DinoV
commented
2 weeks ago Some questions comments below. Biggest questions is that now that we are accessing the
dictdirectly instead of through_PyObject_GenericSetAttrWithDict, how do we know that the other cases handled by_PyObject_GenericSetAttrWithDictare not necessary in_Py_type_getattro?
This is part of the reason why I assert:
assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_INLINE_VALUES));
assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_MANAGED_DICT));In type_setattro. Type objects will always have a dictptr, so we'll either go to _PyObjectDict_SetItem if the dict doesn't exist yet or do the get/set if they do. _PyObjectDict_SetItem is just going to do a get/set as well. And because types don't have a managed dict they don't have shared keys.
Also, I think this merits a NEWS entry.
bedevere-bot
Lookups from the type cache can be exposed to some races. One of those is that
_PyType_Lookupisn't increfing the result, so the resulting value can become invalid at any point. This adds a new_PyType_FetchAPI which does the incref. Most places are updated to use the new API except for the specializer which isn't safe in free-threaded builds yet.But there are also issues around when we do a store into the dict and signal that the type is modified. A race can occur where the old value is removed from the dict and before we invalidate the type version. This can be seen today w/ the GIL when the value in the dict has a finalizer - accessing the type cache and the type's mapping proxy return inconsistent results.
This fixes these issues by making the update to the dict and the type modified update an atomic operation. We also capture the previous value in the dictionary and don't free it until after we've made the atomic update. This basically inlines a much simplified version of
_PyObject_GenericSetAttrWithDictintotype_setattrowhich can lock mutation against types as well as against the type's dict.When updating we first clear the type version so concurrent reads of the type cache won't hit. We then replace the existing value and finish the type modified process.