[Prerender] Restore request priorities when swapped in

chrome/browser/prerender/prerender_resource_throttle.cc

Index: chrome/browser/prerender/prerender_resource_throttle.cc
diff --git a/chrome/browser/prerender/prerender_resource_throttle.cc b/chrome/browser/prerender/prerender_resource_throttle.cc
index afd908fe6db2c2b387927c3fea5619109a906278..89668163038c4d37d26cea3b77d5b4eaf08e3057 100644
--- a/chrome/browser/prerender/prerender_resource_throttle.cc
+++ b/chrome/browser/prerender/prerender_resource_throttle.cc
@@ -12,6 +12,7 @@
 #include "chrome/browser/prerender/prerender_manager.h"
 #include "chrome/browser/prerender/prerender_util.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/resource_dispatcher_host.h"
 #include "content/public/browser/web_contents.h"
 #include "net/base/load_flags.h"
 #include "net/http/http_response_headers.h"
@@ -85,6 +86,7 @@ void PrerenderResourceThrottle::OverridePrerenderContentsForTesting(
 PrerenderResourceThrottle::PrerenderResourceThrottle(net::URLRequest* request)
     : request_(request),
       load_flags_(net::LOAD_NORMAL),
+      original_request_priority_(net::IDLE),
       prerender_throttle_info_(new PrerenderThrottleInfo()) {}
 
 PrerenderResourceThrottle::~PrerenderResourceThrottle() {}
@@ -94,6 +96,25 @@ void PrerenderResourceThrottle::WillStartRequest(bool* defer) {
   const content::ResourceRequestInfo* info =
       content::ResourceRequestInfo::ForRequest(request_);
   *defer = true;
+
+// Priorities for prerendering requests are lowered, to avoid competing with
+// other page loads, except on Android where this is less likely to be a
+// problem. In some cases, this may negatively impact the performance of
+// prerendering, see https://crbug.com/652746 for details.
+#if !defined(OS_ANDROID)
+  // Requests with the IGNORE_LIMITS flag set (i.e., sync XHRs)
+  // should remain at MAXIMUM_PRIORITY.
+  if (request_->load_flags() & net::LOAD_IGNORE_LIMITS) {
+    DCHECK_EQ(request_->priority(), net::MAXIMUM_PRIORITY);
+    // original_request_priority_ can be kept to IDLE, so that the priority will
+    // never be touched again.
+  } else if (request_->priority() != net::IDLE) {
+    original_request_priority_ = request_->priority();
+    content::ResourceDispatcherHost::Get()->ReprioritizeRequest(request_,
+                                                                net::IDLE);
+  }
+#endif  // OS_ANDROID
+
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&PrerenderResourceThrottle::WillStartRequestOnUI, AsWeakPtr(),
@@ -149,6 +170,17 @@ void PrerenderResourceThrottle::ResumeHandler() {
   Resume();
 }
 
+void PrerenderResourceThrottle::ResetResourcePriority() {
+  if (original_request_priority_ == net::IDLE) {
+    // IDLE means that the original request was either IDLE or must not be
+    // changed.

[Charlie Harrison, 2017/04/12 12:37:26]

You could use a base::Optional to make this distinction more clear.

[droger, 2017/04/12 13:20:54]

Done.

+    return;
+  }
+
+  content::ResourceDispatcherHost::Get()->ReprioritizeRequest(
+      request_, original_request_priority_);
+}
+
 // static
 void PrerenderResourceThrottle::WillStartRequestOnUI(
     const base::WeakPtr<PrerenderResourceThrottle>& throttle,
@@ -202,9 +234,15 @@ void PrerenderResourceThrottle::WillStartRequestOnUI(
       // Delay icon fetching until the contents are getting swapped in
       // to conserve network usage in mobile devices.
       prerender_contents->AddResourceThrottle(throttle);

[Charlie Harrison, 2017/04/12 12:37:26]

nit: newline below

[droger, 2017/04/12 13:20:54]

Done.

+      // No need to add to call AddIdleResource() on Android.
       return;
 #endif
     }
+
+#if !defined(OS_ANDROID)
+    if (!cancel)
+      prerender_contents->AddIdleResource(throttle);
+#endif
   }
 
   BrowserThread::PostTask(