From d350158e060c01acf49759dcbdd1f4d72530111b Mon Sep 17 00:00:00 2001
From: Viktor Klang <vklang@openjdk.org>
Date: Thu, 27 Nov 2025 09:42:46 +0000
Subject: [PATCH] 8371740: LinkedTransferQueue.poll() returns null even though
 queue is not empty

Co-authored-by: Doug Lea <dl@openjdk.org>
Co-authored-by: Dr Heinz M. Kabutz <heinz@javaspecialists.eu>
Reviewed-by: alanb
---
 .../util/concurrent/LinkedTransferQueue.java  | 16 ++--
 .../concurrent/BlockingQueue/MissedPoll.java  | 78 +++++++++++++++++++
 2 files changed, 87 insertions(+), 7 deletions(-)
 create mode 100644 test/jdk/java/util/concurrent/BlockingQueue/MissedPoll.java

diff --git a/src/java.base/share/classes/java/util/concurrent/LinkedTransferQueue.java b/src/java.base/share/classes/java/util/concurrent/LinkedTransferQueue.java
index 118d648c7a2..787d8d5fecd 100644
--- a/src/java.base/share/classes/java/util/concurrent/LinkedTransferQueue.java
+++ b/src/java.base/share/classes/java/util/concurrent/LinkedTransferQueue.java
@@ -588,13 +588,15 @@ public class LinkedTransferQueue<E> extends AbstractQueue<E>
             do {
                 m = p.item;
                 q = p.next;
-                if (p.isData != haveData && haveData != (m != null) &&
-                    p.cmpExItem(m, e) == m) {
-                    Thread w = p.waiter;    // matched complementary node
-                    if (p != h && h == cmpExHead(h, (q == null) ? p : q))
-                        h.next = h;         // advance head; self-link old
-                    LockSupport.unpark(w);
-                    return m;
+                if (p.isData != haveData && haveData != (m != null)) {
+                    if (p.cmpExItem(m, e) == m) {
+                        Thread w = p.waiter; // matched complementary node
+                        if (p != h && h == cmpExHead(h, (q == null) ? p : q))
+                            h.next = h;     // advance head; self-link old
+                        LockSupport.unpark(w);
+                        return m;
+                    }
+                    continue restart;
                 } else if (q == null) {
                     if (ns == 0L)           // try to append unless immediate
                         break restart;
diff --git a/test/jdk/java/util/concurrent/BlockingQueue/MissedPoll.java b/test/jdk/java/util/concurrent/BlockingQueue/MissedPoll.java
new file mode 100644
index 00000000000..9eb39634ce6
--- /dev/null
+++ b/test/jdk/java/util/concurrent/BlockingQueue/MissedPoll.java
@@ -0,0 +1,78 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8371740
+ * @summary Checks for poll() (or peek) returning null when an element must exist.
+ */
+import java.util.*;
+import java.util.concurrent.*;
+
+public class MissedPoll {
+    public static void main(String... args) throws Throwable {
+        test(new LinkedTransferQueue<>());
+        test(new LinkedBlockingQueue<>());
+        test(new LinkedBlockingDeque<>());
+        test(new ArrayBlockingQueue<>(10));
+    }
+
+    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<Integer>>();
+            var phaser = new Phaser(4) {
+                @Override
+                protected boolean onAdvance(int phase, int registeredParties) {
+                    q.clear();
+                    return super.onAdvance(phase, registeredParties);
+                }
+            };
+            for (var i = 0; i < 4; i++) {
+                futures.add(pool.submit(() -> {
+                    int errors = 0;
+                    for (int k = 0; k < 10; k++) {
+                        for (int j = 0; j < 1000; j++) {
+                            q.offer(j);
+                            if (q.peek() == null)
+                                ++errors;
+                            if (q.poll() == null)
+                                ++errors;
+                        }
+                        phaser.arriveAndAwaitAdvance();
+                    }
+                    return errors;
+                }));
+            }
+            for (var future : futures) {
+                Integer res;
+                if ((res = future.get()) != 0)
+                    throw new AssertionError("Expected 0 but got " + res);
+            }
+        }
+        if (!q.isEmpty())
+            throw new AssertionError("Queue is not empty: " + q);
+    }
+}