#[pymethod]
fn __length_hint__(&self, vm: &VirtualMachine) -> PyObjectRef {
    let internal = self.internal.lock(); // holds PyMutex across user callbacks
    if let IterStatus::Active(obj) = &internal.status {
        let seq = PySequence { obj, methods: self.seq_methods };
        seq.length(vm) // invokes user __len__. PoC calls iterator.__length_hint__ again while lock is held
            .map(|x| PyInt::from(x).into_pyobject(vm))
            .unwrap_or_else(|_| vm.ctx.not_implemented())
    } else {
        PyInt::from(0).into_pyobject(vm)
    }
}
// Inner __length_hint__ call tries to lock the same mutex again, blocking forever → deadlock.