[NoStatePrefetch] Do not send responses to renderer in prefetch mode.

content/browser/loader/resource_dispatcher_host_impl.cc

Index: content/browser/loader/resource_dispatcher_host_impl.cc
diff --git a/content/browser/loader/resource_dispatcher_host_impl.cc b/content/browser/loader/resource_dispatcher_host_impl.cc
index 847f0fd5f9d82e6dd5d54a4e4db3d75360c35fde..84c79752fba4c3982399688c5565ee399077b282 100644
--- a/content/browser/loader/resource_dispatcher_host_impl.cc
+++ b/content/browser/loader/resource_dispatcher_host_impl.cc
@@ -1638,12 +1638,17 @@ ResourceDispatcherHostImpl::CreateResourceHandler(
     }
   }
 
+  // In prefetch mode, only let the main resource go through.
+  bool start_detached = request_data.is_prefetch_only &&

[mmenke, 2016/08/23 15:11:41]

|is_prefetch_only| seems confusing, given that we also have
RESOURCE_TYPE_PREFETCH, which also seems to indicate a request is only a
prefetch.  I guess we can't just make a new resource type, since we want to get
the accept header right, which is set based on the resource type, but I think we
at least need to switch to different terminology.

|do_not_send_to_renderer|?  |download_to_[network_]cache_only|?

+                        request_data.resource_type != RESOURCE_TYPE_MAIN_FRAME;

[mmenke, 2016/08/23 15:11:42]

I'm not a fan of this MAIN_FRAME logic here.  Perhaps it should be in the
renderer, based on the main frame being one of the new prerender types?

+
   // Prefetches and <a ping> requests outlive their child process.
-  if (!sync_result && IsDetachableResourceType(request_data.resource_type)) {
+  if (!sync_result && (start_detached ||
+                       IsDetachableResourceType(request_data.resource_type))) {
     handler.reset(new DetachableResourceHandler(
         request,
         base::TimeDelta::FromMilliseconds(kDefaultDetachableCancelDelayMs),
-        std::move(handler)));
+        start_detached ? nullptr : std::move(handler)));

[mmenke, 2016/08/23 15:11:41]

We need to tell the renderer the request was aborted.  Otherwise, it will keep
on waiting for a response.  We should have a unit test for this (And it should
also make sure the entire resource is read).

[mmenke, 2016/08/23 15:11:42]

Could you update the DetachableResourceHandler docs to mention this works?

   }
 
   // PlzNavigate: If using --enable-browser-side-navigation, the
@@ -2640,6 +2645,9 @@ int ResourceDispatcherHostImpl::BuildLoadFlagsForRequest(
   if (is_sync_load)
     load_flags |= net::LOAD_IGNORE_LIMITS;
 
+  if (request_data.is_prefetch_only)
+    load_flags |= net::LOAD_PREFETCH;
+
   return load_flags;
 }