From 79e9ddc5247ae154d5f63ceeeb02afb293dbc91f Mon Sep 17 00:00:00 2001
From: Leif Åstrand <leif@vaadin.com>
Date: Thu, 2 May 2013 14:19:06 +0300
Subject: Make access() throw if another session is already locked (#11757)

Change-Id: Ic8571b0d078c3ff64e8b9914e55c51766ae49024
---
 server/src/com/vaadin/server/VaadinService.java | 19 +++++++++++++++++++
 server/src/com/vaadin/server/VaadinSession.java | 12 ++++++++++++
 server/src/com/vaadin/ui/UI.java                | 10 ++++++++++
 3 files changed, 41 insertions(+)

diff --git a/server/src/com/vaadin/server/VaadinService.java b/server/src/com/vaadin/server/VaadinService.java
index f6b520d4b0..083bd4bedf 100644
--- a/server/src/com/vaadin/server/VaadinService.java
+++ b/server/src/com/vaadin/server/VaadinService.java
@@ -1540,6 +1540,25 @@ public abstract class VaadinService implements Serializable {
         return false;
     }
 
+    /**
+     * Checks that another {@link VaadinSession} instance is not locked. This is
+     * internally used by {@link VaadinSession#access(Runnable)} and
+     * {@link UI#access(Runnable)} to help avoid causing deadlocks.
+     * 
+     * @since 7.1
+     * @param session
+     *            the session that is being locked
+     * @throws IllegalStateException
+     *             if the current thread holds the lock for another session
+     */
+    public static void verifyNoOtherSessionLocked(VaadinSession session) {
+        VaadinSession otherSession = VaadinSession.getCurrent();
+        if (otherSession != null && otherSession != session) {
+            throw new IllegalStateException(
+                    "Can't access session while another session is locked by the same thread. This restriction is intended to help avoid deadlocks.");
+        }
+    }
+
     /**
      * Verifies that the given CSRF token (aka double submit cookie) is valid
      * for the given session. This is used to protect against Cross Site Request
diff --git a/server/src/com/vaadin/server/VaadinSession.java b/server/src/com/vaadin/server/VaadinSession.java
index fe786ad52b..317ea6cf7b 100644
--- a/server/src/com/vaadin/server/VaadinSession.java
+++ b/server/src/com/vaadin/server/VaadinSession.java
@@ -1075,15 +1075,27 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
      * as the session is automatically locked by the framework during request
      * handling.
      * </p>
+     * <p>
+     * Note that calling this method while another session is locked by the
+     * current thread will cause an exception. This is to prevent deadlock
+     * situations when two threads have locked one session each and are both
+     * waiting for the lock for the other session.
+     * </p>
      * 
      * @param runnable
      *            the runnable which accesses the session
      * 
+     * @throws IllegalStateException
+     *             if the current thread holds the lock for another session
+     * 
+     * 
      * @see #lock()
      * @see #getCurrent()
      * @see UI#access(Runnable)
      */
     public void access(Runnable runnable) {
+        VaadinService.verifyNoOtherSessionLocked(this);
+
         Map<Class<?>, CurrentInstance> old = null;
         lock();
         try {
diff --git a/server/src/com/vaadin/ui/UI.java b/server/src/com/vaadin/ui/UI.java
index f260a6af79..6433bebbe4 100644
--- a/server/src/com/vaadin/ui/UI.java
+++ b/server/src/com/vaadin/ui/UI.java
@@ -1103,12 +1103,20 @@ public abstract class UI extends AbstractSingleComponentContainer implements
      * RPC handlers for components inside this UI do not need this method as the
      * session is automatically locked by the framework during request handling.
      * </p>
+     * <p>
+     * Note that calling this method while another session is locked by the
+     * current thread will cause an exception. This is to prevent deadlock
+     * situations when two threads have locked one session each and are both
+     * waiting for the lock for the other session.
+     * </p>
      * 
      * @param runnable
      *            the runnable which accesses the UI
      * @throws UIDetachedException
      *             if the UI is not attached to a session (and locking can
      *             therefore not be done)
+     * @throws IllegalStateException
+     *             if the current thread holds the lock for another session
      * 
      * @see #getCurrent()
      * @see VaadinSession#access(Runnable)
@@ -1123,6 +1131,8 @@ public abstract class UI extends AbstractSingleComponentContainer implements
             throw new UIDetachedException();
         }
 
+        VaadinService.verifyNoOtherSessionLocked(session);
+
         session.lock();
         try {
             if (getSession() == null) {
-- 
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