nicely report any errors involving finding and using ifconfig

[zooko]

A couple of users have recently ​reported errors from ​iputil when it couldn't find their ifconfig executable, e.g.:

[Failure instance: Traceback: <type 'exceptions.OSError'>: [Errno 2] No such file or directory
/usr/lib/python2.7/threading.py:525:__bootstrap
/usr/lib/python2.7/threading.py:552:__bootstrap_inner
/usr/lib/python2.7/threading.py:505:run
- --- <exception caught here> ---
/usr/lib/python2.7/site-packages/twisted/python/threadpool.py:207:_worker
/usr/lib/python2.7/site-packages/twisted/python/context.py:118:callWithContext
/usr/lib/python2.7/site-packages/twisted/python/context.py:81:callWithContext
/home/kytv/tahoe/allmydata-tahoe-1.8.3/src/allmydata/util/iputil.py:222:_synchronously_find_addresses_via_config
/home/kytv/tahoe/allmydata-tahoe-1.8.3/src/allmydata/util/iputil.py:236:_query
/usr/lib/python2.7/subprocess.py:679:__init__
/usr/lib/python2.7/subprocess.py:1228:_execute_child
]
calling os.abort()

We should catch this exception and report a useful error message, saying what the sys.platform was and asking them to report about their ifconfig, like David-Sarah ​did on the list:

which ifconfig
ifconfig --help
ifconfig

[Brian Warner <warner@…>]

In 8e69b94588c1c0e7:

Make platform-detection code tolerate linux-3.0, patch by zooko.

Otherwise address-autodetection can't find ifconfig. refs #1536

[davidsarah]

Re: 8e69b94588c1c0e7, I would be tempted to use .startswith('linux') and .startswith('irix'), rather than testing for more specific strings.

[killyourtv]

​https://bbs.archlinux.org/viewtopic.php?pid=945602#p945602 & ​http://www.archlinux.org/news/deprecation-of-net-tools/

Apparently Arch deprecated ifconfig and "ip addr" is supposed to be used.

[mk.fg]

I've just created ​pull request (#14) on github with a simple patch to use $PATH to find ifconfig. A few questions that went through my mind during that process:

• Is there any particular reason why it's ($PATH) used only on windows?
• Why OS-native path resolution mechanics (which Popen() will use through os.execvp()) are overidden with twisted which() util?

I considered adding optional iproute2 support, as suggested above (and indeed, net-tools are deprecated in many distros), but I think it's a wrong way to do it - iproute2 docs clearly state that it's output is a subject to change (and it's usually updated once per kernel release) and is not designed to be parseable, so maybe some specialized netlink module should be used on linux instead?

[zooko]

Hi mk.fg! I was just investigating related bugs and I discovered this comment you added to this ticket. I'm sorry I didn't see it when you first posted it! I need to get more reliable monitoring of updates to trac+github...