[analyzer] Default to IPv4 if IPv6 is not available (#158).

This commit re-defines the IPv6 HTTP server class to attempt to open a
server listing only on IPv4 if IPv6 is not available.

analyzer/python/ikos/http.py

@@ -42,23 +42,47 @@
 try:
     # Python 3
     from http.server import HTTPServer, BaseHTTPRequestHandler
+    from errno import EAFNOSUPPORT
     from urllib.parse import parse_qs, urlencode
     from urllib.request import urlopen
     from socket import AF_INET6
+    import os # OSError
 except ImportError:
     # Python 2
     from BaseHTTPServer import HTTPServer, BaseHTTPRequestHandler
+    from errno import EAFNOSUPPORT
     from urlparse import parse_qs
     from urllib import urlencode
     from urllib2 import urlopen
     from socket import AF_INET6
+    import os # OSError
 
 
 class HTTPServerIPv6(HTTPServer):
     '''
     HTTP server that listens in IPv6.
 
     In systems with dual stack, this also listens in IPv4.
+
+    This method tries to listed on IPv6 first (and IPv4 if available). If that
+    is not supported, then it tries IPv4 only.
     '''
 
     address_family = AF_INET6
+
+    def __init__(self, server_address, RequestHandlerClass, bind_and_activate=True):
+        try:
+            # Initially try to open the server with IPv6. This will also select
+            # IPv4 on systems with dual stack.
+            HTTPServer(server_address, RequestHandlerClass, bind_and_activate)
+
+        except OSError as e:
+            if e.errno == errno.EAFNOSUPPORT:
+                # If the exception is due to IPv6 not being supported, we
+                # select IPv4 only and try to re-open the server again.
+                address_family = AF_INET
+                HTTPServer(server_address, RequestHandlerClass, bind_and_activate)
+            else:
+                # If the exception is for any other reason other than IPv6 not
+                # being supported, then we cannot do anything about it.
+                raise e