title: running subprocess.Popen under apache+mod_wsgi is always returning an error with a returncode of -6
body: <p>I'm hoping someone's seen this - </p>
  
  <p>I'm running django-compressor, taking advantage of the lessc setup to render/compress the less into CSS on the file. It works perfectly when invoked from the development server, but when run underneath apache+mod_wsgi it is consistently returning an error.</p>
  
  <p>To debug this, I have run the exact command that the filter is invoking as the www-data user (which is defined as the wsgi user in the WSGIDaemonProcess directive) and verified that it works correctly, including permissions to read and write the files that it's manipulating.</p>
  
  <p>I have also hacked on the django-compressor code in compressor/filters/base.py on that system, and it seems that ANY command attempting to get invoked is getting a returncode of -6 after the proc.communicate() invocation.</p>
  
  <p>I'm hoping someone's seen this before - or that it rings some bell. It works fine on this machine outside of the apache+mod_wsgi process (i.e. running the process as a dev server) as well. I'm just not clear on what might be blocking the subprocess.Popen() invocations.</p>