fix: preserve str subclass map keys in fallback unpacker

Only intern exact str keys so decoding hooks can return str subclasses
when strict_map_key=False, matching the C extension.

Add a regression test for key identity and strict_map_key validation.

Author: SakayanagiYuna

msgpack/fallback.py

@@ -550,7 +550,7 @@ def _unpack(self, execute=EX_CONSTRUCT):
                     key = self._unpack(EX_CONSTRUCT)
                     if self._strict_map_key and type(key) not in (str, bytes):
                         raise ValueError("%s is not allowed for map key" % str(type(key)))
-                    if isinstance(key, str):
+                    if type(key) is str:
                         key = sys.intern(key)
                     ret[key] = self._unpack(EX_CONSTRUCT)
                 if self._object_hook is not None:

test/test_obj.py

@@ -2,7 +2,7 @@
 
 from pytest import raises
 
-from msgpack import packb, unpackb
+from msgpack import ExtType, packb, unpackb
 
 
 def _decode_complex(obj):
@@ -92,3 +92,19 @@ def test_an_exception_in_objecthook2():
     with raises(DecodeError):
         packed = packb({1: [{"__complex__": True, "real": 1, "imag": 2}]})
         unpackb(packed, list_hook=bad_complex_decoder, use_list=1, strict_map_key=False)
+
+
+def test_str_subclass_map_key_from_ext_hook():
+    class Key(str):
+        pass
+
+    key = Key("name")
+    packed = packb({ExtType(1, b"name"): 42})
+    options = {"ext_hook": lambda code, data: key}
+
+    result = unpackb(packed, strict_map_key=False, **options)
+    assert result == {key: 42}
+    assert next(iter(result)) is key
+
+    with raises(ValueError):
+        unpackb(packed, strict_map_key=True, **options)