Null checks to prevent occasional crash

chrome/browser/ui/browser_list.cc

 // Copyright (c) 2012 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/ui/browser_list.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "base/logging.h"
@@ skipping 145 matching lines @@
                        browsers_to_close,
                        on_close_success,
                        profile_path))) {
       return;
     }
   }
 
   on_close_success.Run(profile_path);
 
   for (BrowserVector::const_iterator it = browsers_to_close.begin();
-       it != browsers_to_close.end(); ++it)
-    (*it)->window()->Close();
+       it != browsers_to_close.end(); ++it) {
+    if (*it && (*it)->window())

[Peter Kasting, 2014/10/29 18:07:01]

Can both these be NULL, or just one?

Why do we need checks here but not above?

Is there a precise scenario that causes NULLs that we can describe in a comment
here explaining why the NULL checks are necessary?

[Mike Lerman, 2014/10/29 18:26:06]

I don't know what scenario is causing the NULLs unfortunately - we just have a
crash report, no steps to reproduce. I can't think of what could cause this.
Perhaps contrived javascript, such as an OnBeforeUnload that closes a browser
window, though I don't think you can do that.

The first *it check may not be necessary, but since I don't have steps to
reproduce I'm just playing it safe. My intuition is that the window() is what's
null, though, and can only check (*it)->window() if you prefer.

+      (*it)->window()->Close();
+  }
 }
 
 // static
 void BrowserList::PostBeforeUnloadHandlers(
     const BrowserVector& browsers_to_close,
     const base::Callback<void(const base::FilePath&)>& on_close_success,
     const base::FilePath& profile_path,
     bool tab_close_confirmed) {
   // We need this bool to avoid infinite recursion when resetting the
   // BeforeUnload handlers, since doing that will trigger calls back to this
@@ skipping 55 matching lines @@
 }
 
 // static
 void BrowserList::RemoveBrowserFrom(Browser* browser,
                                     BrowserVector* browser_list) {
   BrowserVector::iterator remove_browser =
       std::find(browser_list->begin(), browser_list->end(), browser);
   if (remove_browser != browser_list->end())
     browser_list->erase(remove_browser);
 }