Issue #23209, #23225: selectors.BaseSelector.get_key() now raises a

RuntimeError if the selector is closed. And selectors.BaseSelector.close() now
clears its internal reference to the selector mapping to break a reference
cycle. Initial patch written by Martin Richard.

Lib/selectors.py

@@ -174,6 +174,8 @@ class BaseSelector(metaclass=ABCMeta):
         SelectorKey for this file object
         """
         mapping = self.get_map()
+        if mapping is None:
+            raise RuntimeError('Selector is closed')
         try:
             return mapping[fileobj]
         except KeyError:
@@ -256,6 +258,7 @@ class _BaseSelectorImpl(BaseSelector):
 
     def close(self):
         self._fd_to_key.clear()
+        self._map = None
 
     def get_map(self):
         return self._map