swarming: use df's -l argument

appengine/swarming/swarming_bot/api/platforms/posix.py

 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """POSIX specific utility functions."""
 
 import os
 import subprocess
 import sys
 
 
 def _run_df():
   """Runs df and returns the output."""
   proc = subprocess.Popen(
-      ['/bin/df', '-k', '-P'], env={'LANG': 'C'},
+      ['/bin/df', '-k', '-P', '-l'], env={'LANG': 'C'},
       stdout=subprocess.PIPE, stderr=subprocess.PIPE)
   for l in proc.communicate()[0].splitlines():
     l = l.decode('utf-8')
     if l.startswith(u'/dev/'):
       items = l.split()
       if (sys.platform == 'darwin' and
           items[5].startswith(u'/Volumes/firmwaresyncd.')):
         # There's an issue on OSX where sometimes a small volume is mounted
         # during boot time and may be caught here by accident. Just ignore it as
         # it could trigger the low free disk space check and cause an unexpected
@@ skipping 18 matching lines @@
       # Do not use the value reported by 'df' since it includes all the free
       # space, including the free space reserved by root. Since the Swarming bot
       # is likely not running as root, it present an inflated value of what is
       # usable.
       #u'free_mb': round(float(items[3]) / 1024., 1),
       u'free_mb': get_free_space_mb(items[5]),
       u'size_mb': round(float(items[1]) / 1024., 1),
     }
     for items in _run_df()
   }