--- a/nashorn/src/jdk/nashorn/api/scripting/ScriptUtils.java	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/src/jdk/nashorn/api/scripting/ScriptUtils.java	Tue Oct 15 17:37:47 2013 +0200
@@ -25,6 +25,7 @@
 
 package jdk.nashorn.api.scripting;
 
+import jdk.nashorn.internal.runtime.ScriptFunction;
 import jdk.nashorn.internal.runtime.ScriptRuntime;
 
 /**
@@ -57,4 +58,17 @@
     public static String format(final String format, final Object[] args) {
         return Formatter.format(format, args);
     }
+
+    /**
+     * Create a wrapper function that calls {@code func} synchronized on {@code sync} or, if that is undefined,
+     * {@code self}. Used to implement "sync" function in resources/mozilla_compat.js.
+     *
+     * @param func the function to invoke
+     * @param sync the object to synchronize on
+     * @return a synchronizing wrapper function
+     */
+    public static Object makeSynchronizedFunction(final ScriptFunction func, final Object sync) {
+        return func.makeSynchronizedFunction(sync);
+    }
+
 }

--- a/nashorn/src/jdk/nashorn/internal/objects/ScriptFunctionImpl.java	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/src/jdk/nashorn/internal/objects/ScriptFunctionImpl.java	Tue Oct 15 17:37:47 2013 +0200
@@ -25,6 +25,7 @@
 
 package jdk.nashorn.internal.objects;
 
+import static jdk.nashorn.internal.lookup.Lookup.MH;
 import static jdk.nashorn.internal.runtime.ScriptRuntime.UNDEFINED;
 
 import java.lang.invoke.MethodHandle;
@@ -255,6 +256,12 @@
         return makeFunction(name, methodHandle, null);
     }
 
+    @Override
+    public ScriptFunction makeSynchronizedFunction(final Object sync) {
+        final MethodHandle mh = MH.insertArguments(ScriptFunction.INVOKE_SYNC, 0, this, sync);
+        return makeFunction(getName(), mh);
+    }
+
     /**
      * Same as {@link ScriptFunction#makeBoundFunction(Object, Object[])}. The only reason we override it is so that we
      * can expose it to methods in this package.

--- a/nashorn/src/jdk/nashorn/internal/runtime/RecompilableScriptFunctionData.java	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/src/jdk/nashorn/internal/runtime/RecompilableScriptFunctionData.java	Tue Oct 15 17:37:47 2013 +0200
@@ -53,7 +53,7 @@
 public final class RecompilableScriptFunctionData extends ScriptFunctionData {
 
     /** FunctionNode with the code for this ScriptFunction */
-    private volatile FunctionNode functionNode;
+    private FunctionNode functionNode;
 
     /** Source from which FunctionNode was parsed. */
     private final Source source;
@@ -65,7 +65,7 @@
     private final PropertyMap allocatorMap;
 
     /** Code installer used for all further recompilation/specialization of this ScriptFunction */
-    private volatile CodeInstaller<ScriptEnvironment> installer;
+    private CodeInstaller<ScriptEnvironment> installer;
 
     /** Name of class where allocator function resides */
     private final String allocatorClassName;
@@ -178,7 +178,7 @@
     }
 
     @Override
-    protected void ensureCodeGenerated() {
+    protected synchronized void ensureCodeGenerated() {
          if (!code.isEmpty()) {
              return; // nothing to do, we have code, at least some.
          }
@@ -336,7 +336,7 @@
     }
 
     @Override
-    MethodHandle getBestInvoker(final MethodType callSiteType, final Object[] args) {
+    synchronized MethodHandle getBestInvoker(final MethodType callSiteType, final Object[] args) {
         final MethodType runtimeType = runtimeType(callSiteType, args);
         assert runtimeType.parameterCount() == callSiteType.parameterCount();
 

--- a/nashorn/src/jdk/nashorn/internal/runtime/ScriptFunction.java	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/src/jdk/nashorn/internal/runtime/ScriptFunction.java	Tue Oct 15 17:37:47 2013 +0200
@@ -59,6 +59,9 @@
     /** Method handle for name getter for this ScriptFunction */
     public static final MethodHandle G$NAME = findOwnMH("G$name", Object.class, Object.class);
 
+    /** Method handle used for implementing sync() in mozilla_compat */
+    public static final MethodHandle INVOKE_SYNC = findOwnMH("invokeSync", Object.class, ScriptFunction.class, Object.class, Object.class, Object[].class);
+
     /** Method handle for allocate function for this ScriptFunction */
     static final MethodHandle ALLOCATE = findOwnMH("allocate", Object.class);
 
@@ -301,6 +304,14 @@
     public abstract void setPrototype(Object prototype);
 
     /**
+     * Create a function that invokes this function synchronized on {@code sync} or the self object
+     * of the invocation.
+     * @param sync the Object to synchronize on, or undefined
+     * @return synchronized function
+     */
+    public abstract ScriptFunction makeSynchronizedFunction(Object sync);
+
+    /**
      * Return the most appropriate invoke handle if there are specializations
      * @param type most specific method type to look for invocation with
      * @param args args for trampoline invocation
@@ -614,6 +625,15 @@
         return result;
     }
 
+    @SuppressWarnings("unused")
+    private static Object invokeSync(final ScriptFunction func, final Object sync, final Object self, final Object... args)
+            throws Throwable {
+        final Object syncObj = sync == UNDEFINED ? self : sync;
+        synchronized (syncObj) {
+            return func.invoke(self, args);
+        }
+    }
+
     private static MethodHandle findOwnMH(final String name, final Class<?> rtype, final Class<?>... types) {
         final Class<?>   own = ScriptFunction.class;
         final MethodType mt  = MH.type(rtype, types);

--- a/nashorn/src/jdk/nashorn/internal/runtime/ScriptFunctionData.java	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/src/jdk/nashorn/internal/runtime/ScriptFunctionData.java	Tue Oct 15 17:37:47 2013 +0200
@@ -675,7 +675,7 @@
 
     /**
      * Heuristic to figure out if the method handle has a callee argument. If it's type is either
-     * {@code (boolean, Object, ScriptFunction, ...)} or {@code (Object, ScriptFunction, ...)}, then we'll assume it has
+     * {@code (boolean, ScriptFunction, ...)} or {@code (ScriptFunction, ...)}, then we'll assume it has
      * a callee argument. We need this as the constructor above is not passed this information, and can't just blindly
      * assume it's false (notably, it's being invoked for creation of new scripts, and scripts have scopes, therefore
      * they also always receive a callee).
@@ -692,11 +692,11 @@
             return false;
         }
 
-        if (type.parameterType(0) == boolean.class) {
-            return length > 1 && type.parameterType(1) == ScriptFunction.class;
+        if (type.parameterType(0) == ScriptFunction.class) {
+            return true;
         }
 
-        return type.parameterType(0) == ScriptFunction.class;
+        return length > 1 && type.parameterType(0) == boolean.class && type.parameterType(1) == ScriptFunction.class;
     }
 
     /**

--- a/nashorn/src/jdk/nashorn/internal/runtime/resources/mozilla_compat.js	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/src/jdk/nashorn/internal/runtime/resources/mozilla_compat.js	Tue Oct 15 17:37:47 2013 +0200
@@ -98,6 +98,17 @@
 
 }
 
+// sync
+Object.defineProperty(this, "sync", {
+    configurable: true, enumerable: false, writable: true,
+    value: function(func, syncobj) {
+        if (arguments.length < 1 || arguments.length > 2 ) {
+            throw "sync(function [,object]) parameter count mismatch";
+        }
+        return Packages.jdk.nashorn.api.scripting.ScriptUtils.makeSynchronizedFunction(func, syncobj);
+    }
+});
+
 // Object.prototype.__defineGetter__
 Object.defineProperty(Object.prototype, "__defineGetter__", {
     configurable: true, enumerable: false, writable: true,

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/nashorn/test/script/basic/JDK-8026367.js	Tue Oct 15 17:37:47 2013 +0200
@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2010, 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.
+ * 
+ * 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.
+ */
+
+/**
+ * JDK-8026367: Add a sync keyword to mozilla_compat
+ *
+ * @test
+ * @run
+ */
+
+if (typeof sync === "undefined") {
+    load("nashorn:mozilla_compat.js");
+}
+
+var obj = {
+    count: 0,
+    // Sync called with one argument will synchronize on this-object of invocation
+    inc: sync(function(d) {
+        this.count += d;
+    }),
+    // Pass explicit object to synchronize on as second argument
+    dec: sync(function(d) {
+        this.count -= d;
+    }, obj)
+};
+
+var t1 = new java.lang.Thread(function() {
+    for (var i = 0; i < 100000; i++) obj.inc(1);
+});
+var t2 = new java.lang.Thread(function() {
+    for (var i = 0; i < 100000; i++) obj.dec(1);
+});
+
+t1.start();
+t2.start();
+t1.join();
+t2.join();
+
+if (obj.count !== 0) {
+    throw new Error("Expected count == 0, got " + obj.count);
+}

--- a/nashorn/test/script/sandbox/loadcompat.js	Tue Oct 15 15:57:14 2013 +0200
+++ b/nashorn/test/script/sandbox/loadcompat.js	Tue Oct 15 17:37:47 2013 +0200
@@ -48,3 +48,7 @@
 if (typeof importPackage != 'function') {
     fail("importPackage function is missing in compatibility script");
 }
+
+if (typeof sync != 'function') {
+    fail("sync function is missing in compatibility script");
+}