Make failing to import netifaces not a load-time error

Only raise an exception if we try to execute a validator that would
require `netifaces` (the default.)

advocate/addrvalidator.py

@@ -3,9 +3,14 @@
 import ipaddress
 import re
 
-import netifaces
+try:
+    import netifaces
+    HAVE_NETIFACES = True
+except ImportError:
+    netifaces = None
+    HAVE_NETIFACES = False
 
-from .exceptions import NameserverException
+from .exceptions import NameserverException, ConfigException
 
 
 def canonicalize_hostname(hostname):
@@ -19,6 +24,9 @@ def canonicalize_hostname(hostname):
 
 def determine_local_addresses():
     """Get all IPs that refer to this machine according to netifaces"""
+    if not HAVE_NETIFACES:
+        raise ConfigException("Tried to determine local addresses, "
+                              "but netifaces module was not importable")
     ips = []
     for interface in netifaces.interfaces():
         if_families = netifaces.ifaddresses(interface)
@@ -68,6 +76,8 @@ def __init__(
             allow_teredo=False,
             allow_6to4=False,
             allow_dns64=False,
+            # Must be explicitly set to "False" if you don't want to try
+            # detecting local interface addresses with netifaces.
             autodetect_local_addresses=True,
     ):
         if not port_blacklist and not port_whitelist:

advocate/exceptions.py

@@ -16,3 +16,7 @@ class MountDisabledException(AdvocateException):
 
 class ProxyDisabledException(NotImplementedError, AdvocateException):
     pass
+
+
+class ConfigException(AdvocateException):
+    pass

test/test_advocate.py

@@ -20,6 +20,7 @@
 from advocate.api import RequestsAPIWrapper
 from advocate.connection import advocate_getaddrinfo
 from advocate.exceptions import (
+    ConfigException,
     MountDisabledException,
     NameserverException,
     UnacceptableAddressException,
@@ -311,6 +312,25 @@ def test_local_address_handling(self, mock_determine_local_addresses):
         ))
         mock_determine_local_addresses.assert_not_called()
 
+    def test_netifaces_presence_optional(self):
+        # Advocate should still work without netifaces, but only if you've specifically
+        # said you don't care about checking against local interface addresses.
+        with patch("advocate.addrvalidator.HAVE_NETIFACES", False):
+            with self.assertRaises(ConfigException):
+                self._is_addrinfo_allowed("200.1.1.1", 80, autodetect_local_addresses=True)
+            with self.assertRaises(ConfigException):
+                advocate.addrvalidator.determine_local_addresses()
+            # Should be fine if you specifically asked to not look at the local addrs
+            self.assertTrue(self._is_addrinfo_allowed(
+                "200.1.1.1",
+                80,
+                autodetect_local_addresses=False,
+            ))
+
+        # These shouldn't `raise`
+        self._is_addrinfo_allowed("200.1.1.1", 80, autodetect_local_addresses=True)
+        advocate.addrvalidator.determine_local_addresses()
+
 
 @unittest.skipIf(
     not canonname_supported(),
@@ -341,7 +361,7 @@ def test_idn(self):
             fake_lookup=True,
             hostname_blacklist={"*.org"}
         ))
-        # case insensitive, please
+        # case-insensitive, please
         self.assertFalse(self._is_hostname_allowed(
             u"中国.example.oRg",
             fake_lookup=True,