[Python-Dev] Issue 13524: subprocess on Windows (original) (raw)

Tim Golden mail at timgolden.me.uk
Sun Dec 4 11:59:13 CET 2011

http://bugs.python.org/issue13524

Someone raised issue13524 yesterday to illustrate that a subprocess will crash immediately if an environment block is passed which does not contain a valid SystemRoot environment variable.

Note that the calling (Python) process is unaffected; this isn't - strictly - a Python crash. The issue is essentially a Windows one where a fairly unusual cornercase -- passing an empty environment -- has unforseen effects.

The smallest reproducible example is this:

import os, sys import subprocess subprocess.Popen( [sys.executable], env={} )

and it can be prevented like this:

import os, sys import subprocess subprocess.Popen( [sys.executable], env={"SystemRoot" : os.environ['SystemRoot']} )

There's a blog post here which gives a worked example:

http://jpassing.com/2009/12/28/the-hidden-danger-of-forgetting-to-specify-systemroot-in-a-custom-environment-block/

but as the author points out, nowhere on MSDN is there a warning that SystemRoot is mandatory. (And, in effect, it's not as it would just be possible to write code which had no need of it).

So... what's our take on this? As I see it we could:

  1. Do nothing: it's the caller's responsibility to understand the complications of the chosen Operating System.

  2. Add a doc warning (ironically, considering the recent to-and-fro on doc warnings in this very module).

  3. Add a check into the subprocess.Popen code which would raise some exception if the environment block is empty (or doesn't contain SystemRoot) on the grounds that this probably wasn't what the user thought they were doing.

  4. Automatically add an entry for SystemRoot to the env block if it's not present already.

It's tempting to opt for (1) and if we were exposing an API called CreateProcess which mimicked the underlying Windows API I would be inclined to go that way. But we're abstracting a little bit away from that and I think that that layer of abstraction carries its own responsibilities.

Option (3) seems to give the best balance. It is a cornercase, but at the same time it's easy to misunderstand that the env block you're passing in replaces rather than augments that of the current process.

Thoughts?

TJG

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