reading from /sys/class/net/eth0/carriers raises IOError if interface is down

[Lars Kellogg-Stedman]

If an interface is "down", reading from several of the sysfs files for that interface will return EINVAL, which causes Python to raise an IOError like this:

Dec 02 18:11:09 citest.novalocal cloud-init[413]: [CLOUDINIT] util.py[DEBUG]: failed stage init-local
                                                  Traceback (most recent call last):
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/cmd/main.py", line 521, in status_wr
apper
                                                      ret = functor(name, args)
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/cmd/main.py", line 265, in main_init
                                                      init.apply_network_config(bring_up=not args.local)
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/stages.py", line 631, in apply_netwo
rk_config
                                                      netcfg, src = self._find_networking_config()
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/stages.py", line 628, in _find_netwo
rking_config
                                                      return (net.generate_fallback_config(), "fallback")
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/net/__init__.py", line 146, in gener
ate_fallback_config
                                                      carrier = int(sys_netdev_info(interface, 'carrier'))
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/net/__init__.py", line 119, in sys_n
etdev_info
                                                      data = util.load_file(fname)
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/util.py", line 1268, in load_file
                                                      pipe_in_out(ifh, ofh, chunk_cb=read_cb)
                                                    File "/usr/lib/python2.7/site-packages/cloudinit/util.py", line 1311, in pipe_in_out
                                                      data = in_fh.read(chunk_size)
                                                  IOError: [Errno 22] Invalid argument

The code calling sys_netdev_info should catch IOError as well as OSError.

A simple reproducer is:

  # ip link set eth0 down
  # cat /sys/class/net/eth0/carrier
  cat: /sys/class/net/eth0/carrier: Invalid argument