Unavoidable race condition when using QPromise::setException()?

https://doc.qt.io/qt-6/qpromise.html#setException says, "This method must not be used after QFuture::cancel() or QPromise::finish()". But how can we write code that guarantees that QPromise::setException() won't be called after QFuture::cancel()? The former is meant to be called from the "promise thread" while the latter is meant to be called from the "future thread".

Consider this sequence of events:

1. Thread A creates a QPromise + QFuture, then passes the QPromise to Thread B
2. Thread B starts doing work
3. Thread A requests cancellation
4. Thread B has an error and wants to call setException()

// Inside Thread B

if (!promise->isCanceled()) { // isCanceled() returns false here
    // ...

    // Uh-oh, isCanceled() could return true here!
    promise->setException(MyException());
}

Suggestion

We need a function that "atomically" checks the cancellation flag and stores the exception if not cancelled. For example, bool setExceptionIfNotCanceled(const QException &) – return true if the exception was set, false otherwise.