Replace PruneAllButActive with PruneAllButVisible.

chrome/browser/prerender/prerender_manager.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/prerender/prerender_manager.h"
 
 #include <algorithm>
 #include <functional>
 #include <string>
 #include <vector>
@@ skipping 409 matching lines @@
   if (!prerender_data)
     return false;
   DCHECK(prerender_data->contents());
   if (IsNoSwapInExperiment(prerender_data->contents()->experiment_id()))
     return false;
 
   if (WebContents* new_web_contents =
       prerender_data->contents()->prerender_contents()) {
     if (web_contents == new_web_contents)
       return false;  // Do not swap in to ourself.
+
+    // We cannot swap in if there is no last committed entry, because we would
+    // show a blank page under an existing entry from the current tab.  Even if
+    // there is a pending entry, it may not commit.
+    // TODO(creis): If there is a pending navigation and no last committed
+    // entry, we might be able to transfer the network request instead.

[cbentzel, 2013/05/17 00:32:15]

I'd like to add UMA to see how frequently this happens - possibly even by
canceling the prerender contents. I'm happy to do it in a followup CL. 

[Charlie Reis, 2013/05/17 23:50:21]

Yes, that's a good idea.  Just increment a stat here if there's no last
committed entry but there is a pending entry.  (I can do it in this CL if you
let me know what name you want.  There's a bunch for prerendering and I'm not
sure where this would fit in.)

[cbentzel, 2013/05/18 21:11:08]

For now, I would actually destroy the prerender and add a new final status - see
many examples below like FINAL_STATUS_WINDOW_OPENER.

But I think it's easier to do that in a followup CL to keep this small - that
will require final_status.[h|cc] changes, plus histograms.xml change, etc.

+    if (!new_web_contents->GetController().CanPruneAllButVisible())
+      return false;
   }
 
   // Do not use the prerendered version if there is an opener object.
   if (web_contents->HasOpener()) {
     prerender_data->contents()->Destroy(FINAL_STATUS_WINDOW_OPENER);
     return false;
   }
 
   // If we are just in the control group (which can be detected by noticing
   // that prerendering hasn't even started yet), record that |web_contents| now
@@ skipping 73 matching lines @@
 
   WebContents* new_web_contents =
       prerender_contents->ReleasePrerenderContents();
   WebContents* old_web_contents = web_contents;
   DCHECK(new_web_contents);
   DCHECK(old_web_contents);
 
   MarkWebContentsAsPrerendered(new_web_contents, prerender_contents->origin());
 
   // Merge the browsing history.
-  new_web_contents->GetController().CopyStateFromAndPrune(
-      &old_web_contents->GetController());
+  CHECK(new_web_contents->GetController().CopyStateFromAndPrune(
+            &old_web_contents->GetController()));
   CoreTabHelper::FromWebContents(old_web_contents)->delegate()->
       SwapTabContents(old_web_contents, new_web_contents);
   prerender_contents->CommitHistory(new_web_contents);
 
   GURL icon_url = prerender_contents->icon_url();
 
   if (!icon_url.is_empty()) {
 #if defined(OS_ANDROID)
     // Do the delayed icon fetch since we didn't download
     // the favicon during prerendering on mobile devices.
@@ skipping 1002 matching lines @@
     logged_in_state_->erase(domain_key);
 }
 
 void PrerenderManager::LoggedInPredictorDataReceived(
     scoped_ptr<LoggedInStateMap> new_map) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   logged_in_state_.swap(new_map);
 }
 
 }  // namespace prerender