The following change is a major change on how Python handles undecodable filenames on Windows:

-    return PyUnicode_DecodeMBCS(s, size, NULL);
+    return PyUnicode_DecodeMBCS(s, size, "surrogateescape");

I disagree with this change, Python should not generate surrogates *on Windows*.

By the way, there is also os.fsdecode(), it has the same behaviour than PyUnicode_DecodeFSDefault() and PyUnicode_DecodeFSDefaultAndSize() (it uses the "strict" error handler on Windows).