--- ../cpython/Lib/contextlib.py	2021-06-26 16:28:03.835372955 +1000
+++ contextlib2.py	2021-06-26 17:40:30.047079570 +1000
@@ -1,19 +1,32 @@
-"""Utilities for with-statement contexts.  See PEP 343."""
+"""contextlib2 - backports and enhancements to the contextlib module"""
+
 import abc
 import sys
+import warnings
 import _collections_abc
 from collections import deque
 from functools import wraps
-from types import MethodType, GenericAlias
+from types import MethodType
+
+# Python 3.6/3.7/3.8 compatibility: GenericAlias may not be defined
+try:
+    from types import GenericAlias
+except ImportError:
+    # If the real GenericAlias type doesn't exist, __class_getitem__ won't be used,
+    # so the fallback placeholder doesn't need to provide any meaningful behaviour
+    class GenericAlias:
+        pass
+
 
 __all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
            "AbstractContextManager", "AbstractAsyncContextManager",
            "AsyncExitStack", "ContextDecorator", "ExitStack",
            "redirect_stdout", "redirect_stderr", "suppress", "aclosing"]
 
+# Backwards compatibility
+__all__ += ["ContextStack"]
 
 class AbstractContextManager(abc.ABC):
-
     """An abstract base class for context managers."""
 
     __class_getitem__ = classmethod(GenericAlias)
@@ -60,6 +73,23 @@
 class ContextDecorator(object):
     "A base class or mixin that enables context managers to work as decorators."
 
+    def refresh_cm(self):
+        """Returns the context manager used to actually wrap the call to the
+        decorated function.
+
+        The default implementation just returns *self*.
+
+        Overriding this method allows otherwise one-shot context managers
+        like _GeneratorContextManager to support use as decorators via
+        implicit recreation.
+
+        DEPRECATED: refresh_cm was never added to the standard library's
+                    ContextDecorator API
+        """
+        warnings.warn("refresh_cm was never added to the standard library",
+                      DeprecationWarning)
+        return self._recreate_cm()
+
     def _recreate_cm(self):
         """Return a recreated instance of self.
 
@@ -430,7 +460,9 @@
         return MethodType(cm_exit, cm)
 
     @staticmethod
-    def _create_cb_wrapper(callback, /, *args, **kwds):
+    def _create_cb_wrapper(*args, **kwds):
+        # Python 3.6/3.7 compatibility: no native positional-only args syntax
+        callback, *args = args
         def _exit_wrapper(exc_type, exc, tb):
             callback(*args, **kwds)
         return _exit_wrapper
@@ -479,11 +511,18 @@
         self._push_cm_exit(cm, _exit)
         return result
 
-    def callback(self, callback, /, *args, **kwds):
+    def callback(*args, **kwds):
         """Registers an arbitrary callback and arguments.
 
         Cannot suppress exceptions.
         """
+        # Python 3.6/3.7 compatibility: no native positional-only args syntax
+        try:
+            self, callback, *args = args
+        except ValueError as exc:
+            exc_details = str(exc).partition("(")[2]
+            msg = "Not enough positional arguments {}".format(exc_details)
+            raise TypeError(msg) from None
         _exit_wrapper = self._create_cb_wrapper(callback, *args, **kwds)
 
         # We changed the signature, so using @wraps is not appropriate, but
@@ -589,7 +628,9 @@
         return MethodType(cm_exit, cm)
 
     @staticmethod
-    def _create_async_cb_wrapper(callback, /, *args, **kwds):
+    def _create_async_cb_wrapper(*args, **kwds):
+        # Python 3.6/3.7 compatibility: no native positional-only args syntax
+        callback, *args = args
         async def _exit_wrapper(exc_type, exc, tb):
             await callback(*args, **kwds)
         return _exit_wrapper
@@ -624,11 +665,18 @@
             self._push_async_cm_exit(exit, exit_method)
         return exit  # Allow use as a decorator
 
-    def push_async_callback(self, callback, /, *args, **kwds):
+    def push_async_callback(*args, **kwds):
         """Registers an arbitrary coroutine function and arguments.
 
         Cannot suppress exceptions.
         """
+        # Python 3.6/3.7 compatibility: no native positional-only args syntax
+        try:
+            self, callback, *args = args
+        except ValueError as exc:
+            exc_details = str(exc).partition("(")[2]
+            msg = "Not enough positional arguments {}".format(exc_details)
+            raise TypeError(msg) from None
         _exit_wrapper = self._create_async_cb_wrapper(callback, *args, **kwds)
 
         # We changed the signature, so using @wraps is not appropriate, but
@@ -729,3 +777,22 @@
 
     async def __aexit__(self, *excinfo):
         pass
+
+
+# Preserve backwards compatibility
+class ContextStack(ExitStack):
+    """Backwards compatibility alias for ExitStack"""
+
+    def __init__(self):
+        warnings.warn("ContextStack has been renamed to ExitStack",
+                      DeprecationWarning)
+        super(ContextStack, self).__init__()
+
+    def register_exit(self, callback):
+        return self.push(callback)
+
+    def register(self, callback, *args, **kwds):
+        return self.callback(callback, *args, **kwds)
+
+    def preserve(self):
+        return self.pop_all()