fix: move thread start out of DevServerWatchDog constructor (#23898) (#23919)

Starting a thread in a constructor can expose a partially constructed
object. Extract the thread start into a separate start() method that
callers invoke after construction completes.

Co-authored-by: Artur Signell <artur@vaadin.com>
Co-authored-by: Marco Collovati <marco@vaadin.com>

Author: 3 people

vaadin-dev-server/src/main/java/com/vaadin/base/devserver/AbstractDevServerRunner.java

@@ -197,7 +197,9 @@ void doStartDevModeServer() throws ExecutionFailedException {
         long start = System.nanoTime();
         getLogger().info("Starting " + getServerName());
 
-        watchDog.set(new DevServerWatchDog());
+        DevServerWatchDog newWatchDog = new DevServerWatchDog();
+        newWatchDog.start();
+        watchDog.set(newWatchDog);
 
         // Look for a free port
         port = getFreePort();

vaadin-dev-server/src/main/java/com/vaadin/base/devserver/DevServerWatchDog.java

@@ -79,7 +79,9 @@ private Logger getLogger() {
 
     DevServerWatchDog() {
         watchDogServer = new WatchDogServer();
+    }
 
+    void start() {
         Thread serverThread = new Thread(watchDogServer);
         serverThread.setDaemon(true);
         serverThread.start();

vaadin-dev-server/src/test/java/com/vaadin/base/devserver/AbstractDevServerRunnerTest.java

@@ -190,6 +190,7 @@ private void assertOnDevProcessEnvironment(
             Class<? extends InetAddress> loopbackAddressType,
             Consumer<Map<String, String>> op) {
         final DevServerWatchDog watchDog = new DevServerWatchDog();
+        watchDog.start();
         final InetAddress loopbackAddress = findLocalhostAddress(
                 loopbackAddressType);
         try {