On 12/4/2011 5:59 AM, Tim Golden wrote:
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?
My inclination would be #4 on Windows, certainly for 3.3, unless there
is a clear reason not to.
For 2.7/3.2, at least say (not warn, just say) in the doc that that a
subprocess on Windows may require that SystemRoot be set.
The blog post says the problem is worse on Win 7. So it is not going away.
The blog post has a comment from Martin Loewis a year ago linking to
http://mail.python.org/pipermail/python-dev/2010-November/105866.html
That thread refers to a bug that was not posted on the tracker. This
makes at least three (including #3440).
--
Terry Jan Reedy
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