[Mac] Tighten up objc zombie dealloc implementation.

chrome/browser/cocoa/objc_zombie.mm

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "chrome/browser/cocoa/objc_zombie.h"
 
 #include <dlfcn.h>
 #include <mach-o/dyld.h>
 #include <mach-o/nlist.h>
 
@@ skipping 126 matching lines @@
 
   Class wasa = object_getClass(self);
   const size_t size = class_getInstanceSize(wasa);
 
   // Destroy the instance by calling C++ destructors and clearing it
   // to something unlikely to work well if someone references it.
   (*g_object_cxxDestruct)(self);
   memset(self, '!', size);
 
   // If the instance is big enough, make it into a fat zombie and have
-  // it remember the old isa.  Otherwise make it a regular zombie.
+  // it remember the old |isa|.  Otherwise make it a regular zombie.
+  // Setting |isa| rather than using |object_setClass()| because that
+  // function is implemented with a memory barrier.  The runtime's
+  // |_internal_object_dispose()| (in objc-class.m) does this, so it
+  // should be safe (messaging free'd objects shouldn't be expected to
+  // be thread-safe in the first place).
   if (size >= g_fatZombieSize) {
-    object_setClass(self, g_fatZombieClass);
+    self->isa = g_fatZombieClass;
     static_cast<CrFatZombie*>(self)->wasa = wasa;
   } else {
-    object_setClass(self, g_zombieClass);
+    self->isa = g_zombieClass;

[Avi (use Gerrit), 2010/06/03 19:16:51]

Huh. isa is still public in the objc2 runtime? If this compiles on 64, go for
it. If not you might have to ifdef __LP64__ it.

[Scott Hess - ex-Googler, 2010/06/03 19:46:33]

Not sure what you mean.  struct objc_class has OBJC2_UNAVAILABLE annotations,
but struct objc_object just has Class isa with no annotations.

This code should never be called in 64-bit due to the nlist stuff not being
adapted yet.  I'm trying to figure out how to compile 64-bit, but:
   GYP_DEFINES="target_arch=x64" gclient sync
doesn't seem to change from i386.

   }
 
   // The new record to swap into |g_zombies|.  If |g_zombieCount| is
   // zero, then |self| will be freed immediately.
   ZombieRecord zombieToFree = {self, wasa};
 
   {
     AutoLock pin(lock_);
     if (g_zombieCount > 0) {
       // Put the current object on the treadmill and keep the previous
@@ skipping 227 matching lines @@
   if (oldZombies) {
     for (size_t i = 0; i < oldCount; ++i) {
       if (oldZombies[i].object)
         free(oldZombies[i].object);
     }
     free(oldZombies);
   }
 }
 
 }  // namespace ObjcEvilDoers