/**
    * This file Copyright (c) 2011-2016 Magnolia International
    * Ltd.  (http://www.magnolia-cms.com). All rights reserved.
    *
    *
    * This file is dual-licensed under both the Magnolia
    * Network Agreement and the GNU General Public License.
    * You may elect to use one or the other of these licenses.
    *
   * This file is distributed in the hope that it will be
   * useful, but AS-IS and WITHOUT ANY WARRANTY; without even the
   * implied warranty of MERCHANTABILITY or FITNESS FOR A
   * PARTICULAR PURPOSE, TITLE, or NONINFRINGEMENT.
   * Redistribution, except as permitted by whichever of the GPL
   * or MNA you select, is prohibited.
   *
   * 1. For the GPL license (GPL), you can redistribute and/or
   * modify this file under the terms of the GNU General
   * Public License, Version 3, as published by the Free Software
   * Foundation.  You should have received a copy of the GNU
   * General Public License, Version 3 along with this program;
   * if not, write to the Free Software Foundation, Inc., 51
   * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * 2. For the Magnolia Network Agreement (MNA), this file
   * and the accompanying materials are made available under the
   * terms of the MNA which accompanies this distribution, and
   * is available at http://www.magnolia-cms.com/mna.html
   *
   * Any modifications to this file must keep this entire header
   * intact.
   *
   */
  package info.magnolia.cms.filters;
  
  import java.io.IOException;
  import java.util.concurrent.locks.ReentrantReadWriteLock;
  
  import javax.servlet.FilterChain;
  import javax.servlet.FilterConfig;
  import javax.servlet.ServletException;
  import javax.servlet.ServletRequest;
  import javax.servlet.ServletResponse;
  import javax.servlet.http.HttpServletRequest;
  
  
  /**
   * Wrapper for a MgnlFilter that using a read/write lock ensures that the destroy method waits for requests to complete
   * before destroying the target filter. All use of the wrapper must be done while holding the read lock. The exception
   * to this is the destroy method that MUST NOT be called while holding the read lock, doing so would result in dead
   * lock.
   */
  public class SafeDestroyMgnlFilterWrapper implements MgnlFilter {
  
      private final MgnlFilter target;
      private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);
  
      public SafeDestroyMgnlFilterWrapper(MgnlFilter target) {
          if (target == null) {
              throw new NullPointerException("Target filter must not be null");
          }
          this.target = target;
      }
  
      public void acquireReadLock() {
          lock.readLock().lock();
      }
  
      public void releaseReadLock() {
          lock.readLock().unlock();
      }
  
      @Override
      public void init(FilterConfig filterConfig) throws ServletException {
          target.init(filterConfig);
      }
  
      /**
       * Read lock must be held when invoking this method.
       */
      @Override
      public String getName() {
          return target.getName();
      }
  
      /**
       * Read lock must be held when invoking this method.
       */
      @Override
      public void setName(String name) {
          target.setName(name);
      }
  
      /**
       * Read lock must be held when invoking this method.
       */
      @Override
      public boolean matches(HttpServletRequest request) {
          return target.matches(request);
     }
 
     /**
      * Read lock must be held when invoking this method.
      */
     @Override
     public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
         target.doFilter(request, response, chain);
     }
 
     /**
      * Destroys the target filter after waiting for all requests to complete. Calling this method while holding the read
      * lock will result in dead lock. Therefore a request that has passed through this filter MUST NEVER call this
      * method.
      */
     @Override
     public void destroy() {
 
         // We could employ a ThreadLocal to throw an exception instead of dead locking. There is no other way
         // to see if the calling thread is holding the read lock.
 
         lock.writeLock().lock();
         try {
             target.destroy();
         } finally {
             lock.writeLock().unlock();
         }
     }
 
     public MgnlFilter getTargetFilter() {
         return target;
     }
 
     /**
      * Construct for keeping a reference to a {@link SafeDestroyMgnlFilterWrapper} and change it with the guarantee that
      * the returned previous reference will not get more read locks. It's then safe to destroy the returned reference.
      */
     public static class Switcher {
 
         private SafeDestroyMgnlFilterWrapper filter;
 
         /**
          * Replaces the current filter with a new one and returns the previous filter. The returned filter is possibly still
          * in use by currently executing requests but it will not be used by any more requests after this method returns.
          * <p/>
          * Notes about the returned filter:
          * <ul>
          * <li>No methods other than destroy() should be called on the returned filter</li>
          * <li>The destroy() method will wait for all requests to complete before it is destroyed</li>
          * <li>Calling the destroy() method from a request that has entered the filter WILL RESULT IN DEAD LOCK.</li>
          * </ul>
          */
         public synchronized SafeDestroyMgnlFilterWrapper replaceFilter(SafeDestroyMgnlFilterWrapper newFilter) {
             SafeDestroyMgnlFilterWrapper oldFilter = filter;
             filter = newFilter;
             return oldFilter;
         }
 
         /**
          * Returns the current filter with a read lock held for the current thread. After use the thread must relinquish the
          * lock by calling releaseReadLock(). If no filter has been set this method returns null and no lock is taken.
          */
         public synchronized SafeDestroyMgnlFilterWrapper getFilterAndAcquireReadLock() {
             if (filter == null) {
                 return null;
             }
             filter.acquireReadLock();
             return filter;
         }
 
         /**
          * This method is provided for introspection only. It is inherently UNSAFE as using this method and then
          * acquiring the read lock on the returned filter breaks the guarantee that a replaced reference wont get more
          * read locks.
          */
         public synchronized SafeDestroyMgnlFilterWrapper getFilter() {
             return filter;
         }
     }
 }