gh-141749: fix incorrect exceptions in pure Python Unpickler

Lib/pickle.py

@@ -1322,12 +1322,17 @@ def load(self):
                 if not key:
                     raise EOFError
                 assert isinstance(key, bytes_types)
-                dispatch[key[0]](self)
+                try:
+                    dispatch[key[0]](self)
+                except:
+                    raise UnpicklingError(f"invalid load key, '\\x{key[0]:02x}'.")
         except _Stop as stopinst:
             return stopinst.value
 
     # Return a list of items pushed in the stack after last MARK instruction.
     def pop_mark(self):
+        if not self.metastack:
+            raise UnpicklingError("could not find MARK")
         items = self.stack
         self.stack = self.metastack.pop()
         self.append = self.stack.append

Lib/test/test_pickle.py

@@ -55,7 +55,7 @@ class PyPickleTests(AbstractPickleModuleTests, unittest.TestCase):
 class PyUnpicklerTests(AbstractUnpickleTests, unittest.TestCase):
 
     unpickler = pickle._Unpickler
-    bad_stack_errors = (IndexError,)
+    bad_stack_errors = (IndexError, pickle.UnpicklingError)
     truncated_errors = (pickle.UnpicklingError, EOFError,
                         AttributeError, ValueError,
                         struct.error, IndexError, ImportError)

Misc/NEWS.d/next/Library/2025-11-19-14-35-36.gh-issue-141749.s86RHS.rst

@@ -0,0 +1 @@
+Fixed incorrect exception types occurring in the CPython Unpickler implementation when loading invalid pickle data. In some cases, the CPython fallback raised internal KeyError and IndexError exceptions instead of UnpicklingError.