defaultdict.__missing__ has data races #142495
Description
Bug report
Bug description:
Ideally, defaultdict would provide atomic "compute and add only if key is absent" behavior. When __missing__ is called, another thread might insert the key. Or, it can be inserted between the call to the factory function and the PyObject_SetItem() call.
It would help to call PyDict_SetDefaultRef() so that an existing value is not replaced. That still has the issue that the factory function is called when a value already exists for that key but that is arguably a less serious problem. We should not be replacing an existing value.
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux