fix(queue): prevent consumer busy-wait when partition assignment is empty

AbstractTbQueueConsumerTemplate.poll() returned emptyList() immediately
  when partitions was empty, bypassing both doPoll() and the secondary
  sleep guard (which is also skipped for backends that report
  isLongPollingSupported()==true, e.g. Kafka). The result was a
  permanent CPU-burning loop on consumers whose partition assignment
  ended up empty after a rebalance cascade — observed on 26
  ie-downlink-consumer threads (~244% total CPU) until container restart.

  Route the empty-partition path through sleepAndReturnEmpty() so the
  caller honors durationInMillis regardless of long-polling support.

common/queue/src/main/java/org/thingsboard/server/queue/common/AbstractTbQueueConsumerTemplate.java

@@ -98,7 +98,10 @@ public abstract class AbstractTbQueueConsumerTemplate<R, T extends TbQueueMsg> i
                 doSubscribe(partitions);
                 subscribed = true;
             }
-            records = partitions.isEmpty() ? emptyList() : doPoll(durationInMillis);
+            if (partitions.isEmpty()) {
+                return sleepAndReturnEmpty(startNanos, durationInMillis);
+            }
+            records = doPoll(durationInMillis);
         } finally {
             consumerLock.unlock();
         }

common/queue/src/test/java/org/thingsboard/server/queue/common/AbstractTbQueueConsumerTemplateTest.java

@@ -0,0 +1,146 @@
+/**
+ * Copyright © 2016-2026 The Thingsboard Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.thingsboard.server.queue.common;
+
+import lombok.extern.slf4j.Slf4j;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.thingsboard.server.common.msg.queue.TopicPartitionInfo;
+import org.thingsboard.server.queue.TbQueueMsg;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.TimeUnit;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.greaterThanOrEqualTo;
+import static org.hamcrest.Matchers.is;
+import static org.mockito.ArgumentMatchers.anyLong;
+import static org.mockito.BDDMockito.never;
+import static org.mockito.BDDMockito.spy;
+import static org.mockito.BDDMockito.times;
+import static org.mockito.BDDMockito.verify;
+
+@Slf4j
+@ExtendWith(MockitoExtension.class)
+public class AbstractTbQueueConsumerTemplateTest {
+
+    private static final long POLL_DURATION_MS = 100L;
+    private static final long SLEEP_TOLERANCE_MS = 20L;
+
+    @Test
+    public void givenEmptyPartitionsAndLongPollingSupported_whenPoll_thenSleepsAndDoesNotCallDoPoll() {
+        // Regression: with empty partitions AND isLongPollingSupported()==true (e.g. Kafka),
+        // poll() previously returned instantly with no sleep, causing the consumer loop to busy-spin.
+        TestConsumer consumer = spy(new TestConsumer("test-topic", true));
+        consumer.subscribe(Collections.emptySet());
+
+        long startNs = System.nanoTime();
+        List<TbQueueMsg> result = consumer.poll(POLL_DURATION_MS);
+        long elapsedMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startNs);
+
+        assertThat(result, is(empty()));
+        verify(consumer, never()).doPoll(anyLong());
+        assertThat("poll() must sleep ~durationInMillis when partitions are empty (no busy-wait)",
+                elapsedMs, greaterThanOrEqualTo(POLL_DURATION_MS - SLEEP_TOLERANCE_MS));
+    }
+
+    @Test
+    public void givenEmptyPartitionsAndNoLongPolling_whenPoll_thenSleepsAndDoesNotCallDoPoll() {
+        TestConsumer consumer = spy(new TestConsumer("test-topic", false));
+        consumer.subscribe(Collections.emptySet());
+
+        long startNs = System.nanoTime();
+        List<TbQueueMsg> result = consumer.poll(POLL_DURATION_MS);
+        long elapsedMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startNs);
+
+        assertThat(result, is(empty()));
+        verify(consumer, never()).doPoll(anyLong());
+        assertThat(elapsedMs, greaterThanOrEqualTo(POLL_DURATION_MS - SLEEP_TOLERANCE_MS));
+    }
+
+    @Test
+    public void givenNonEmptyPartitions_whenPoll_thenCallsDoPoll() {
+        TestConsumer consumer = spy(new TestConsumer("test-topic", true));
+        consumer.subscribe(Collections.singleton(new TopicPartitionInfo("test-topic", null, 0, true)));
+
+        List<TbQueueMsg> result = consumer.poll(POLL_DURATION_MS);
+
+        assertThat(result, is(empty()));
+        verify(consumer, times(1)).doPoll(POLL_DURATION_MS);
+    }
+
+    @Test
+    public void givenPartitionsBecomeEmptyAfterRebalance_whenPollAgain_thenStopsCallingDoPoll() {
+        // Reproduces the observed trigger: a rebalance leaves the consumer with an empty
+        // partition assignment. Subsequent poll() calls must not busy-spin or call doPoll().
+        TestConsumer consumer = spy(new TestConsumer("test-topic", true));
+        consumer.subscribe(Collections.singleton(new TopicPartitionInfo("test-topic", null, 0, true)));
+        consumer.poll(POLL_DURATION_MS);
+        verify(consumer, times(1)).doPoll(POLL_DURATION_MS);
+
+        consumer.subscribe(Collections.emptySet());
+
+        long startNs = System.nanoTime();
+        List<TbQueueMsg> result = consumer.poll(POLL_DURATION_MS);
+        long elapsedMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startNs);
+
+        assertThat(result, is(empty()));
+        verify(consumer, times(1)).doPoll(anyLong());
+        assertThat(elapsedMs, greaterThanOrEqualTo(POLL_DURATION_MS - SLEEP_TOLERANCE_MS));
+    }
+
+    static class TestConsumer extends AbstractTbQueueConsumerTemplate<Object, TbQueueMsg> {
+
+        private final boolean longPollingSupported;
+
+        TestConsumer(String topic, boolean longPollingSupported) {
+            super(topic);
+            this.longPollingSupported = longPollingSupported;
+        }
+
+        @Override
+        protected List<Object> doPoll(long durationInMillis) {
+            return Collections.emptyList();
+        }
+
+        @Override
+        protected TbQueueMsg decode(Object record) {
+            return null;
+        }
+
+        @Override
+        protected void doSubscribe(Set<TopicPartitionInfo> partitions) {
+        }
+
+        @Override
+        protected void doCommit() {
+        }
+
+        @Override
+        protected void doUnsubscribe() {
+        }
+
+        @Override
+        protected boolean isLongPollingSupported() {
+            return longPollingSupported;
+        }
+    }
+
+}