/* 
    Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.  
 */
package org.apache.wiki;

import java.lang.ref.WeakReference;
import java.util.EmptyStackException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.Stack;

import org.apache.log4j.Logger;


/**
 *  WatchDog is a general system watchdog.  You can attach any Watchable
 *  or a Thread object to it, and it will notify you if a timeout has been
 *  exceeded.
 *  <p>
 *  The notification of the timeouts is done from a separate WatchDog thread,
 *  of which there is one per watched thread.  This Thread is named 'WatchDog for
 *  XXX', where XXX is your Thread name.
 *  <p>
 *  The suggested method of obtaining a WatchDog is via the static factory
 *  method, since it will return you the correct watchdog for the current
 *  thread.  However, we do not prevent you from creating your own watchdogs
 *  either.
 *  <p>
 *  If you create a WatchDog for a Thread, the WatchDog will figure out when
 *  the Thread is dead, and will stop itself accordingly.  However, this object
 *  is not automatically released, so you might want to check it out after a while.
 *
 *  @since  2.4.92
 */
public final class WatchDog {
    
    private Watchable m_watchable;
    private Stack< State > m_stateStack = new Stack< State >();
    private boolean m_enabled = true;
    private WikiEngine m_engine;

    private static final Logger log = Logger.getLogger( WatchDog.class );

    private static HashMap< Integer, WeakReference< WatchDog > > c_kennel = new HashMap< Integer, WeakReference< WatchDog > >();
    
    private static WikiBackgroundThread c_watcherThread;

    /**
     *  Returns the current watchdog for the current thread. This
     *  is the preferred method of getting you a Watchdog, since it
     *  keeps an internal list of Watchdogs for you so that there
     *  won't be more than one watchdog per thread.
     *
     *  @param engine The WikiEngine to which the Watchdog should
     *                be bonded to.
     *  @return A usable WatchDog object.
     */
    public static WatchDog getCurrentWatchDog( WikiEngine engine ) {
        Thread t = Thread.currentThread();
        WatchDog wd = null;

        WeakReference< WatchDog > w = c_kennel.get( t.hashCode() );

        if( w != null ) {
            wd = w.get();
        }

        if( w == null || wd == null ) {
            wd = new WatchDog( engine, t );
            w = new WeakReference< WatchDog >( wd );

            synchronized( c_kennel ) {
                c_kennel.put( t.hashCode(), w );
            }
        }

        return wd;
    }

    /**
     *  Creates a new WatchDog for a Watchable.
     *
     *  @param engine  The WikiEngine.
     *  @param watch   A Watchable object.
     */
    public WatchDog( WikiEngine engine, Watchable watch ) {
        m_engine    = engine;
        m_watchable = watch;

        synchronized( this.getClass() ) {
            if( c_watcherThread == null ) {
                c_watcherThread = new WatchDogThread( engine );
                c_watcherThread.start();
            }
        }
    }

    /**
     *  Creates a new WatchDog for a Thread.  The Thread is wrapped in a Watchable wrapper for this purpose.
     *
     *  @param engine The WikiEngine
     *  @param thread A Thread for watching.
     */
    public WatchDog(WikiEngine engine, Thread thread) {
        this( engine, new ThreadWrapper(thread) );
    }

    /**
     *  Hopefully finalizes this properly.  This is rather untested for now...
     */
    private static void scrub() {
        //
        //  During finalization, the object may already be cleared (depending
        //  on the finalization order).  Therefore, it's possible that this
        //  method is called from another thread after the WatchDog itself
        //  has been cleared.
        //
        if( c_kennel == null ) {
            return;
        }

        synchronized( c_kennel ) {
            for( Iterator< Map.Entry< Integer, WeakReference< WatchDog > > > i = c_kennel.entrySet().iterator(); i.hasNext(); ) {
                Map.Entry< Integer, WeakReference< WatchDog > > e = i.next();

                WeakReference< WatchDog > w = e.getValue();

                //
                //  Remove expired as well
                //
                if( w.get() == null ) {
                    c_kennel.remove( e.getKey() );
                    scrub();
                    break;
                }
            }
        }
    }

    /**
     *  Can be used to enable the WatchDog.  Will cause a new Thread to be created, if none was existing previously.
     */
    public void enable() {
        synchronized( this.getClass() ) {
            if( !m_enabled ) {
                m_enabled = true;
                c_watcherThread = new WatchDogThread( m_engine );
                c_watcherThread.start();
            }
        }
    }

    /**
     *  Is used to disable a WatchDog.  The watchdog thread is shut down and resources released.
     */
    public void disable() {
        synchronized( this.getClass() ) {
            if( m_enabled ) {
                m_enabled = false;
                c_watcherThread.shutdown();
                c_watcherThread = null;
            }
        }
    }

    /**
     *  Enters a watched state with no expectation of the expected completion time. In practice this method is 
     *  used when you have no idea, but would like to figure out, e.g. via debugging, where exactly your Watchable is.
     *
     *  @param state A free-form string description of your state.
     */
    public void enterState( String state ) {
        enterState( state, Integer.MAX_VALUE );
    }

    /**
     *  Enters a watched state which has an expected completion time.  This is the main method for using the 
     *  WatchDog.  For example:
     *
     *  <code>
     *     WatchDog w = m_engine.getCurrentWatchDog();
     *     w.enterState("Processing Foobar", 60);
     *     foobar();
     *     w.exitState();
     *  </code>
     *
     *  If the call to foobar() takes more than 60 seconds, you will receive an ERROR in the log stream.
     *
     *  @param state A free-form string description of the state
     *  @param expectedCompletionTime The timeout in seconds.
     */
    public void enterState( String state, int expectedCompletionTime ) {
        if( log.isDebugEnabled() ){
            log.debug( m_watchable.getName() + ": Entering state " + state + 
                                               ", expected completion in " + expectedCompletionTime + " s");
        }

        synchronized( m_stateStack ) {
            State st = new State( state, expectedCompletionTime );
            m_stateStack.push( st );
        }
    }

    /**
     *  Exits a state entered with enterState().  This method does not check that the state is correct, it'll just 
     *  pop out whatever is on the top of the state stack.
     */
    public void exitState() {
        exitState( null );
    }

    /**
     *  Exits a particular state entered with enterState().  The state is checked against the current state, and if 
     *  they do not match, an error is flagged.
     *
     *  @param state The state you wish to exit.
     */
    public void exitState( String state ) {
        try {
            synchronized( m_stateStack ) {
                State st = m_stateStack.peek();
                if( state == null || st.getState().equals( state ) ) {
                    m_stateStack.pop();

                    if( log.isDebugEnabled() ) {
                        log.debug( m_watchable.getName() + ": Exiting state " + st.getState() );
                    }
                } else {
                    // FIXME: should actually go and fix things for that
                    log.error( "exitState() called before enterState()" );
                }
            }
        } catch( EmptyStackException e ) {
            log.error( "Stack for " + m_watchable.getName() + " is empty!", e );
        }
    }
    
    /**
     * helper to see if the associated stateStack is not empty.
     * 
     * @return {@code true} if not empty, {@code false} otherwise.
     */
    public boolean isStateStackNotEmpty() {
        return m_stateStack != null && !m_stateStack.isEmpty();
    }
    
    /**
     * helper to see if the associated watchable is alive.
     * 
     * @return {@code true} if it's alive, {@code false} otherwise.
     */
    public boolean isWatchableAlive() {
        return m_watchable != null && m_watchable.isAlive();
    }

    private void check() {
        if( log.isDebugEnabled() ) {
            log.debug( "Checking watchdog '" + m_watchable.getName() + "'" );
        }

        synchronized( m_stateStack ) {
            try {
                State st = m_stateStack.peek();
                long now = System.currentTimeMillis();

                if( now > st.getExpiryTime() ) {
                    log.info( "Watchable '" + m_watchable.getName() + "' exceeded timeout in state '" + st.getState() + 
                              "' by " + (now - st.getExpiryTime()) / 1000 + " seconds" + 
                             ( log.isDebugEnabled() ? "" : "Enable DEBUG-level logging to see stack traces." ) );
                    dumpStackTraceForWatchable();
                    
                    m_watchable.timeoutExceeded(st.getState());
                }
            } catch( EmptyStackException e ) {
                log.error( "Stack for " + m_watchable.getName() + " is empty!", e );
            }
        }
    }

    /**
     *  Dumps the stack traces as DEBUG level events.
     */
    private void dumpStackTraceForWatchable() {
        if( !log.isDebugEnabled() ) {
            return;
        }
        
        Map<Thread, StackTraceElement[]> stackTraces = Thread.getAllStackTraces();
        Set<Thread> threads = stackTraces.keySet();
        Iterator<Thread> threadIterator = threads.iterator();
        StringBuilder stacktrace = new StringBuilder();

        while ( threadIterator.hasNext() ) {
            Thread t = threadIterator.next();
            if( t.getName().equals( m_watchable.getName() ) ) {
                if( t.getName().equals( m_watchable.getName() ) ) {
                    stacktrace.append( "dumping stacktrace for too long running thread : " + t );
                } else {
                    stacktrace.append( "dumping stacktrace for other running thread : " + t );
                }
                StackTraceElement[] ste = stackTraces.get( t );
                for( int i = 0; i < ste.length; i++ ) {
                    stacktrace.append( "\n" + ste[i] );
                }
            }
        }
        
        log.debug( stacktrace.toString() );
    }

    /**
     *  Strictly for debugging/informative purposes.
     *
     *  @return Random ramblings.
     */
    public String toString() {
        synchronized( m_stateStack ) {
            String state = "Idle";

            try {
                State st = m_stateStack.peek();
                state = st.getState();
            } catch( EmptyStackException e ) {}
            return "WatchDog state=" + state;
        }
    }

    /**
     *  This is the chief watchdog thread.
     */
    private static class WatchDogThread extends WikiBackgroundThread {
        /** How often the watchdog thread should wake up (in seconds) */
        private static final int CHECK_INTERVAL = 30;

        public WatchDogThread( WikiEngine engine ) {
            super( engine, CHECK_INTERVAL );
            setName( "WatchDog for '" + engine.getApplicationName() + "'" );
        }

        public void startupTask() {
        }

        public void shutdownTask() {
            WatchDog.scrub();
        }

        /**
         *  Checks if the watchable is alive, and if it is, checks if
         *  the stack is finished.
         *
         *  If the watchable has been deleted in the mean time, will
         *  simply shut down itself.
         */
        public void backgroundTask() throws Exception {
            if( c_kennel == null ) {
                return;
            }
            
            synchronized( c_kennel ) {
                for( Iterator< Map.Entry< Integer, WeakReference< WatchDog > > > i = c_kennel.entrySet().iterator(); i.hasNext(); ) {
                    Map.Entry< Integer, WeakReference< WatchDog > > entry = i.next();

                    WeakReference< WatchDog > wr = entry.getValue();
                    WatchDog w = wr.get();
                    if( w != null ) {
                        if( w.isWatchableAlive() && w.isStateStackNotEmpty() ) {
                            w.check();
                        } else {
                            c_kennel.remove( entry.getKey() );
                            break;
                        }
                    }
                } // for
            } // synchronized

            WatchDog.scrub();
        }

    }

    /**
     *  A class which just stores the state in our State stack.
     */
    private static class State {
        
        protected String m_state;
        protected long   m_enterTime;
        protected long   m_expiryTime;

        protected State( String state, int expiry ) {
            m_state      = state;
            m_enterTime  = System.currentTimeMillis();
            m_expiryTime = m_enterTime + (expiry * 1000L);
        }

        protected String getState() {
            return m_state;
        }

        protected long getExpiryTime() {
            return m_expiryTime;
        }
    }

    /**
     *  This class wraps a Thread so that it can become Watchable.
     */
    private static class ThreadWrapper implements Watchable {
        private Thread m_thread;

        public ThreadWrapper( Thread thread ) {
            m_thread = thread;
        }

        public void timeoutExceeded( String state ) {
            // TODO: Figure out something sane to do here.
        }

        public String getName() {
            return m_thread.getName();
        }

        public boolean isAlive() {
            return m_thread.isAlive();
        }
    }

}