic the Windows approach; the code that sets errwrite wouldn't change, but instead of a "pass", when stdout is None, we'd explicitly set it to os.STDOUT_FILENO, and the stderr=subprocess.STDOUT (stdout unset) case would work automatically, and the code would be more similar.

Haven't explored the negative consequences of that change, if any.