add function to find local IP addresses

This is a stripped-down copy of the synchronous/blocking portion of
Tahoe-LAFS's src/allmydata/utils/iputil.py .

src/wormhole/ipaddrs.py

@@ -0,0 +1,71 @@
+
+# Find all of our ip addresses. From tahoe's src/allmydata/util/iputil.py
+
+import os, re, subprocess, errno
+from sys import platform
+
+# Wow, I'm really amazed at home much mileage we've gotten out of calling
+# the external route.exe program on windows...  It appears to work on all
+# versions so far.  Still, the real system calls would much be preferred...
+# ... thus wrote Greg Smith in time immemorial...
+_win32_re = re.compile(r'^\s*\d+\.\d+\.\d+\.\d+\s.+\s(?P<address>\d+\.\d+\.\d+\.\d+)\s+(?P<metric>\d+)\s*$', flags=re.M|re.I|re.S)
+_win32_commands = (('route.exe', ('print',), _win32_re),)
+
+# These work in most Unices.
+_addr_re = re.compile(r'^\s*inet [a-zA-Z]*:?(?P<address>\d+\.\d+\.\d+\.\d+)[\s/].+$', flags=re.M|re.I|re.S)
+_unix_commands = (('/bin/ip', ('addr',), _addr_re),
+                  ('/sbin/ifconfig', ('-a',), _addr_re),
+                  ('/usr/sbin/ifconfig', ('-a',), _addr_re),
+                  ('/usr/etc/ifconfig', ('-a',), _addr_re),
+                  ('ifconfig', ('-a',), _addr_re),
+                  ('/sbin/ifconfig', (), _addr_re),
+                 )
+
+
+def find_addresses():
+    # originally by Greg Smith, hacked by Zooko and then Daira
+
+    # We don't reach here for cygwin.
+    if platform == 'win32':
+        commands = _win32_commands
+    else:
+        commands = _unix_commands
+
+    for (pathtotool, args, regex) in commands:
+        assert os.path.isabs(pathtotool)
+        if not os.path.isfile(pathtotool):
+            continue
+        try:
+            addresses = _query(pathtotool, args, regex)
+        except Exception:
+            addresses = []
+        if addresses:
+            return addresses
+    return []
+
+def _query(path, args, regex):
+    env = {'LANG': 'en_US.UTF-8'}
+    TRIES = 5
+    for trial in xrange(TRIES):
+        try:
+            p = subprocess.Popen([path] + list(args),
+                                 stdout=subprocess.PIPE,
+                                 stderr=subprocess.PIPE,
+                                 env=env)
+            (output, err) = p.communicate()
+            break
+        except OSError, e:
+            if e.errno == errno.EINTR and trial < TRIES-1:
+                continue
+            raise
+
+    addresses = []
+    outputsplit = output.split('\n')
+    for outline in outputsplit:
+        m = regex.match(outline)
+        if m:
+            addr = m.group('address')
+            if addr not in addresses:
+                addresses.append(addr)
+
+    return addresses