src/main/java/io/reactivex/internal/queue/MpscLinkedQueue.java
@Override
    public boolean offer(final T e) {
        if (null == e) {
            throw new NullPointerException("Null is not a valid element");
        }
        final LinkedQueueNode<T> nextNode = new LinkedQueueNode<T>(e);
        final LinkedQueueNode<T> prevProducerNode = xchgProducerNode(nextNode);
        // Should a producer thread get interrupted here the chain WILL be broken until that thread is resumed
        // and completes the store in prev.next.
        prevProducerNode.soNext(nextNode); // StoreStore
        return true;
    }
@Override
    public boolean offer(final T e) {
        if (null == e) {
            throw new NullPointerException("Null is not a valid element");
        }
        final LinkedQueueNode<T> nextNode = new LinkedQueueNode<T>(e);
        final LinkedQueueNode<T> prevProducerNode = xchgProducerNode(nextNode);
        // Should a producer thread get interrupted here the chain WILL be broken until that thread is resumed
        // and completes the store in prev.next.
        prevProducerNode.soNext(nextNode); // StoreStore
        return true;
    }
{@inheritDoc} <br>
<p>
IMPLEMENTATION NOTES:<br>
Offer is allowed from multiple threads.<br>
Offer allocates a new node and:
<ol>
<li>Swaps it atomically with current producer node (only one producer 'wins')
<li>Sets the new node as the node following from the swapped producer node
</ol>
This works because each producer is guaranteed to 'plant' a new node and link the old node. No 2 producers can
get the same producer node as part of XCHG guarantee.

@see java.util.Queue#offer(java.lang.Object)