/*******************************************************************************
 *
 * Copyright (c) 2004-2009 Oracle Corporation.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 * 
 *    Kohsuke Kawaguchi
 *
 *
 *******************************************************************************/ 

package hudson.util;

import hudson.model.Hudson;
import org.eclipse.hudson.WebAppController;
import hudson.triggers.SafeTimerTask;
import hudson.triggers.Trigger;
import org.apache.commons.io.FileUtils;
import org.kohsuke.stapler.StaplerRequest;
import org.kohsuke.stapler.StaplerResponse;

import javax.servlet.ServletException;
import javax.servlet.ServletContext;
import static javax.servlet.http.HttpServletResponse.SC_INTERNAL_SERVER_ERROR;
import java.io.File;
import java.io.IOException;
import java.util.Random;
import java.util.logging.Logger;
import java.util.logging.Level;
import java.lang.management.ManagementFactory;
import java.lang.reflect.Method;

/**
 * Makes sure that no other Hudson uses our <tt>HUDSON_HOME</tt> directory, to
 * forestall the problem of running multiple instances of Hudson that point to
 * the same data directory.
 *
 * <p> This set up error occasionally happens especialy when the user is trying
 * to reassign the context path of the app, and it results in a hard-to-diagnose
 * error, so we actively check this.
 *
 * <p> The mechanism is simple. This class occasionally updates a known file
 * inside the hudson home directory, and whenever it does so, it monitors the
 * timestamp of the file to make sure no one else is updating this file. In this
 * way, while we cannot detect the problem right away, within a reasonable time
 * frame we can detect the collision.
 *
 * <p> More traditional way of doing this is to use a lock file with PID in it,
 * but unfortunately in Java, there's no reliabe way to obtain PID.
 *
 * @author Kohsuke Kawaguchi
 * @since 1.178
 */
public class DoubleLaunchChecker {

    /**
     * The timestamp of the owner file when we updated it for the last time. 0
     * to indicate that there was no update before.
     */
    private long lastWriteTime = 0L;
    /**
     * Once the error is reported, the user can choose to ignore and proceed
     * anyway, in which case the flag is set to true.
     */
    private boolean ignore = false;
    private final Random random = new Random();
    public final File home;
    /**
     * ID string of the other Hudson that we are colliding with. Can be null.
     */
    private String collidingId;

    public DoubleLaunchChecker() {
        home = Hudson.getInstance().getRootDir();
    }

    protected void execute() {
        File timestampFile = new File(home, ".owner");

        long t = timestampFile.lastModified();
        if (t != 0 && lastWriteTime != 0 && t != lastWriteTime && !ignore) {
            try {
                collidingId = FileUtils.readFileToString(timestampFile);
            } catch (IOException e) {
                LOGGER.log(Level.SEVERE, "Failed to read collision file", e);
            }
            // we noticed that someone else have updated this file.
            // switch GUI to display this error.
            WebAppController.get().install(this);
            LOGGER.severe("Collision detected. timestamp=" + t + ", expected=" + lastWriteTime);
            // we need to continue updating this file, so that the other Hudson would notice the problem, too.
        }

        try {
            FileUtils.writeStringToFile(timestampFile, getId());
            lastWriteTime = timestampFile.lastModified();
        } catch (IOException e) {
            // if failed to write, err on the safe side and assume things are OK.
            lastWriteTime = 0;
        }

        schedule();
    }

    /**
     * Figures out a string that identifies this instance of Hudson.
     */
    public String getId() {
        Hudson h = Hudson.getInstance();

        // in servlet 2.5, we can get the context path
        String contextPath = "";
        try {
            Method m = ServletContext.class.getMethod("getContextPath");
            contextPath = " contextPath=\"" + m.invoke(h.servletContext) + "\"";
        } catch (Exception e) {
            // maybe running with Servlet 2.4
        }

        return h.hashCode() + contextPath + " at " + ManagementFactory.getRuntimeMXBean().getName();
    }

    public String getCollidingId() {
        return collidingId;
    }

    /**
     * Schedules the next execution.
     */
    public void schedule() {
        // randomize the scheduling so that multiple Hudson instances will write at the file at different time
        long MINUTE = 1000 * 60;
        Trigger.timer.schedule(new SafeTimerTask() {
            protected void doRun() {
                execute();
            }
        }, (random.nextInt(30) + 60) * MINUTE);
    }

    /**
     * Serve all URLs with the index view.
     */
    public void doDynamic(StaplerRequest req, StaplerResponse rsp) throws IOException, ServletException {
        rsp.setStatus(SC_INTERNAL_SERVER_ERROR);
        req.getView(this, "index.jelly").forward(req, rsp);
    }

    /**
     * Ignore the problem and go back to using Hudson.
     */
    public void doIgnore(StaplerRequest req, StaplerResponse rsp) throws IOException {
        ignore = true;
        WebAppController.get().install(Hudson.getInstance());
        rsp.sendRedirect2(req.getContextPath() + '/');
    }
    private static final Logger LOGGER = Logger.getLogger(DoubleLaunchChecker.class.getName());
}