/*
 * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package sun.misc;

import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.ProtectionDomain;
import java.security.PrivilegedAction;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A thread that has no permissions, is not a member of any user-defined
 * ThreadGroup and supports the ability to erase ThreadLocals.
 */
public final class InnocuousThread extends ManagedLocalsThread {
    private static final Unsafe UNSAFE;
    private static final ThreadGroup INNOCUOUSTHREADGROUP;
    private static final AccessControlContext ACC;
    private static final long INHERITEDACCESSCONTROLCONTEXT;
    private static final long CONTEXTCLASSLOADER;

    private static final AtomicInteger threadNumber = new AtomicInteger(1);

    public InnocuousThread(Runnable target) {
        this(INNOCUOUSTHREADGROUP, target,
             "InnocuousThread-" + threadNumber.getAndIncrement());
    }

    public InnocuousThread(Runnable target, String name) {
        this(INNOCUOUSTHREADGROUP, target, name);
    }

    public InnocuousThread(ThreadGroup group, Runnable target, String name) {
        super(group, target, name);
        UNSAFE.putOrderedObject(this, INHERITEDACCESSCONTROLCONTEXT, ACC);
        UNSAFE.putOrderedObject(this, CONTEXTCLASSLOADER, ClassLoader.getSystemClassLoader());
    }

    @Override
    public void setUncaughtExceptionHandler(UncaughtExceptionHandler x) {
        // silently fail
    }

    @Override
    public void setContextClassLoader(ClassLoader cl) {
        // Allow clearing of the TCCL to remove the reference to the system classloader.
        if (cl == null)
            super.setContextClassLoader(null);
        else
            throw new SecurityException("setContextClassLoader");
    }

    // ensure run method is run only once
    private volatile boolean hasRun;

    @Override
    public void run() {
        if (Thread.currentThread() == this && !hasRun) {
            hasRun = true;
            super.run();
        }
    }

    // Use Unsafe to access Thread group and ThreadGroup parent fields
    static {
        try {
            ACC = new AccessControlContext(new ProtectionDomain[] {
                new ProtectionDomain(null, null)
            });

            // Find and use topmost ThreadGroup as parent of new group
            UNSAFE = Unsafe.getUnsafe();
            Class<?> tk = Thread.class;
            Class<?> gk = ThreadGroup.class;

            INHERITEDACCESSCONTROLCONTEXT = UNSAFE.objectFieldOffset
                (tk.getDeclaredField("inheritedAccessControlContext"));
            CONTEXTCLASSLOADER = UNSAFE.objectFieldOffset
                (tk.getDeclaredField("contextClassLoader"));

            long tg = UNSAFE.objectFieldOffset(tk.getDeclaredField("group"));
            long gp = UNSAFE.objectFieldOffset(gk.getDeclaredField("parent"));
            ThreadGroup group = (ThreadGroup)
                UNSAFE.getObject(Thread.currentThread(), tg);

            while (group != null) {
                ThreadGroup parent = (ThreadGroup)UNSAFE.getObject(group, gp);
                if (parent == null)
                    break;
                group = parent;
            }
            final ThreadGroup root = group;
            INNOCUOUSTHREADGROUP = AccessController.doPrivileged(
                (PrivilegedAction<ThreadGroup>) () ->
                    { return new ThreadGroup(root, "InnocuousThreadGroup"); });
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}