selectors: avoid reference cycle with _SelectorMapping

See #117

Lib/selectors.py

@@ -210,8 +210,7 @@ class _BaseSelectorImpl(BaseSelector):
     def __init__(self):
         # this maps file descriptors to keys
         self._fd_to_key = {}
-        # read-only mapping returned by get_map()
-        self._map = _SelectorMapping(self)
+        self._closed = False
 
     def _fileobj_lookup(self, fileobj):
         """Return a file descriptor from a file object.
@@ -268,10 +267,13 @@ def modify(self, fileobj, events, data=None):
 
     def close(self):
         self._fd_to_key.clear()
-        self._map = None
+        self._closed = True
 
     def get_map(self):
-        return self._map
+        # read-only mapping returned by get_map()
+        if self._closed:
+            return None
+        return _SelectorMapping(self)
 
     def _key_from_fd(self, fd):
         """Return the key associated to a given file descriptor.

Lib/test/test_selectors.py

@@ -198,10 +198,10 @@ def test_modify_unregister(self):
             self.addCleanup(s.close)
             rd, wr = self.make_socketpair()
             s.register(rd, selectors.EVENT_READ)
-            self.assertEqual(len(s._map), 1)
+            self.assertEqual(len(s.get_map()), 1)
             with self.assertRaises(ZeroDivisionError):
                 s.modify(rd, selectors.EVENT_WRITE)
-            self.assertEqual(len(s._map), 0)
+            self.assertEqual(len(s.get_map()), 0)
 
     def test_close(self):
         s = self.SELECTOR()