hudson.slaves.SlaveComputer.java Source code

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/*
 * The MIT License
 * 
 * Copyright (c) 2004-2009, Sun Microsystems, Inc., Kohsuke Kawaguchi, Stephen Connolly
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package hudson.slaves;

import hudson.AbortException;
import hudson.FilePath;
import hudson.Util;
import hudson.model.Computer;
import hudson.model.Executor;
import hudson.model.ExecutorListener;
import hudson.model.Node;
import hudson.model.Queue;
import hudson.model.Slave;
import hudson.model.TaskListener;
import hudson.model.User;
import hudson.remoting.Callable;
import hudson.remoting.Channel;
import hudson.remoting.Launcher;
import hudson.remoting.VirtualChannel;
import hudson.security.ACL;
import hudson.slaves.OfflineCause.ChannelTermination;
import hudson.util.Futures;
import hudson.util.NullStream;
import hudson.util.RingBufferLogHandler;
import hudson.util.StreamTaskListener;
import hudson.util.io.ReopenableFileOutputStream;
import hudson.util.io.ReopenableRotatingFileOutputStream;
import jenkins.model.Jenkins;
import jenkins.slaves.EncryptedSlaveAgentJnlpFile;
import jenkins.slaves.JnlpSlaveAgentProtocol;
import jenkins.slaves.systemInfo.SlaveSystemInfo;
import org.acegisecurity.context.SecurityContext;
import org.acegisecurity.context.SecurityContextHolder;
import org.apache.commons.io.IOUtils;
import org.kohsuke.stapler.HttpRedirect;
import org.kohsuke.stapler.HttpResponse;
import org.kohsuke.stapler.QueryParameter;
import org.kohsuke.stapler.StaplerRequest;
import org.kohsuke.stapler.StaplerResponse;
import org.kohsuke.stapler.WebMethod;
import org.kohsuke.stapler.interceptor.RequirePOST;

import javax.servlet.ServletException;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintStream;
import java.nio.charset.Charset;
import java.security.Security;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Future;
import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.LogRecord;
import java.util.logging.Logger;

import javax.annotation.CheckForNull;
import jenkins.model.Jenkins;
import static hudson.slaves.SlaveComputer.LogHolder.*;

/**
 * {@link Computer} for {@link Slave}s.
 *
 * @author Kohsuke Kawaguchi
 */
public class SlaveComputer extends Computer {
    private volatile Channel channel;
    private volatile transient boolean acceptingTasks = true;
    private Charset defaultCharset;
    private Boolean isUnix;
    /**
     * Effective {@link ComputerLauncher} that hides the details of
     * how we launch a slave agent on this computer.
     *
     * <p>
     * This is normally the same as {@link Slave#getLauncher()} but
     * can be different. See {@link #grabLauncher(Node)}. 
     */
    private ComputerLauncher launcher;

    /**
     * Perpetually writable log file.
     */
    private final ReopenableFileOutputStream log;

    /**
     * {@link StreamTaskListener} that wraps {@link #log}, hence perpetually writable.
     */
    private final TaskListener taskListener;

    /**
     * Number of failed attempts to reconnect to this node
     * (so that if we keep failing to reconnect, we can stop
     * trying.)
     */
    private transient int numRetryAttempt;

    /**
     * Tracks the status of the last launch operation, which is always asynchronous.
     * This can be used to wait for the completion, or cancel the launch activity.
     */
    private volatile Future<?> lastConnectActivity = null;

    private Object constructed = new Object();

    public SlaveComputer(Slave slave) {
        super(slave);
        this.log = new ReopenableRotatingFileOutputStream(getLogFile(), 10);
        this.taskListener = new StreamTaskListener(log);
        assert slave.getNumExecutors() != 0 : "Computer created with 0 executors";
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean isAcceptingTasks() {
        return acceptingTasks;
    }

    /**
     * @since 1.498
     */
    public String getJnlpMac() {
        return JnlpSlaveAgentProtocol.SLAVE_SECRET.mac(getName());
    }

    /**
     * Allows a {@linkplain hudson.slaves.ComputerLauncher} or a {@linkplain hudson.slaves.RetentionStrategy} to
     * suspend tasks being accepted by the slave computer.
     *
     * @param acceptingTasks {@code true} if the slave can accept tasks.
     */
    public void setAcceptingTasks(boolean acceptingTasks) {
        this.acceptingTasks = acceptingTasks;
    }

    /**
     * True if this computer is a Unix machine (as opposed to Windows machine).
     *
     * @return
     *      null if the computer is disconnected and therefore we don't know whether it is Unix or not.
     */
    public Boolean isUnix() {
        return isUnix;
    }

    @CheckForNull
    @Override
    public Slave getNode() {
        Node node = super.getNode();
        if (node == null || node instanceof Slave) {
            return (Slave) node;
        } else {
            logger.log(Level.WARNING, "found an unexpected kind of node {0} from {1} with nodeName={2}",
                    new Object[] { node, this, nodeName });
            return null;
        }
    }

    @Override
    public String getIcon() {
        Future<?> l = lastConnectActivity;
        if (l != null && !l.isDone())
            return "computer-flash.gif";
        return super.getIcon();
    }

    /**
     * @deprecated since 2008-05-20.
     */
    @Deprecated
    @Override
    public boolean isJnlpAgent() {
        return launcher instanceof JNLPLauncher;
    }

    @Override
    public boolean isLaunchSupported() {
        return launcher.isLaunchSupported();
    }

    public ComputerLauncher getLauncher() {
        return launcher;
    }

    protected Future<?> _connect(boolean forceReconnect) {
        if (channel != null)
            return Futures.precomputed(null);
        if (!forceReconnect && isConnecting())
            return lastConnectActivity;
        if (forceReconnect && isConnecting())
            logger.fine("Forcing a reconnect on " + getName());

        closeChannel();
        return lastConnectActivity = Computer.threadPoolForRemoting
                .submit(new java.util.concurrent.Callable<Object>() {
                    public Object call() throws Exception {
                        // do this on another thread so that the lengthy launch operation
                        // (which is typical) won't block UI thread.

                        ACL.impersonate(ACL.SYSTEM); // background activity should run like a super user

                        try {
                            log.rewind();
                            try {
                                for (ComputerListener cl : ComputerListener.all())
                                    cl.preLaunch(SlaveComputer.this, taskListener);

                                launcher.launch(SlaveComputer.this, taskListener);
                            } catch (AbortException e) {
                                taskListener.error(e.getMessage());
                                throw e;
                            } catch (IOException e) {
                                Util.displayIOException(e, taskListener);
                                e.printStackTrace(taskListener.error(Messages.ComputerLauncher_unexpectedError()));
                                throw e;
                            } catch (InterruptedException e) {
                                e.printStackTrace(taskListener.error(Messages.ComputerLauncher_abortedLaunch()));
                                throw e;
                            }
                        } finally {
                            if (channel == null) {
                                offlineCause = new OfflineCause.LaunchFailed();
                                for (ComputerListener cl : ComputerListener.all())
                                    cl.onLaunchFailure(SlaveComputer.this, taskListener);
                            }
                        }

                        if (channel == null)
                            throw new IOException(
                                    "Slave failed to connect, even though the launcher didn't report it. See the log output for details.");
                        return null;
                    }
                });
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void taskAccepted(Executor executor, Queue.Task task) {
        super.taskAccepted(executor, task);
        if (launcher instanceof ExecutorListener) {
            ((ExecutorListener) launcher).taskAccepted(executor, task);
        }

        //getNode() can return null at indeterminate times when nodes go offline
        Slave node = getNode();
        if (node != null && node.getRetentionStrategy() instanceof ExecutorListener) {
            ((ExecutorListener) node.getRetentionStrategy()).taskAccepted(executor, task);
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void taskCompleted(Executor executor, Queue.Task task, long durationMS) {
        super.taskCompleted(executor, task, durationMS);
        if (launcher instanceof ExecutorListener) {
            ((ExecutorListener) launcher).taskCompleted(executor, task, durationMS);
        }
        RetentionStrategy r = getRetentionStrategy();
        if (r instanceof ExecutorListener) {
            ((ExecutorListener) r).taskCompleted(executor, task, durationMS);
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void taskCompletedWithProblems(Executor executor, Queue.Task task, long durationMS, Throwable problems) {
        super.taskCompletedWithProblems(executor, task, durationMS, problems);
        if (launcher instanceof ExecutorListener) {
            ((ExecutorListener) launcher).taskCompletedWithProblems(executor, task, durationMS, problems);
        }
        RetentionStrategy r = getRetentionStrategy();
        if (r instanceof ExecutorListener) {
            ((ExecutorListener) r).taskCompletedWithProblems(executor, task, durationMS, problems);
        }
    }

    @Override
    public boolean isConnecting() {
        Future<?> l = lastConnectActivity;
        return isOffline() && l != null && !l.isDone();
    }

    public OutputStream openLogFile() {
        try {
            log.rewind();
            return log;
        } catch (IOException e) {
            logger.log(Level.SEVERE, "Failed to create log file " + getLogFile(), e);
            return new NullStream();
        }
    }

    private final Object channelLock = new Object();

    public void setChannel(InputStream in, OutputStream out, TaskListener taskListener, Channel.Listener listener)
            throws IOException, InterruptedException {
        setChannel(in, out, taskListener.getLogger(), listener);
    }

    /**
     * Creates a {@link Channel} from the given stream and sets that to this slave.
     *
     * @param in
     *      Stream connected to the remote "slave.jar". It's the caller's responsibility to do
     *      buffering on this stream, if that's necessary.
     * @param out
     *      Stream connected to the remote peer. It's the caller's responsibility to do
     *      buffering on this stream, if that's necessary.
     * @param launchLog
     *      If non-null, receive the portion of data in <tt>is</tt> before
     *      the data goes into the "binary mode". This is useful
     *      when the established communication channel might include some data that might
     *      be useful for debugging/trouble-shooting.
     * @param listener
     *      Gets a notification when the channel closes, to perform clean up. Can be null.
     *      By the time this method is called, the cause of the termination is reported to the user,
     *      so the implementation of the listener doesn't need to do that again.
     */
    public void setChannel(InputStream in, OutputStream out, OutputStream launchLog, Channel.Listener listener)
            throws IOException, InterruptedException {
        Channel channel = new Channel(nodeName, threadPoolForRemoting, Channel.Mode.NEGOTIATE, in, out, launchLog);
        setChannel(channel, launchLog, listener);
    }

    /**
     * Shows {@link Channel#classLoadingCount}.
     * @since 1.495
     */
    public int getClassLoadingCount() throws IOException, InterruptedException {
        return channel.call(new LoadingCount(false));
    }

    /**
     * Shows {@link Channel#classLoadingPrefetchCacheCount}.
     * @return -1 in case that capability is not supported
     * @since 1.519
     */
    public int getClassLoadingPrefetchCacheCount() throws IOException, InterruptedException {
        if (!channel.remoteCapability.supportsPrefetch()) {
            return -1;
        }
        return channel.call(new LoadingPrefetchCacheCount());
    }

    /**
     * Shows {@link Channel#resourceLoadingCount}.
     * @since 1.495
     */
    public int getResourceLoadingCount() throws IOException, InterruptedException {
        return channel.call(new LoadingCount(true));
    }

    /**
     * Shows {@link Channel#classLoadingTime}.
     * @since 1.495
     */
    public long getClassLoadingTime() throws IOException, InterruptedException {
        return channel.call(new LoadingTime(false));
    }

    /**
     * Shows {@link Channel#resourceLoadingTime}.
     * @since 1.495
     */
    public long getResourceLoadingTime() throws IOException, InterruptedException {
        return channel.call(new LoadingTime(true));
    }

    static class LoadingCount implements Callable<Integer, RuntimeException> {
        private final boolean resource;

        LoadingCount(boolean resource) {
            this.resource = resource;
        }

        @Override
        public Integer call() {
            Channel c = Channel.current();
            return resource ? c.resourceLoadingCount.get() : c.classLoadingCount.get();
        }
    }

    static class LoadingPrefetchCacheCount implements Callable<Integer, RuntimeException> {
        @Override
        public Integer call() {
            return Channel.current().classLoadingPrefetchCacheCount.get();
        }
    }

    static class LoadingTime implements Callable<Long, RuntimeException> {
        private final boolean resource;

        LoadingTime(boolean resource) {
            this.resource = resource;
        }

        @Override
        public Long call() {
            Channel c = Channel.current();
            return resource ? c.resourceLoadingTime.get() : c.classLoadingTime.get();
        }
    }

    /**
     * Sets up the connection through an existing channel.
     *
     * @since 1.444
     */
    public void setChannel(Channel channel, OutputStream launchLog, Channel.Listener listener)
            throws IOException, InterruptedException {
        if (this.channel != null)
            throw new IllegalStateException("Already connected");

        final TaskListener taskListener = new StreamTaskListener(launchLog);
        PrintStream log = taskListener.getLogger();

        channel.addListener(new Channel.Listener() {
            @Override
            public void onClosed(Channel c, IOException cause) {
                // Orderly shutdown will have null exception
                if (cause != null) {
                    offlineCause = new ChannelTermination(cause);
                    cause.printStackTrace(taskListener.error("Connection terminated"));
                } else {
                    taskListener.getLogger().println("Connection terminated");
                }
                closeChannel();
                launcher.afterDisconnect(SlaveComputer.this, taskListener);
            }
        });
        if (listener != null)
            channel.addListener(listener);

        String slaveVersion = channel.call(new SlaveVersion());
        log.println("Slave.jar version: " + slaveVersion);

        boolean _isUnix = channel.call(new DetectOS());
        log.println(_isUnix ? hudson.model.Messages.Slave_UnixSlave() : hudson.model.Messages.Slave_WindowsSlave());

        String defaultCharsetName = channel.call(new DetectDefaultCharset());

        Slave node = getNode();
        if (node == null) { // Node has been disabled/removed during the connection
            throw new IOException("Node " + nodeName + " has been deleted during the channel setup");
        }

        String remoteFs = node.getRemoteFS();
        if (_isUnix && !remoteFs.contains("/") && remoteFs.contains("\\"))
            log.println("WARNING: " + remoteFs + " looks suspiciously like Windows path. Maybe you meant "
                    + remoteFs.replace('\\', '/') + "?");
        FilePath root = new FilePath(channel, remoteFs);

        // reference counting problem is known to happen, such as JENKINS-9017, and so as a preventive measure
        // we pin the base classloader so that it'll never get GCed. When this classloader gets released,
        // it'll have a catastrophic impact on the communication.
        channel.pinClassLoader(getClass().getClassLoader());

        channel.call(new SlaveInitializer());
        SecurityContext old = ACL.impersonate(ACL.SYSTEM);
        try {
            for (ComputerListener cl : ComputerListener.all()) {
                cl.preOnline(this, channel, root, taskListener);
            }
        } finally {
            SecurityContextHolder.setContext(old);
        }

        offlineCause = null;

        // update the data structure atomically to prevent others from seeing a channel that's not properly initialized yet
        synchronized (channelLock) {
            if (this.channel != null) {
                // check again. we used to have this entire method in a big sycnhronization block,
                // but Channel constructor blocks for an external process to do the connection
                // if CommandLauncher is used, and that cannot be interrupted because it blocks at InputStream.
                // so if the process hangs, it hangs the thread in a lock, and since Hudson will try to relaunch,
                // we'll end up queuing the lot of threads in a pseudo deadlock.
                // This implementation prevents that by avoiding a lock. HUDSON-1705 is likely a manifestation of this.
                channel.close();
                throw new IllegalStateException("Already connected");
            }
            isUnix = _isUnix;
            numRetryAttempt = 0;
            this.channel = channel;
            defaultCharset = Charset.forName(defaultCharsetName);

            synchronized (statusChangeLock) {
                statusChangeLock.notifyAll();
            }
        }
        old = ACL.impersonate(ACL.SYSTEM);
        try {
            for (ComputerListener cl : ComputerListener.all()) {
                cl.onOnline(this, taskListener);
            }
        } finally {
            SecurityContextHolder.setContext(old);
        }
        log.println("Slave successfully connected and online");
        Jenkins.getInstance().getQueue().scheduleMaintenance();
    }

    @Override
    public Channel getChannel() {
        return channel;
    }

    public Charset getDefaultCharset() {
        return defaultCharset;
    }

    public List<LogRecord> getLogRecords() throws IOException, InterruptedException {
        if (channel == null)
            return Collections.emptyList();
        else
            return channel.call(new SlaveLogFetcher());
    }

    @RequirePOST
    public HttpResponse doDoDisconnect(@QueryParameter String offlineMessage) throws IOException, ServletException {
        if (channel != null) {
            //does nothing in case computer is already disconnected
            checkPermission(DISCONNECT);
            offlineMessage = Util.fixEmptyAndTrim(offlineMessage);
            disconnect(new OfflineCause.UserCause(User.current(), offlineMessage));
        }
        return new HttpRedirect(".");
    }

    @Override
    public Future<?> disconnect(OfflineCause cause) {
        super.disconnect(cause);
        return Computer.threadPoolForRemoting.submit(new Runnable() {
            public void run() {
                // do this on another thread so that any lengthy disconnect operation
                // (which could be typical) won't block UI thread.
                launcher.beforeDisconnect(SlaveComputer.this, taskListener);
                closeChannel();
                launcher.afterDisconnect(SlaveComputer.this, taskListener);
            }
        });
    }

    @RequirePOST
    public void doLaunchSlaveAgent(StaplerRequest req, StaplerResponse rsp) throws IOException, ServletException {
        if (channel != null) {
            req.getView(this, "already-launched.jelly").forward(req, rsp);
            return;
        }

        connect(true);

        // TODO: would be nice to redirect the user to "launching..." wait page,
        // then spend a few seconds there and poll for the completion periodically.
        rsp.sendRedirect("log");
    }

    public void tryReconnect() {
        numRetryAttempt++;
        if (numRetryAttempt < 6 || (numRetryAttempt % 12) == 0) {
            // initially retry several times quickly, and after that, do it infrequently.
            logger.info("Attempting to reconnect " + nodeName);
            connect(true);
        }
    }

    /**
     * Serves jar files for JNLP slave agents.
     *
     * @deprecated since 2008-08-18.
     *      This URL binding is no longer used and moved up directly under to {@link jenkins.model.Jenkins},
     *      but it's left here for now just in case some old JNLP slave agents request it.
     */
    public Slave.JnlpJar getJnlpJars(String fileName) {
        return new Slave.JnlpJar(fileName);
    }

    @WebMethod(name = "slave-agent.jnlp")
    public HttpResponse doSlaveAgentJnlp(StaplerRequest req, StaplerResponse res)
            throws IOException, ServletException {
        return new EncryptedSlaveAgentJnlpFile(this, "slave-agent.jnlp.jelly", getName(), CONNECT);
    }

    @Override
    protected void kill() {
        super.kill();
        closeChannel();
        IOUtils.closeQuietly(log);
    }

    public RetentionStrategy getRetentionStrategy() {
        Slave n = getNode();
        return n == null ? RetentionStrategy.INSTANCE : n.getRetentionStrategy();
    }

    /**
     * If still connected, disconnect.
     */
    private void closeChannel() {
        // TODO: race condition between this and the setChannel method.
        Channel c = channel;
        channel = null;
        isUnix = null;
        if (c != null) {
            try {
                c.close();
            } catch (IOException e) {
                logger.log(Level.SEVERE, "Failed to terminate channel to " + getDisplayName(), e);
            }
            for (ComputerListener cl : ComputerListener.all())
                cl.onOffline(this);
        }
    }

    @Override
    protected void setNode(Node node) {
        super.setNode(node);
        launcher = grabLauncher(node);

        // maybe the configuration was changed to relaunch the slave, so try to re-launch now.
        // "constructed==null" test is an ugly hack to avoid launching before the object is fully
        // constructed.
        if (constructed != null) {
            if (node instanceof Slave)
                ((Slave) node).getRetentionStrategy().check(this);
            else
                connect(false);
        }
    }

    /**
     * Grabs a {@link ComputerLauncher} out of {@link Node} to keep it in this {@link Computer}.
     * The returned launcher will be set to {@link #launcher} and used to carry out the actual launch operation.
     *
     * <p>
     * Subtypes that needs to decorate {@link ComputerLauncher} can do so by overriding this method.
     * This is useful for {@link SlaveComputer}s for clouds for example, where one normally needs
     * additional pre-launch step (such as waiting for the provisioned node to become available)
     * before the user specified launch step (like SSH connection) kicks in.
     *
     * @see ComputerLauncherFilter
     */
    protected ComputerLauncher grabLauncher(Node node) {
        return ((Slave) node).getLauncher();
    }

    /**
     * Get the slave version
     */
    public String getSlaveVersion() throws IOException, InterruptedException {
        return channel.call(new SlaveVersion());
    }

    /**
     * Get the OS description.
     */
    public String getOSDescription() throws IOException, InterruptedException {
        return channel.call(new DetectOS()) ? "Unix" : "Windows";
    }

    private static final Logger logger = Logger.getLogger(SlaveComputer.class.getName());

    private static final class SlaveVersion implements Callable<String, IOException> {
        public String call() throws IOException {
            try {
                return Launcher.VERSION;
            } catch (Throwable ex) {
                return "< 1.335";
            } // Older slave.jar won't have VERSION
        }
    }

    private static final class DetectOS implements Callable<Boolean, IOException> {
        public Boolean call() throws IOException {
            return File.pathSeparatorChar == ':';
        }
    }

    private static final class DetectDefaultCharset implements Callable<String, IOException> {
        public String call() throws IOException {
            return Charset.defaultCharset().name();
        }
    }

    /**
     * Puts the {@link #SLAVE_LOG_HANDLER} into a separate class so that loading this class
     * in JVM doesn't end up loading tons of additional classes.
     */
    static final class LogHolder {
        /**
         * This field is used on each slave node to record log records on the slave.
         */
        static final RingBufferLogHandler SLAVE_LOG_HANDLER = new RingBufferLogHandler();
    }

    private static class SlaveInitializer implements Callable<Void, RuntimeException> {
        public Void call() {
            // avoid double installation of the handler. JNLP slaves can reconnect to the master multiple times
            // and each connection gets a different RemoteClassLoader, so we need to evict them by class name,
            // not by their identity.
            for (Handler h : LOGGER.getHandlers()) {
                if (h.getClass().getName().equals(SLAVE_LOG_HANDLER.getClass().getName()))
                    LOGGER.removeHandler(h);
            }
            LOGGER.addHandler(SLAVE_LOG_HANDLER);

            // remove Sun PKCS11 provider if present. See http://wiki.jenkins-ci.org/display/JENKINS/Solaris+Issue+6276483
            try {
                Security.removeProvider("SunPKCS11-Solaris");
            } catch (SecurityException e) {
                // ignore this error.
            }

            Channel.current().setProperty("slave", Boolean.TRUE); // indicate that this side of the channel is the slave side.

            return null;
        }

        private static final long serialVersionUID = 1L;
        private static final Logger LOGGER = Logger.getLogger("");
    }

    /**
     * Obtains a {@link VirtualChannel} that allows some computation to be performed on the master.
     * This method can be called from any thread on the master, or from slave (more precisely,
     * it only works from the remoting request-handling thread in slaves, which means if you've started
     * separate thread on slaves, that'll fail.)
     *
     * @return null if the calling thread doesn't have any trace of where its master is.
     * @since 1.362
     */
    public static VirtualChannel getChannelToMaster() {
        if (Jenkins.getInstance() != null)
            return FilePath.localChannel;

        // if this method is called from within the slave computation thread, this should work
        Channel c = Channel.current();
        if (c != null && Boolean.TRUE.equals(c.getProperty("slave")))
            return c;

        return null;
    }

    /**
     * Helper method for Jelly.
     */
    public static List<SlaveSystemInfo> getSystemInfoExtensions() {
        return SlaveSystemInfo.all();
    }

    private static class SlaveLogFetcher implements Callable<List<LogRecord>, RuntimeException> {
        public List<LogRecord> call() {
            return new ArrayList<LogRecord>(SLAVE_LOG_HANDLER.getView());
        }
    }
}