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gh-148891: prevent crash in HAMT #148928

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gh-148891: prevent crash in HAMT #148928

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14 changes: 13 additions & 1 deletion Python/context.c
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Original file line number Diff line number Diff line change
Expand Up @@ -791,8 +791,16 @@ contextvar_set(PyContextVar *var, PyObject *val)
return -1;
}

// gh-148891: _PyHamt_Assoc can allocate HAMT nodes, which may trigger
// GC. A weakref callback or finalizer could then re-enter
// contextvar_set, replacing ctx->ctx_vars via Py_SETREF and freeing
// the old HAMT while _PyHamt_Assoc is still traversing it.
// Prevent this by holding a strong reference to the current vars.
PyHamtObject *old_vars = ctx->ctx_vars;
Py_INCREF(old_vars);
PyHamtObject *new_vars = _PyHamt_Assoc(
ctx->ctx_vars, (PyObject *)var, val);
old_vars, (PyObject *)var, val);
Py_DECREF(old_vars);
if (new_vars == NULL) {
return -1;
}
Expand All @@ -819,8 +827,12 @@ contextvar_del(PyContextVar *var)
return -1;
}

// gh-148891: same borrowed-reference hazard as contextvar_set:
// _PyHamt_Without allocates nodes, which may trigger GC re-entrancy.
PyHamtObject *vars = ctx->ctx_vars;
Py_INCREF(vars);
PyHamtObject *new_vars = _PyHamt_Without(vars, (PyObject *)var);
Py_DECREF(vars);
if (new_vars == NULL) {
return -1;
}
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