src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final ParallelFlowable<T> doOnCancel(@NonNull Action onCancel) {
        ObjectHelper.requireNonNull(onCancel, "onCancel is null");
        return RxJavaPlugins.onAssembly(new ParallelPeek<T>(this,
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION,
                Functions.emptyConsumer(),
                Functions.EMPTY_LONG_CONSUMER,
                onCancel
                ));
    }
@CheckReturnValue
    @NonNull
    public final ParallelFlowable<T> doOnCancel(@NonNull Action onCancel) {
        ObjectHelper.requireNonNull(onCancel, "onCancel is null");
        return RxJavaPlugins.onAssembly(new ParallelPeek<T>(this,
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.emptyConsumer(),
                Functions.EMPTY_ACTION,
                Functions.EMPTY_ACTION,
                Functions.emptyConsumer(),
                Functions.EMPTY_LONG_CONSUMER,
                onCancel
                ));
    }
Run the specified Action when a 'rail' receives a cancellation.

@param onCancel the callback
@return the new ParallelFlowable instance