src/main/java/io/reactivex/internal/disposables/ArrayCompositeDisposable.java
public Disposable replaceResource(int index, Disposable resource) {
        for (;;) {
            Disposable o = get(index);
            if (o == DisposableHelper.DISPOSED) {
                resource.dispose();
                return null;
            }
            if (compareAndSet(index, o, resource)) {
                return o;
            }
        }
    }
public Disposable replaceResource(int index, Disposable resource) {
        for (;;) {
            Disposable o = get(index);
            if (o == DisposableHelper.DISPOSED) {
                resource.dispose();
                return null;
            }
            if (compareAndSet(index, o, resource)) {
                return o;
            }
        }
    }
Replaces the resource at the specified index and returns the old resource.
@param index the index of the resource to replace
@param resource the new resource
@return the old resource, can be null