avoid dereferencing NULL extension in ExtensionHost

chrome/browser/extensions/extension_process_manager.cc

 // Copyright (c) 2011 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.
 
 #include "chrome/browser/extensions/extension_process_manager.h"
 
 #include "chrome/browser/ui/browser_window.h"
 #include "content/browser/browsing_instance.h"
 #if defined(OS_MACOSX)
 #include "chrome/browser/extensions/extension_host_mac.h"
@@ skipping 273 matching lines @@
       }
       break;
     }
 
     case NotificationType::EXTENSION_UNLOADED: {
       const Extension* extension =
           Details<UnloadedExtensionInfo>(details)->extension;
       for (ExtensionHostSet::iterator iter = background_hosts_.begin();
            iter != background_hosts_.end(); ++iter) {
         ExtensionHost* host = *iter;
-        if (host->extension()->id() == extension->id()) {
+        if (host->extension() == NULL ||

[Matt Perry, 2011/05/05 17:17:59]

This seems a little dangerous to me. Here's an alternative approach:
1. remove this change.
2. change ExtensionHost to hold a scoped_refptr<const Extension> rather than a
raw ptr.
3. change ExtensionHost so that it doesn't NULL out its extension_ ptr in
EXTENSION_UNLOADED. (Extension objects were not refcounted when that was
written, so it was not possible then.)
4. have ExtensionHost set a flag when it receives EXTENSION_UNLOADED, and check
that flag in places where we currently NULL-check the extension ptr.

+            host->extension()->id() == extension->id()) {
           delete host;
           // |host| should deregister itself from our structures.
           DCHECK(background_hosts_.find(host) == background_hosts_.end());
           break;
         }
       }
       break;
     }
 
     case NotificationType::EXTENSION_HOST_DESTROYED: {
@@ skipping 147 matching lines @@
         if (service && service->is_ready())
           CreateBackgroundHosts(this, service->extensions());
       }
       break;
     }
     default:
       ExtensionProcessManager::Observe(type, source, details);
       break;
   }
 }