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[3.14] gh-142831: Fix use-after-free in json encoder during re-entrant mutation (gh-142851) - #150078
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… mutation (pythongh-142851) User callbacks invoked during JSON encoding (e.g. the `default` callback or a custom string encoder) can mutate or clear the dict or sequence being encoded, invalidating borrowed references to items, keys, and values. Hold strong references unconditionally while iterating. Co-authored-by: Kumar Aditya <kumaraditya@python.org> Co-authored-by: Gregory P. Smith <greg@krypto.org> (cherry picked from commit 235fa72)
This was referenced May 19, 2026
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Manual backport of #142851 to 3.14 (the bot couldn't cleanly cherry-pick due to refactors in main).
The fix had to be adapted because in 3.14 the dict/list iteration loops were not yet extracted into the _encoder_iterate_*_lock_held helpers introduced later in main. Added the Py_INCREF/Py_DECREF around each borrowed item/key/value inline in encoder_listencode_dict (both the PyMapping_Items and PyDict_Next paths) and encoder_listencode_list.
The two regression tests from the original PR were cherry-picked as-is.
Built --with-pydebug and ran ./python -m test test_json -v locally — all 218 tests pass.