ARTEMIS-6271 Fix Core client reconnection delay

artemis-core-client/src/main/java/org/apache/activemq/artemis/core/client/impl/ServerLocatorImpl.java

@@ -877,7 +877,9 @@ private boolean checkOnDiscovery() throws ActiveMQException {
                   // and we should ignore UDP restarts here.
                   if (!disableDiscoveryRetries) {
                      if (discoveryGroup != null) {
-                        discoveryGroup.stop();
+                        // stop(0L) — skip thread join on reconnect retry to avoid blocking
+                        // for stoppingTimeout on every failed attempt.
+                        discoveryGroup.stop(0L);
                      }
 
                      logger.debug("Restarting discovery");

artemis-core-client/src/main/java/org/apache/activemq/artemis/core/cluster/DiscoveryGroup.java

@@ -143,6 +143,16 @@ public void internalRunning() throws Exception {
 
    @Override
    public void stop() {
+      stop(stoppingTimeout);
+   }
+
+   /**
+    * Stops this discovery group, waiting at most {@code joinTimeout} ms for the discovery thread.
+    * Pass {@code 0} to skip the join entirely (use in reconnect-retry paths to avoid blocking).
+    *
+    * @param joinTimeout max ms to wait for the thread; {@code 0} means do not wait
+    */
+   public void stop(long joinTimeout) {
 
       if (logger.isDebugEnabled()) {
          logger.debug("Stopping discovery. There's an exception just as a trace where it happened", new Exception("trace"));
@@ -169,9 +179,11 @@ public void stop() {
       try {
          if (thread != null) {
             thread.interrupt();
-            thread.join(stoppingTimeout);
-            if (thread.isAlive()) {
-               ActiveMQClientLogger.LOGGER.timedOutStoppingDiscovery();
+            if (joinTimeout > 0) {
+               thread.join(joinTimeout);
+               if (thread.isAlive()) {
+                  ActiveMQClientLogger.LOGGER.timedOutStoppingDiscovery();
+               }
             }
          }
       } catch (InterruptedException e) {

artemis-core-client/src/test/java/org/apache/activemq/artemis/core/cluster/DiscoveryGroupStopTimeoutTest.java

@@ -0,0 +1,159 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.apache.activemq.artemis.core.cluster;
+
+import java.util.concurrent.TimeUnit;
+
+import org.apache.activemq.artemis.api.core.BroadcastEndpoint;
+import org.apache.activemq.artemis.api.core.BroadcastEndpointFactory;
+import org.apache.activemq.artemis.utils.RandomUtil;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Verifies that {@code stop(0L)} returns immediately without joining the discovery thread,
+ * while the no-arg {@code stop()} still delegates to the configured {@code stoppingTimeout}.
+ */
+public class DiscoveryGroupStopTimeoutTest {
+
+   // Fake endpoint blocks 8s after close() — longer than the 3s pass threshold,
+   // shorter than the 30s stoppingTimeout, so a blocked join is clearly visible.
+   private static final long SLOW_ENDPOINT_BLOCK_MS = 8_000L;
+   private static final long LARGE_STOPPING_TIMEOUT = 30_000L;
+   private static final long FAST_STOP_THRESHOLD_MS = 3_000L;
+
+   private DiscoveryGroup dg;
+
+   @AfterEach
+   public void tearDown() {
+      if (dg != null && dg.isStarted()) {
+         dg.stop(0L); // use 0 to avoid blocking teardown
+      }
+   }
+
+   /** stop(0L) must return without waiting for the thread join, even with a slow endpoint. */
+   @Test
+   public void testStopWithZeroTimeoutDoesNotBlock() throws Exception {
+      dg = new DiscoveryGroup(RandomUtil.randomUUIDString(), RandomUtil.randomUUIDString(),
+                              500L, LARGE_STOPPING_TIMEOUT,
+                              new SlowEndpointFactory(SLOW_ENDPOINT_BLOCK_MS),
+                              null);
+      dg.start();
+      assertTrue(dg.isStarted());
+
+      long before = System.currentTimeMillis();
+      dg.stop(0L);
+      long elapsed = System.currentTimeMillis() - before;
+
+      assertFalse(dg.isStarted(), "DiscoveryGroup should be stopped after stop(0L)");
+      assertTrue(elapsed < FAST_STOP_THRESHOLD_MS,
+                 "stop(0L) blocked for " + elapsed + " ms — expected immediate return (< " + FAST_STOP_THRESHOLD_MS + " ms).");
+   }
+
+   /** stop() with stoppingTimeout=0 must also return immediately — no hang, no NPE. */
+   @Test
+   public void testNoArgStopDelegatesToStoppingTimeout() throws Exception {
+      dg = new DiscoveryGroup(RandomUtil.randomUUIDString(), RandomUtil.randomUUIDString(),
+                              500L, 0L,
+                              new SlowEndpointFactory(SLOW_ENDPOINT_BLOCK_MS),
+                              null);
+      dg.start();
+      assertTrue(dg.isStarted());
+
+      long before = System.currentTimeMillis();
+      dg.stop(); // stoppingTimeout=0 → stop(0L) → must not hang
+      long elapsed = System.currentTimeMillis() - before;
+
+      assertFalse(dg.isStarted(), "DiscoveryGroup should be stopped after stop()");
+      assertTrue(elapsed < FAST_STOP_THRESHOLD_MS,
+                 "stop() with stoppingTimeout=0 blocked for " + elapsed + " ms — expected immediate return");
+   }
+
+   // Fake endpoint: blocks until close(), then sleeps SLOW_ENDPOINT_BLOCK_MS ignoring interrupts,
+   // simulating a thread that is slow to exit so that thread.join() would actually block.
+
+   private static final class SlowEndpointFactory implements BroadcastEndpointFactory {
+
+      private final long blockMs;
+
+      SlowEndpointFactory(long blockMs) {
+         this.blockMs = blockMs;
+      }
+
+      @Override
+      public BroadcastEndpoint createBroadcastEndpoint() {
+         return new SlowBroadcastEndpoint(blockMs);
+      }
+   }
+
+   private static final class SlowBroadcastEndpoint implements BroadcastEndpoint {
+
+      private final long blockMs;
+      private volatile boolean closed = false;
+
+      SlowBroadcastEndpoint(long blockMs) {
+         this.blockMs = blockMs;
+      }
+
+      @Override
+      public void openClient() {
+         closed = false;
+      }
+
+      @Override
+      public void openBroadcaster() {
+      }
+
+      @Override
+      public void close(boolean isBroadcaster) {
+         closed = true;
+      }
+
+      // InterruptedException is deliberately swallowed so the thread stays alive
+      // for blockMs after close() — forcing thread.join() to actually wait.
+      @Override
+      public byte[] receiveBroadcast() throws Exception {
+         // wait until close() is called
+         while (!closed) {
+            try {
+               Thread.sleep(10);
+            } catch (InterruptedException ignored) {
+               // swallow — the thread won't exit until close() + blockMs have elapsed
+            }
+         }
+         // simulate slow teardown after close
+         try {
+            Thread.sleep(blockMs);
+         } catch (InterruptedException ignored) {
+            // swallow — stays blocked
+         }
+         return null;
+      }
+
+      @Override
+      public byte[] receiveBroadcast(long time, TimeUnit unit) throws Exception {
+         return receiveBroadcast();
+      }
+
+      @Override
+      public void broadcast(byte[] data) {
+      }
+   }
+}