Fix race condition that might leave access queue unpurged (#12277)

Merge: no
Change-Id: Iee1012486906d8c2c46cef94cfcd6d2b399d7a6b

server/src/com/vaadin/server/VaadinService.java

@@ -1655,6 +1655,23 @@ public abstract class VaadinService implements Serializable {
         FutureAccess future = new FutureAccess(session, runnable);
         session.getPendingAccessQueue().add(future);
 
+        ensureAccessQueuePurged(session);
+
+        return future;
+    }
+
+    /**
+     * Makes sure the pending access queue is purged for the provided session.
+     * If the session is currently locked by the current thread or some other
+     * thread, the queue will be purged when the session is unlocked. If the
+     * lock is not held by any thread, it is acquired and the queue is purged
+     * right away.
+     * 
+     * @since 7.1.2
+     * @param session
+     *            the session for which the access queue should be purged
+     */
+    public void ensureAccessQueuePurged(VaadinSession session) {
         /*
          * If no thread is currently holding the lock, pending changes for UIs
          * with automatic push would not be processed and pushed until the next
@@ -1677,8 +1694,6 @@ public abstract class VaadinService implements Serializable {
         } catch (InterruptedException e) {
             // Just ignore
         }
-
-        return future;
     }
 
     /**

server/src/com/vaadin/server/VaadinSession.java

@@ -926,6 +926,18 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
         } finally {
             getLockInstance().unlock();
         }
+
+        /*
+         * If the session is locked when a new access task is added, it is
+         * assumed that the queue will be purged when the lock is released. This
+         * might however not happen if a task is enqueued between the moment
+         * when unlock() purges the queue and the moment when the lock is
+         * actually released. This means that the queue should be purged again
+         * if it is not empty after unlocking.
+         */
+        if (!getPendingAccessQueue().isEmpty()) {
+            getService().ensureAccessQueuePurged(this);
+        }
     }
 
     /**