Fix root servers query handling

aiodnsresolver.py

@@ -99,6 +99,10 @@ class DnsRecordDoesNotExist(DnsError):
     pass
 
 
+class DnsNoMatchingAnswers(DnsRecordDoesNotExist):
+    pass
+
+
 class DnsPointerLoop(DnsError):
     pass
 
@@ -153,7 +157,7 @@ def pack_name(name):
         return b''.join(
             bytes((len(part),)) + part
             for part in name.split(b'.')
-        ) + b'\0'
+        ) + b'\0' if name else b'\0'
 
     def pack_resource(record):
         rdata = \
@@ -589,11 +593,12 @@ async def req():
                     last_exception = DnsResponseCode(res.rcode)
                     set_timeout_cause(last_exception)
                     logger.debug('Error from %s', addr_port)
-                elif name_error or (not cname_answers and not qtype_answers):
-                    # a name error can be returned by some non-authoritative
-                    # servers on not-existing, contradicting RFC 1035
+                elif name_error:
                     logger.debug('Record not found from %s', addr_port)
                     raise DnsRecordDoesNotExist()
+                elif not cname_answers and not qtype_answers:
+                    logger.debug('No answers from %s', addr_port)
+                    raise DnsNoMatchingAnswers()
                 else:
                     return cname_answers, qtype_answers
 

test.py

@@ -21,6 +21,7 @@
     TYPES,
     DnsCnameChainTooLong,
     DnsError,
+    DnsNoMatchingAnswers,
     DnsRecordDoesNotExist,
     DnsSocketError,
     DnsTimeout,
@@ -1772,6 +1773,18 @@ async def test_localhost_aaaa(self):
             self.assertEqual(str(res[0]), '::1')
             self.assertEqual(res[0].expires_at, loop.time())
 
+    @async_test
+    async def test_root_servers_queries(self):
+        # Upstream DNS server should return NOERROR status with an empty `A` records set.
+        # The default Github's does not do this, so we need to switch to Google's server.
+        self.addCleanup(patch_open(upstream_ip='8.8.8.8'))
+
+        resolve, _ = Resolver()
+        with self.assertRaises(DnsNoMatchingAnswers):
+            await resolve('', TYPES.A)
+        with self.assertRaises(DnsNoMatchingAnswers):
+            await resolve('root-servers.net', TYPES.A)
+
 
 class TestMemoizedMutex(unittest.TestCase):
     @async_test
@@ -1808,11 +1821,11 @@ async def func():
         self.assertEqual(await task3, 42)
 
 
-def patch_open():
+def patch_open(upstream_ip='127.0.0.1'):
     def mock_open(file_name, _):
         lines = \
-            ['127.0.0.1 localhost'] if file_name == '/etc/hosts' else \
-            ['nameserver 127.0.0.1']
+            [f'{upstream_ip} localhost'] if file_name == '/etc/hosts' else \
+            [f'nameserver {upstream_ip}']
 
         context_manager = MagicMock()
         context_manager.__enter__.return_value = lines