sandboxlib: Allow None stdout/stderr to retain forwarding

[fishface60]

I couldn't work out a way to make sandboxlib let me keep my stdout/stderr otherwise.
I'm not a fan of changing the default behaviour of this, especially without a documented way to preserve it.

[ssssam]

There is a documented way to do this already, see: https://github.com/CodethinkLabs/sandboxlib/blob/master/sandboxlib/__init__.py#L100. It's hard to find because there's no documentation generated from this source yet.

I think it's bad design to make the user 'opt out' of inheriting the parent file descriptions -- it should be 'opt in' if we are trying to sandbox things. I know it's different from how subprocess.Popen() works and that needs to be clear.

[richardmaw-codethink]

On Wed, Nov 25, 2015 at 02:52:34AM -0800, Sam Thursfield wrote:

There is a documented way to do this already, see: https://github.com/CodethinkLabs/sandboxlib/blob/master/sandboxlib/__init__.py#L100. It's hard to find because there's no documentation generated from this source yet.

I think it's bad design to make the user 'opt out' of inheriting the parent file descriptions -- it should be 'opt in' if we are trying to sandbox things. I know it's different from how subprocess.Popen() works and that needs to be clear.

You already need to opt-into not inheriting file descriptors by virtue
of the fact that you have changed the default value to CAPTURE.

By forcing None to mean devnull this makes subprocess.Popen have
to do a lot more work to shuffle file descriptors,
and makes it more difficult to allow file descriptor inheritance for
reasons I don't find compelling.