diff -r 000000000000 -r 4f2f89ce4247 WebKit/mac/Misc/WebNSObjectExtras.mm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/WebKit/mac/Misc/WebNSObjectExtras.mm	Fri Sep 17 09:02:29 2010 +0300
@@ -0,0 +1,118 @@
+/*
+ * Copyright (C) 2008, 2009 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1.  Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ * 2.  Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ * 3.  Neither the name of Apple Computer, Inc. ("Apple") nor the names of
+ *     its contributors may be used to endorse or promote products derived
+ *     from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#import "WebNSObjectExtras.h"
+
+#import <wtf/Assertions.h>
+
+@interface WebMainThreadInvoker : NSProxy
+{
+    id target;
+    id exception;
+}
+@end
+
+static bool returnTypeIsObject(NSInvocation *invocation)
+{
+    // Could use either _C_ID or NSObjCObjectType, but it seems that neither is
+    // both available and non-deprecated on all versions of Mac OS X we support.
+    return strchr([[invocation methodSignature] methodReturnType], '@');
+}
+
+@implementation WebMainThreadInvoker
+
+- (id)initWithTarget:(id)passedTarget
+{
+    target = passedTarget;
+    return self;
+}
+
+- (void)forwardInvocation:(NSInvocation *)invocation
+{
+    [invocation setTarget:target];
+    [invocation performSelectorOnMainThread:@selector(_webkit_invokeAndHandleException:) withObject:self waitUntilDone:YES];
+    if (exception) {
+        id exceptionToThrow = [exception autorelease];
+        exception = nil;
+        @throw exceptionToThrow;
+    } else if (returnTypeIsObject(invocation)) {
+        // _webkit_invokeAndHandleException retained the return value on the main thread.
+        // Now autorelease it on the calling thread.
+        id returnValue;
+        [invocation getReturnValue:&returnValue];
+        [returnValue autorelease];
+    }
+}
+
+- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
+{
+    return [target methodSignatureForSelector:selector];
+}
+
+- (void)handleException:(id)passedException
+{
+    ASSERT(!exception);
+    exception = [passedException retain];
+}
+
+@end
+
+@implementation NSInvocation (WebMainThreadInvoker)
+
+- (void)_webkit_invokeAndHandleException:(WebMainThreadInvoker *)exceptionHandler
+{
+    @try {
+        [self invoke];
+    } @catch (id exception) {
+        [exceptionHandler handleException:exception];
+        return;
+    }
+    if (returnTypeIsObject(self)) {
+        // Retain the return value on the main thread.
+        // -[WebMainThreadInvoker forwardInvocation:] will autorelease it on the calling thread.
+        id value;
+        [self getReturnValue:&value];
+        [value retain];
+    }
+}
+
+@end
+
+@implementation NSObject (WebNSObjectExtras)
+
++ (id)_webkit_invokeOnMainThread
+{
+    return [[[WebMainThreadInvoker alloc] initWithTarget:self] autorelease];
+}
+
+- (id)_webkit_invokeOnMainThread
+{
+    return [[[WebMainThreadInvoker alloc] initWithTarget:self] autorelease];
+}
+
+@end