Make restoreInstances clear value when previous value was gc:ed (#17603)

Change-Id: I61992c04cac578dc1158efd9c76484eacc8a6b6a

server/src/com/vaadin/util/CurrentInstance.java

                  * unless it respects null values, will just leave the wrong UI
                  * instance registered.
                  */
-                set(c, null, ci.inheritable);
-            } else {
-                set(c, v, ci.inheritable);
+                v = null;
             }
+            set(c, v, ci.inheritable);
         }
 
         if (removeStale) {

server/tests/src/com/vaadin/util/CurrentInstanceTest.java

 
 import static org.junit.Assert.assertNull;
 
+import java.lang.ref.WeakReference;
 import java.lang.reflect.Field;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
         assertNull(CurrentInstance.get(VaadinSession.class));
         assertNull(CurrentInstance.get(VaadinService.class));
     }
+
+    @Test
+    public void testRestoreWithGarbageCollectedValue()
+            throws InterruptedException {
+        VaadinSession session1 = new VaadinSession(null) {
+            @Override
+            public String toString() {
+                return "First session";
+            }
+        };
+        VaadinSession session2 = new VaadinSession(null) {
+            @Override
+            public String toString() {
+                return "Second session";
+            }
+        };
+
+        VaadinSession.setCurrent(session1);
+        Map<Class<?>, CurrentInstance> previous = CurrentInstance
+                .setCurrent(session2);
+
+        // Use weak ref to verify object is collected
+        WeakReference<VaadinSession> ref = new WeakReference<VaadinSession>(
+                session1);
+
+        session1 = null;
+        waitUntilGarbageCollected(ref);
+
+        CurrentInstance.restoreInstances(previous);
+
+        Assert.assertNull(VaadinSession.getCurrent());
+    }
+
+    private static void waitUntilGarbageCollected(WeakReference<?> ref)
+            throws InterruptedException {
+        for (int i = 0; i < 50; i++) {
+            System.gc();
+            if (ref.get() == null) {
+                return;
+            }
+            Thread.sleep(100);
+        }
+        Assert.fail("Value was not garbage collected.");
+    }
 }