restore Mock._spec_asyncs and match upstream

Summary:
D42039568 (733b7b3) backported an early version of python/cpython#100252,
and there were some changes made before that PR was merged upstream.
Backport those changes so we match upstream behavior. Most importantly,
this restores `_spec_asyncs`, which although a private API does seem to be used.

Reviewed By: itamaro

Differential Revision: D42416767

fbshipit-source-id: a7e7a21

Lib/unittest/mock.py

@@ -493,6 +493,7 @@ def _mock_add_spec(self, spec, spec_set, _spec_as_instance=False,
                        _eat_self=False):
         _spec_class = None
         _spec_signature = None
+        _spec_asyncs = []
 
         if spec is not None and not _is_list(spec):
             if isinstance(spec, type):
@@ -503,13 +504,20 @@ def _mock_add_spec(self, spec, spec_set, _spec_as_instance=False,
                                         _spec_as_instance, _eat_self)
             _spec_signature = res and res[1]
 
-            spec = dir(spec)
+            spec_list = dir(spec)
+
+            for attr in spec_list:
+                if iscoroutinefunction(getattr(spec, attr, None)):
+                    _spec_asyncs.append(attr)
+
+            spec = spec_list
 
         __dict__ = self.__dict__
         __dict__['_spec_class'] = _spec_class
         __dict__['_spec_set'] = spec_set
         __dict__['_spec_signature'] = _spec_signature
         __dict__['_mock_methods'] = spec
+        __dict__['_spec_asyncs'] = _spec_asyncs
 
     def __get_return_value(self):
         ret = self._mock_return_value
@@ -998,8 +1006,7 @@ def _get_child_mock(self, /, **kw):
         For non-callable mocks the callable variant will be used (rather than
         any custom subclass)."""
         _new_name = kw.get("_new_name")
-        _spec_val = getattr(self.__dict__["_spec_class"], _new_name, None)
-        if _spec_val is not None and asyncio.iscoroutinefunction(_spec_val):
+        if _new_name in self.__dict__['_spec_asyncs']:
             return AsyncMock(**kw)
 
         if self._mock_sealed: