Avoid reordering IPv6 interface addresses

When deleting unsuitable addresses in get_interface_address6_list(), to
avoid reordering IPv6 interface addresses and keep the order returned by
the OS, use SMARTLIST_DEL_CURRENT_KEEPORDER() instead of
SMARTLIST_DEL_CURRENT().

This issue was reported by René Mayrhofer.

[Closes ticket 20163; changes file written by teor. This paragraph
added by nickm]

changes/20163

@@ -0,0 +1,6 @@
+  o Minor bugfixes (address discovery):
+    - Stop reordering IP addresses returned by the OS.
+This makes is more likely that Tor will guess the same relay IP address every time.
+Fixes issue 20163; bugfix on 0.2.7.1-alpha, ticket 17027.
+Reported by René Mayrhofer, patch by "cypherpunks".
+

src/common/address.c

@@ -1770,13 +1770,13 @@ MOCK_IMPL(smartlist_t *,get_interface_address6_list,(int severity,
     {
       if (tor_addr_is_loopback(a) ||
           tor_addr_is_multicast(a)) {
-        SMARTLIST_DEL_CURRENT(addrs, a);
+        SMARTLIST_DEL_CURRENT_KEEPORDER(addrs, a);
         tor_free(a);
         continue;
       }
 
       if (!include_internal && tor_addr_is_internal(a, 0)) {
-        SMARTLIST_DEL_CURRENT(addrs, a);
+        SMARTLIST_DEL_CURRENT_KEEPORDER(addrs, a);
         tor_free(a);
         continue;
       }