When multiple threads offer() and poll() items into a LinkedTransferQueue, it can happen that poll() returns null, even though the queue is not actually empty.

Sample code:

import java.util.*;
import java.util.concurrent.*;

public class LinkedTransferQueueTest {
    public static void main(String... args) throws Throwable {
        test(new LinkedTransferQueue<>());
        test(new LinkedBlockingQueue<>());
        test(new LinkedBlockingDeque<>());
        test(new ArrayBlockingQueue<>(10));
        var q = new LinkedTransferQueue<Integer>();
    }

    private static void test(BlockingQueue<Integer> q) throws ExecutionException, InterruptedException {
        System.out.println(q.getClass());
        try (var pool = Executors.newCachedThreadPool()) {
            var futures = new ArrayList<Future<List<Integer>>>();
            var phaser = new Phaser(4);
            for (var i = 0; i < 4; i++) {
                futures.add(pool.submit(() -> {
                    var failedPolls = new ArrayList<Integer>();
                    for (int k = 0; k < 10; k++) {
                        for (int j = 0; j < 1000; j++) {
                            if (!q.offer(j))
                                throw new AssertionError("offer() failed for j=" + j);
                            if (q.peek() == null)
                                throw new AssertionError("peek() failed for j=" + j);
                            if (q.peek() == null)
                                throw new AssertionError("peek() failed for j=" + j);
                            while (q.poll() == null)
                                failedPolls.add(j);
                        }
                        phaser.arriveAndAwaitAdvance();
                        q.clear();
                        phaser.arriveAndAwaitAdvance();
                    }
                    return failedPolls;
                }));
            }
            var futureNumber = 0;
            for (var future : futures) {
                var failedPolls = future.get();
                if (failedPolls.size() > 0) {
                    System.out.println(futureNumber +
                                       ": failedPolls = " + failedPolls);
                }
                futureNumber++;
            }
        }
    }
}

Expected output:

class java.util.concurrent.LinkedTransferQueue
class java.util.concurrent.LinkedBlockingQueue
class java.util.concurrent.LinkedBlockingDeque
class java.util.concurrent.ArrayBlockingQueue

Actual output:

class java.util.concurrent.LinkedTransferQueue
0: failedPolls = [590, 971, 581, 629, 652, 632, 439, 651, 820, 918, 555]
1: failedPolls = [682, 516, 14, 426, 510, 607, 658, 679, 699, 811, 812, 813, 828, 862, 876]
2: failedPolls = [151, 497, 524, 547, 609, 617, 759, 795, 826, 843, 845, 866, 988]
3: failedPolls = [43, 7, 20, 929, 963, 976, 875, 900, 982, 200]
class java.util.concurrent.LinkedBlockingQueue
class java.util.concurrent.LinkedBlockingDeque
class java.util.concurrent.ArrayBlockingQueue