BlockingMpmcQueue

A blocking MPMC (multi-producer, multi-consumer) queue.

Backed by MpmcRingBuffer for the fast path; uses LockSupport.parkNanos for blocking when the buffer is full or empty.

Multiple threads may call offer and take concurrently.

==Single-Waiter Design==

Only one producer and one consumer can be fast-path woken at a time. The queue maintains a single producerWaiter and consumerWaiter volatile slot. When a thread blocks, it registers itself in the appropriate slot and calls LockSupport.parkNanos(this, 1000L) (1 microsecond timeout). When the opposite side has data/space, it unparks the registered waiter. Other waiting threads wake up within 1 microsecond via the timeout and retry.

This design is virtual-thread friendly: parkNanos does not OS-block on virtual threads, allowing bounded re-checks without risk of permanent parking.

==Happens-Before Guarantee==

The pattern producerWaiter = Thread.currentThread() (volatile write) followed by if (closed) return false (volatile read) ensures that a close() call setting closed = true is always visible before the thread parks. This prevents the race where a thread parks after close() has already been called.

==close() Behavior==

close() unparks only the registered producerWaiter and consumerWaiter. Other waiting threads wake up within 1 microsecond via parkNanos timeout and detect the closed flag on retry. This is intentional and documented.

Null elements are not permitted.