gh-151722: Defer GC tracking of frozendict.fromkeys() result until fully built#151967
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gh-151722: Defer GC tracking of frozendict.fromkeys() result until fully built#151967tonghuaroot wants to merge 1 commit into
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…til fully built frozendict.fromkeys() built its result with PyIter_Next() on an already GC-tracked object, so a half-built frozendict was reachable from another thread (using the gc module) and could be observed mutating mid-construction in the free threading build. Untrack the result while it is being filled and re-track it once fully built.
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frozendict.fromkeys()fills its result by iterating the keys and insertingthem one at a time with
PyIter_Next(), which runs user code (the iterable's__next__). In the free-threaded build the result is GC-tracked during thisfill loop, so another thread calling
gc.get_objects()can reach and take areference to the still half-built frozendict. That breaks the
unique-reference invariant the lock-free insert relies on, tripping
assert(can_modify_dict(mp))on a debug build (or an unsynchronized writeconcurrent with a reader on a release build).
This mirrors gh-151740, which applied the same untrack-during-build /
track-when-complete pattern to
frozendict_new()andfrozendict_vectorcall().fromkeys()is the remaining construction path that change did not cover.This is the separate follow-up for the
fromkeys()path; in gh-151740@corona10 suggested submitting it as its own PR once that one was merged.
Issue: #151722