Pass partial answer to NXDOMAIN exception

[cluck]

When an NXDOMAIN is raised in dns.resolver.Resolver.query() the implementor should still have access to the partial result, specifically to find out a dangling CNAME. This is useful for example when chasing the right PTR record to update for a given IP Address (FreeIPA #5033). I propose to construct an Anwer object also for NXDOMAIN exceptions:

--- dns/resolver.py.2   2015-05-26 14:12:13.658045246 +0200
+++ dns/resolver.py 2015-05-26 15:31:26.459862822 +0200
@@ -44,7 +44,8 @@
     import _winreg

 class NXDOMAIN(dns.exception.DNSException):
-    """The query name does not exist."""
+    """The query name does not exist.
+    The Exception argument is the partial answer."""
     pass

 class YXDOMAIN(dns.exception.DNSException):
@@ -907,7 +908,9 @@
             all_nxdomain = False
             break
         if all_nxdomain:
-            raise NXDOMAIN
+            answer = Answer(qnames_to_try[0], rdtype, rdclass,
+                            response, False)
+            raise NXDOMAIN(answer)
         answer = Answer(qname, rdtype, rdclass, response,
                         raise_on_no_answer)
         if self.cache:

[spacekpe]

I generally like the idea but it needs to be polished.

@cluck , beware - the answer might really surprise you if search list in resolv.conf is non-empty. This needs to be carefully tested and documented because search lists make it quite complicated.

Another thing is that raise NXDOMAIN() should use a keyword argument to pass answer to it, please see class NXDOMAIN definition in resolver.py.

Thank you for the idea and your time, @cluck!

[cluck]

This is implemented in pull request #109 which is waiting since one year now.