socket: Actually provide non-blocking DNS methods

The green socket module seemed to have only blocking DNS resolution
methods even with dnspython installed which is inconsistent with the
documentation.

This patch has a few consequences:

* an import cycle is eliminated
* if an import cycle reappears here it'll be visible

Note: eliminating the import cycle revealed an issue related to monkey
patching and the way we perform greendns tests (the test failures were
already present on Python 3.5[1] as that version has some import cycle
handling changes). The failures look like this:

======================================================================
FAIL: test_query_ans_types (tests.greendns_test.TestHostsResolver)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/kuba/projects/eventlet/tests/greendns_test.py", line 97, in test_query_ans_types
    assert isinstance(ans, greendns.dns.resolver.Answer)
AssertionError

======================================================================
FAIL: test_query_unknown_no_raise (tests.greendns_test.TestHostsResolver)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/kuba/projects/eventlet/tests/greendns_test.py", line 129, in test_query_unknown_no_raise
    assert isinstance(ans, greendns.dns.resolver.Answer)
AssertionError

This issue will be addressed in a separate commit.

This patch is contributed by Smarkets Limited.

[1] eventlet#267

eventlet/green/socket.py

@@ -16,7 +16,16 @@
     try:
         from eventlet.support import greendns
     except ImportError as ex:
-        pass
+        try:
+            import dns
+        except ImportError:
+            # greendns import failed because we don't have dnspython - all is well,
+            # that's why we have the conditional import
+            pass
+        else:
+            # If, however, dnspython is importable yet greendns can't be imported
+            # this suggests there's another issue (like an import cycle)
+            raise
 
 if greendns:
     gethostbyname = greendns.gethostbyname

eventlet/support/greendns.py

@@ -43,6 +43,10 @@
 
 
 def import_patched(module_name):
+    # Import cycle note: it's crucial to use _socket_nodns here because
+    # regular evenlet.green.socket imports *this* module and if we imported
+    # it back we'd end with an import cycle (socket -> greendns -> socket).
+    # We break this import cycle by providing a restricted socket module.
     return patcher.import_patched(module_name,
                                   select=select,
                                   time=time,
@@ -55,15 +59,15 @@ def import_patched(module_name):
 
 for pkg in ('dns.entropy', 'dns.inet', 'dns.query'):
     setattr(dns, pkg.split('.')[1], import_patched(pkg))
-del import_patched
 
 import dns.rdtypes
 for pkg in ['dns.rdtypes.IN', 'dns.rdtypes.ANY']:
-    setattr(dns.rdtypes, pkg.split('.')[-1], patcher.import_patched(pkg))
+    setattr(dns.rdtypes, pkg.split('.')[-1], import_patched(pkg))
 for pkg in ['dns.rdtypes.IN.A', 'dns.rdtypes.IN.AAAA']:
-    setattr(dns.rdtypes.IN, pkg.split('.')[-1], patcher.import_patched(pkg))
+    setattr(dns.rdtypes.IN, pkg.split('.')[-1], import_patched(pkg))
 for pkg in ['dns.rdtypes.ANY.CNAME']:
-    setattr(dns.rdtypes.ANY, pkg.split('.')[-1], patcher.import_patched(pkg))
+    setattr(dns.rdtypes.ANY, pkg.split('.')[-1], import_patched(pkg))
+del import_patched
 
 
 socket = _socket_nodns

tests/socket_test.py

@@ -1,5 +1,11 @@
 import eventlet
 from eventlet.green import socket
+try:
+    from eventlet.support import greendns
+    has_greendns = True
+except ImportError:
+    has_greendns = False
+from tests import skip_if
 
 
 def test_create_connection_error():
@@ -29,3 +35,11 @@ def server():
     sock = eventlet.connect(tuple(addr))
     s = sock.recv(1)
     assert isinstance(s, bytes)
+
+
+@skip_if(not has_greendns)
+def test_dns_methods_are_green():
+    assert socket.gethostbyname is greendns.gethostbyname
+    assert socket.gethostbyname_ex is greendns.gethostbyname_ex
+    assert socket.getaddrinfo is greendns.getaddrinfo
+    assert socket.getnameinfo is greendns.getnameinfo