Prerendered pages are swapped in at browser::Navigate time.

chrome/browser/prerender/prerender_contents.cc

Index: chrome/browser/prerender/prerender_contents.cc
diff --git a/chrome/browser/prerender/prerender_contents.cc b/chrome/browser/prerender/prerender_contents.cc
index d81dfcfd9fbbc3ea9b4d87e5a2a5857203bedb32..a6c4773b9178c99e4a4a49c50f8bfbed60c78d22 100644
--- a/chrome/browser/prerender/prerender_contents.cc
+++ b/chrome/browser/prerender/prerender_contents.cc
@@ -21,7 +21,7 @@
 #include "chrome/browser/ui/tab_contents/tab_contents_wrapper.h"
 #include "chrome/common/chrome_notification_types.h"
 #include "chrome/common/icon_messages.h"
-#include "chrome/common/render_messages.h"
+#include "chrome/common/prerender_messages.h"
 #include "chrome/common/url_constants.h"
 #include "content/browser/renderer_host/render_view_host.h"
 #include "content/browser/renderer_host/resource_request_details.h"
@@ -61,6 +61,29 @@ struct PrerenderURLPredicate {
   GURL url_;
 };
 
+// Tells the render process at |child_id| whether |url| is a new prerendered
+// page, or whether |url| is being removed as a prerendered page. Currently
+// this will only inform the render process that created the prerendered page
+// with <link rel="prerender"> tags about it. This means that if the user
+// clicks on a link for a prerendered URL in a different page, the prerender
+// will not be swapped in.

[Charlie Reis, 2012/03/01 19:06:03]

I think this is the right call.  For reference, we may eventually have renderers
check in with the browser on every navigation (e.g., if we enforce strict site
isolation).  In that case the browser would be able to make a more informed
decision.

+void InformRenderProcessAboutPrerender(const GURL& url,
+                                       bool is_add,
+                                       int child_id) {
+  if (child_id < 0)
+    return;
+  content::RenderProcessHost* render_process_host =
+      content::RenderProcessHost::FromID(child_id);
+  if (!render_process_host)
+    return;
+  IPC::Message* message = NULL;
+  if (is_add)
+    message = new PrerenderMsg_AddPrerenderURL(url);
+  else
+    message = new PrerenderMsg_RemovePrerenderURL(url);

[Charlie Reis, 2012/03/01 19:06:03]

Just as a sanity check, we don't need to be concerned about races here, right? 
If a renderer thinks we can prerender it and we've since deleted, we'll just
load it from scratch.  And if it thinks we can't prerender it but we can, it's
just some wasted effort.  Seems ok to me.

[cbentzel, 2012/03/01 22:18:53]

You are correct - there is not a race here.

I commented about the impact of a delayed RemovePrerenderURL case in ShouldFork
- would it help to copy here as well?

[Charlie Reis, 2012/03/01 22:24:38]

That comment is sufficient, thanks.  This file just came up earlier in my
review.

+  render_process_host->Send(message);
+}
+
 }  // end namespace
 
 class PrerenderContentsFactoryImpl : public PrerenderContents::Factory {
@@ -227,7 +250,8 @@ PrerenderContents::PrerenderContents(
       child_id_(-1),
       route_id_(-1),
       origin_(origin),
-      experiment_id_(experiment_id) {
+      experiment_id_(experiment_id),
+      creator_child_id_(-1) {
   DCHECK(prerender_manager != NULL);
 }
 
@@ -248,6 +272,12 @@ void PrerenderContents::StartPrerendering(
   DCHECK(prerender_contents_.get() == NULL);
 
   prerendering_has_started_ = true;
+  DCHECK(creator_child_id_ == -1);
+  if (source_render_view_host)
+    creator_child_id_ = source_render_view_host->process()->GetID();
+  InformRenderProcessAboutPrerender(prerender_url_, true,
+                                    creator_child_id_);
+
   WebContents* new_contents = WebContents::Create(
       profile_, NULL, MSG_ROUTING_NONE, NULL, session_storage_namespace);
   prerender_contents_.reset(new TabContentsWrapper(new_contents));
@@ -363,8 +393,10 @@ PrerenderContents::~PrerenderContents() {
       match_complete_status_,
       final_status_);
 
-  if (child_id_ != -1 && route_id_ != -1)
+  if (child_id_ != -1 && route_id_ != -1) {
     prerender_tracker_->OnPrerenderingFinished(child_id_, route_id_);
+    InformRenderProcessAboutPrerender(prerender_url_, false, creator_child_id_);
+  }
 
   // If we still have a TabContents, clean up anything we need to and then
   // destroy it.
@@ -419,7 +451,7 @@ void PrerenderContents::Observe(int type,
         // first navigation, so there's no need to send the message just after
         // the TabContents is created.
         new_render_view_host->Send(
-            new ChromeViewMsg_SetIsPrerendering(
+            new PrerenderMsg_SetIsPrerendering(
                 new_render_view_host->routing_id(),
                 true));