Safely cancel prerenders on threads other than the UI thread

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
===================================================================
--- chrome/browser/prerender/prerender_manager.cc	(revision 85771)
+++ chrome/browser/prerender/prerender_manager.cc	(working copy)
@@ -14,6 +14,7 @@
 #include "chrome/browser/favicon/favicon_tab_helper.h"
 #include "chrome/browser/prerender/prerender_contents.h"
 #include "chrome/browser/prerender/prerender_final_status.h"
+#include "chrome/browser/prerender/prerender_tracker.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/tab_contents/tab_contents_wrapper.h"
 #include "chrome/browser/ui/tab_contents/tab_contents_wrapper_delegate.h"
@@ -378,9 +379,7 @@
     PrerenderContentsData data = prerender_list_.front();
     if (IsPrerenderElementFresh(data.start_time_))
       return;
-    prerender_list_.pop_front();
-    data.contents_->set_final_status(FINAL_STATUS_TIMED_OUT);
-    delete data.contents_;
+    data.contents_->Destroy(FINAL_STATUS_TIMED_OUT);
   }
   MaybeStopSchedulingPeriodicCleanups();
 }
@@ -427,6 +426,16 @@
     return false;
   }
 
+  int child_id, route_id;
+  CHECK(prerender_contents->GetChildId(&child_id));
+  CHECK(prerender_contents->GetRouteId(&route_id));
+
+  // Try to set the prerendered page as used, so any subsequent attempts to
+  // cancel on other threads will fail.  If this fails because the prerender
+  // was already cancelled, possibly on another thread, fail.
+  if (!PrerenderTracker::GetInstance()->TryUse(child_id, route_id))
+    return false;
+
   if (!prerender_contents->load_start_time().is_null())
     RecordTimeUntilUsed(GetCurrentTimeTicks() -
                         prerender_contents->load_start_time());
@@ -435,11 +444,6 @@
                        ++prerenders_per_session_count_);
   prerender_contents->set_final_status(FINAL_STATUS_USED);
 
-  int child_id;
-  int route_id;
-  CHECK(prerender_contents->GetChildId(&child_id));
-  CHECK(prerender_contents->GetRouteId(&route_id));
-
   RenderViewHost* render_view_host =
       prerender_contents->render_view_host_mutable();
   prerender_contents->set_render_view_host(NULL);
@@ -519,6 +523,16 @@
     return false;
   }
 
+  int child_id, route_id;
+  CHECK(prerender_contents->GetChildId(&child_id));
+  CHECK(prerender_contents->GetRouteId(&route_id));
+
+  // Try to set the prerendered page as used, so any subsequent attempts to
+  // cancel on other threads will fail.  If this fails because the prerender
+  // was already cancelled, possibly on another thread, fail.
+  if (!PrerenderTracker::GetInstance()->TryUse(child_id, route_id))
+    return false;
+
   if (!prerender_contents->load_start_time().is_null())
     RecordTimeUntilUsed(GetCurrentTimeTicks() -
                         prerender_contents->load_start_time());
@@ -527,11 +541,6 @@
                        ++prerenders_per_session_count_);
   prerender_contents->set_final_status(FINAL_STATUS_USED);
 
-  int child_id;
-  int route_id;
-  CHECK(prerender_contents->GetChildId(&child_id));
-  CHECK(prerender_contents->GetRouteId(&route_id));
-
   RenderViewHost* render_view_host =
       prerender_contents->prerender_contents()->render_view_host();
   DCHECK(render_view_host);