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defaultdict.__missing__ has data races #142495
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extension-modulesC modules in the Modules dirC modules in the Modules dirtopic-free-threadingtype-crashA hard crash of the interpreter, possibly with a core dumpA hard crash of the interpreter, possibly with a core dump
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extension-modulesC modules in the Modules dirC modules in the Modules dirtopic-free-threadingtype-crashA hard crash of the interpreter, possibly with a core dumpA hard crash of the interpreter, possibly with a core dump
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 thePyObject_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
Linked PRs
defaultdictkeep existed value when racing with__missing__#142668defaultdictkeep existed value when racing with__missing__(GH-142668) #142832defaultdictkeep existed value when racing with__missing__(GH-142668) #142858