gh-105332: [Enum] Fix unpickling flags in edge-cases (GH-105348)

* revert enum pickling from by-name to by-value

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

1 files changed, 9 insertions, 21 deletions

diff --git a/Lib/enum.py b/Lib/enum.py
index b50fe50..a22bcca 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -12,6 +12,7 @@ __all__ = [
         'FlagBoundary', 'STRICT', 'CONFORM', 'EJECT', 'KEEP',
         'global_flag_repr', 'global_enum_repr', 'global_str', 'global_enum',
         'EnumCheck', 'CONTINUOUS', 'NAMED_FLAGS', 'UNIQUE',
+        'pickle_by_global_name', 'pickle_by_enum_name',
         ]
 
 
@@ -913,7 +914,6 @@ class EnumType(type):
         body['__module__'] = module
         tmp_cls = type(name, (object, ), body)
         cls = _simple_enum(etype=cls, boundary=boundary or KEEP)(tmp_cls)
-        cls.__reduce_ex__ = _reduce_ex_by_global_name
         if as_global:
             global_enum(cls)
         else:
@@ -1216,7 +1216,7 @@ class Enum(metaclass=EnumType):
         return hash(self._name_)
 
     def __reduce_ex__(self, proto):
-        return getattr, (self.__class__, self._name_)
+        return self.__class__, (self._value_, )
 
     # enum.property is used to provide access to the `name` and
     # `value` attributes of enum members while keeping some measure of
@@ -1283,8 +1283,14 @@ class StrEnum(str, ReprEnum):
         return name.lower()
 
 
-def _reduce_ex_by_global_name(self, proto):
+def pickle_by_global_name(self, proto):
+    # should not be used with Flag-type enums
     return self.name
+_reduce_ex_by_global_name = pickle_by_global_name
+
+def pickle_by_enum_name(self, proto):
+    # should not be used with Flag-type enums
+    return getattr, (self.__class__, self._name_)
 
 class FlagBoundary(StrEnum):
     """
@@ -1306,23 +1312,6 @@ class Flag(Enum, boundary=STRICT):
     Support for flags
     """
 
-    def __reduce_ex__(self, proto):
-        cls = self.__class__
-        unknown = self._value_ & ~cls._flag_mask_
-        member_value = self._value_ & cls._flag_mask_
-        if unknown and member_value:
-            return _or_, (cls(member_value), unknown)
-        for val in _iter_bits_lsb(member_value):
-            rest = member_value & ~val
-            if rest:
-                return _or_, (cls(rest), cls._value2member_map_.get(val))
-            else:
-                break
-        if self._name_ is None:
-            return cls, (self._value_,)
-        else:
-            return getattr, (cls, self._name_)
-
     _numeric_repr_ = repr
 
     @staticmethod
@@ -2049,7 +2038,6 @@ def _old_convert_(etype, name, module, filter, source=None, *, boundary=None):
         # unless some values aren't comparable, in which case sort by name
         members.sort(key=lambda t: t[0])
     cls = etype(name, members, module=module, boundary=boundary or KEEP)
-    cls.__reduce_ex__ = _reduce_ex_by_global_name
     return cls
 
 _stdlib_enums = IntEnum, StrEnum, IntFlag