Check BrowsingInstance before swapping prerenders.

content/browser/browsing_instance.h

 // 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.
 
 #ifndef CONTENT_BROWSER_BROWSING_INSTANCE_H_
 #define CONTENT_BROWSER_BROWSING_INSTANCE_H_
 
 #include "base/containers/hash_tables.h"
 #include "base/lazy_instance.h"
 #include "base/memory/ref_counted.h"
@@ skipping 61 matching lines @@
   // Adds the given SiteInstance to our map, to ensure that we do not create
   // another SiteInstance for the same site.
   void RegisterSiteInstance(SiteInstance* site_instance);
 
   // Removes the given SiteInstance from our map, after all references to it
   // have been deleted.  This means it is safe to create a new SiteInstance
   // if the user later visits a page from this site, within this
   // BrowsingInstance.
   void UnregisterSiteInstance(SiteInstance* site_instance);
 
+  size_t active_contents_count() const { return active_contents_count_; }
+  void increment_active_contents_count() { active_contents_count_++; }
+  void decrement_active_contents_count() { active_contents_count_--; }
+
   friend class SiteInstanceImpl;
   friend class SiteInstance;
 
   friend class base::RefCounted<BrowsingInstance>;
 
   // Virtual to allow tests to extend it.
   virtual ~BrowsingInstance();
 
  private:
   // Map of site to SiteInstance, to ensure we only have one SiteInstance per
+  // site.

[Charlie Reis, 2014/04/29 17:31:08]

Oops, wonder how that got deleted.  (Looks like it dates back to the initial
open-sourcing of Chrome.)  Nice catch.

   typedef base::hash_map<std::string, SiteInstance*> SiteInstanceMap;
 
   // Common browser context to which all SiteInstances in this BrowsingInstance
   // must belong.
   BrowserContext* const browser_context_;
 
   // Map of site to SiteInstance, to ensure we only have one SiteInstance per
   // site.  The site string should be the possibly_invalid_spec() of a GURL
   // obtained with SiteInstanceImpl::GetSiteForURL.  Note that this map may not
   // contain every active SiteInstance, because a race exists where two
   // SiteInstances can be assigned to the same site.  This is ok in rare cases.
+  // It also does not contain SiteInstances which have not yet been assigned a
+  // site, such as about:blank.  See NavigatorImpl::ShouldAssignSiteForURL.
   SiteInstanceMap site_instance_map_;
 
+  // Number of WebContentses currently on this SiteInstance.

[Charlie Reis, 2014/04/29 17:31:08]

on this SiteInstance -> in this BrowsingInstance

[davidben, 2014/04/29 21:31:40]

Done.

+  size_t active_contents_count_;
+
   DISALLOW_COPY_AND_ASSIGN(BrowsingInstance);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BROWSING_INSTANCE_H_