Resource warnings

When running pytest with xdist I get a bunch of ResourceWarnings. To reproduce:

touch empty.py
pytest -n 2 empty.py

which produces

=============================================================== test session starts ================================================================
platform linux -- Python 3.6.1, pytest-3.2.5, py-1.5.2, pluggy-0.4.0
rootdir: /home/jgosmann, inifile:
plugins: xdist-1.20.1, forked-0.2
gw0 [0] / gw1 [0]
scheduling tests via LoadScheduling
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=0 mode='r' encoding='UTF-8'>
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=1 mode='w' encoding='UTF-8'>
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=0 mode='r' encoding='UTF-8'>
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=1 mode='w' encoding='UTF-8'>

=========================================================== no tests ran in 0.33 seconds ===========================================================

wheras

pytest empty.py

produces no warnings

=============================================================== test session starts ================================================================
platform linux -- Python 3.6.1, pytest-3.2.5, py-1.5.2, pluggy-0.4.0
rootdir: /home/jgosmann, inifile:
plugins: xdist-1.20.1, forked-0.2
collected 0 items                                                                                                                                   

=========================================================== no tests ran in 0.00 seconds ===========================================================

Relevant version numbers are listed in the console output.

Start by running the reported pytest -n 2 empty.py command and compare it with pytest empty.py using the versions shown in the issue. Trace the xdist execution path responsible for the warnings; done means the parallel command no longer emits the unclosed-file ResourceWarnings while the no-test run still completes normally.