fix(backup): avoid hang when closing unbound channel

framework/src/main/java/org/tron/common/backup/BackupManager.java

@@ -172,6 +172,14 @@ public void handleEvent(UdpEvent udpEvent) {
     }
   }
 
+  /**
+   * Stop the keep-alive scheduler without waiting so the UDP channel can be closed
+   * before a subsequent write() hits a closed channel.
+   */
+  public void stopScheduler() {
+    executorService.shutdown();
+  }
+
   public void stop() {
     ExecutorServiceManager.shutdownAndAwaitTermination(executorService, esName);
     if (dnsExecutorService != null) {

framework/src/main/java/org/tron/common/backup/socket/BackupServer.java

@@ -27,7 +27,8 @@ public class BackupServer implements AutoCloseable {
 
   private BackupManager backupManager;
 
-  private Channel channel;
+  // volatile: Dekker pairing with shutdown so close() observes bind and start() observes close.
+  private volatile Channel channel;
 
   private volatile boolean shutdown = false;
 
@@ -52,6 +53,11 @@ public void initServer() {
     }
   }
 
+  public boolean isBound() {
+    Channel ch = channel;
+    return ch != null && ch.isActive();
+  }
+
   private void start() throws Exception {
     NioEventLoopGroup group = new NioEventLoopGroup(1);
     try {
@@ -77,6 +83,11 @@ public void initChannel(NioDatagramChannel ch)
 
         logger.info("Backup server started, bind port {}", port);
 
+        // close() may have raced bind (Dekker). Close now so closeFuture.sync() cannot hang.
+        if (shutdown) {
+          channel.close();
+        }
+
         channel.closeFuture().sync();
         if (shutdown) {
           logger.info("Shutdown backup BackupServer");
@@ -95,14 +106,16 @@ public void initChannel(NioDatagramChannel ch)
   public void close() {
     logger.info("Closing backup server...");
     shutdown = true;
-    backupManager.stop();
+    // Stop keep-alive first (no wait) so it cannot write() after the channel is closed.
+    backupManager.stopScheduler();
     if (channel != null) {
       try {
         channel.close().await(10, TimeUnit.SECONDS);
       } catch (Exception e) {
         logger.warn("Closing backup server failed.", e);
       }
     }
+    backupManager.stop();
     ExecutorServiceManager.shutdownAndAwaitTermination(executor, name);
     logger.info("Backup server closed.");
   }

framework/src/test/java/org/tron/common/backup/BackupServerTest.java

@@ -3,6 +3,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import org.junit.After;
+import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -46,7 +47,10 @@ public void tearDown() {
   @Test(timeout = 60_000)
   public void test() throws InterruptedException {
     backupServer.initServer();
-    // wait for the server to start so channel is assigned before close() is called
-    Thread.sleep(1000);
+    long deadline = System.currentTimeMillis() + 15_000L;
+    while (!backupServer.isBound() && System.currentTimeMillis() < deadline) {
+      Thread.sleep(50);
+    }
+    Assert.assertTrue("BackupServer UDP channel did not bind", backupServer.isBound());
   }
 }

framework/src/test/java/org/tron/common/utils/PublicMethod.java

@@ -343,8 +343,8 @@ public static int chooseRandomPort(int min, int max) {
   }
 
   private static boolean checkPortAvailable(int port) throws IOException {
-    try (java.net.ServerSocket ss = new java.net.ServerSocket(port)) {
-      ss.setReuseAddress(true);
+    try (java.net.ServerSocket ss = new java.net.ServerSocket(port);
+         java.net.DatagramSocket ds = new java.net.DatagramSocket(port)) {
       return true;
     } catch (IOException e) {
       return false;