fix: make Popover static methods thread-safe (#6864) (CP: 24.5) (#6868)

Co-authored-by: Diego Cardoso <diego@vaadin.com>

vaadin-popover-flow-parent/vaadin-popover-flow/src/main/java/com/vaadin/flow/component/popover/Popover.java

@@ -21,6 +21,7 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.Collectors;
 
 import com.vaadin.flow.component.AttachEvent;
@@ -67,7 +68,8 @@ public class Popover extends Component implements HasAriaLabel, HasComponents,
     private static Integer defaultHideDelay;
     private static Integer defaultFocusDelay;
     private static Integer defaultHoverDelay;
-    private static boolean uiInitListenerRegistered = false;
+    final static AtomicBoolean uiInitListenerRegistered = new AtomicBoolean(
+            false);
 
     private Component target;
     private Registration targetAttachRegistration;
@@ -148,11 +150,10 @@ private static void applyConfiguration() {
             applyConfigurationForUI(UI.getCurrent());
         }
 
-        if (!uiInitListenerRegistered) {
+        if (uiInitListenerRegistered.compareAndSet(false, true)) {
             // Apply the popover configuration for all new UIs
             VaadinService.getCurrent()
                     .addUIInitListener(e -> applyConfigurationForUI(e.getUI()));
-            uiInitListenerRegistered = true;
         }
     }
 

vaadin-popover-flow-parent/vaadin-popover-flow/src/test/java/com/vaadin/flow/component/popover/PopoverTest.java

@@ -15,12 +15,22 @@
  */
 package com.vaadin.flow.component.popover;
 
+import static org.mockito.Mockito.times;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.stream.IntStream;
+
 import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
+import org.mockito.Mockito;
 
 import com.vaadin.flow.component.Text;
 import com.vaadin.flow.component.html.Div;
+import com.vaadin.flow.server.VaadinService;
 
 import elemental.json.JsonArray;
 
@@ -294,4 +304,48 @@ public void popoverWithContent() {
         Assert.assertSame(content,
                 popoverWithContent.getChildren().findFirst().get());
     }
+
+    @Test
+    public void testSetDefaultFocusDelay_threadSafety() {
+        testStaticSettersThreadsSafety(
+                () -> Popover.setDefaultFocusDelay(1000));
+    }
+
+    @Test
+    public void testSetDefaultHoverDelay_threadSafety() {
+        testStaticSettersThreadsSafety(
+                () -> Popover.setDefaultHoverDelay(1000));
+    }
+
+    @Test
+    public void testSetDefaultHideDelay_threadSafety() {
+        testStaticSettersThreadsSafety(() -> Popover.setDefaultHideDelay(1000));
+    }
+
+    private void testStaticSettersThreadsSafety(Runnable tester) {
+        // Reset the static state for each test
+        Popover.uiInitListenerRegistered.set(false);
+        final VaadinService current = Mockito.mock(VaadinService.class);
+
+        ExecutorService executorService = Executors.newFixedThreadPool(10);
+        final CountDownLatch latch = new CountDownLatch(10);
+        final Runnable runnable = () -> {
+            VaadinService.setCurrent(current);
+            latch.countDown();
+            for (int i = 0; i < 100000; i++) {
+                tester.run();
+            }
+        };
+        final var list = IntStream.range(0, 10)
+                .mapToObj(it -> executorService.submit(runnable)).toList();
+        for (var future : list) {
+            try {
+                future.get();
+            } catch (InterruptedException | ExecutionException e) {
+                throw new RuntimeException(e);
+            }
+        }
+        executorService.shutdown();
+        Mockito.verify(current, times(1)).addUIInitListener(Mockito.any());
+    }
 }