android: Disable the startup DNS resolutions.

chrome/browser/net/predictor.cc

Index: chrome/browser/net/predictor.cc
diff --git a/chrome/browser/net/predictor.cc b/chrome/browser/net/predictor.cc
index 566818448a5ca13bba5baa48e56c8e0ad055ca52..be2f921ea9413325aa5fba94618ba67a40c0e31d 100644
--- a/chrome/browser/net/predictor.cc
+++ b/chrome/browser/net/predictor.cc
@@ -27,7 +27,9 @@
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
+#include "base/trace_event/trace_event.h"

[Charlie Harrison, 2017/06/20 13:24:29]

Is it used?

[Benoit L, 2017/06/20 13:30:17]

Yes, there is a TRACE_EVENT0() added below.

[Charlie Harrison, 2017/06/20 13:32:22]

Ahh sorry. It's early :)

 #include "base/values.h"
+#include "build/build_config.h"
 #include "chrome/browser/io_thread.h"
 #include "chrome/browser/prefs/session_startup_pref.h"
 #include "chrome/browser/profiles/profile_io_data.h"
@@ -60,6 +62,15 @@ namespace {
 const base::Feature kNetworkPrediction{"NetworkPrediction",
                                        base::FEATURE_ENABLED_BY_DEFAULT};
 
+#if defined(OS_ANDROID)
+// Disabled on Android, as there are no "pinned tabs", meaning that a startup
+// is unlikely to request the same URL, and hence to resolve the same domains
+// as the previous one.
+constexpr bool kInitialDnsPrefetchListEnabled = false;
+#else
+constexpr bool kInitialDnsPrefetchListEnabled = true;
+#endif  // defined(OS_ANDROID)
+
 }  // namespace
 
 // static
@@ -506,7 +517,7 @@ void Predictor::GetHtmlReferrerLists(std::string* output) {
 
 void Predictor::GetHtmlInfo(std::string* output) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  if (initial_observer_.get())
+  if (initial_observer_)
     initial_observer_->GetFirstResolutionsHtml(output);
   // Show list of subresource predictions and stats.
   GetHtmlReferrerLists(output);
@@ -587,7 +598,7 @@ void Predictor::DeserializeReferrers(const base::ListValue& referral_list) {
 
 void Predictor::DiscardInitialNavigationHistory() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  if (initial_observer_.get())
+  if (initial_observer_)
     initial_observer_->DiscardInitialNavigationHistory();
 }
 
@@ -597,9 +608,11 @@ void Predictor::FinalizeInitializationOnIOThread(
     IOThread* io_thread,
     ProfileIOData* profile_io_data) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  TRACE_EVENT0("net", "Predictor::FinalizeInitializationOnIOThread");
 
   profile_io_data_ = profile_io_data;
-  initial_observer_.reset(new InitialObserver());
+  if (kInitialDnsPrefetchListEnabled)
+    initial_observer_ = base::MakeUnique<InitialObserver>();
 
   net::URLRequestContext* context =
       url_request_context_getter_->GetURLRequestContext();
@@ -612,7 +625,8 @@ void Predictor::FinalizeInitializationOnIOThread(
   io_weak_factory_.reset(new base::WeakPtrFactory<Predictor>(this));
 
   // Prefetch these hostnames on startup.
-  DnsPrefetchMotivatedList(startup_urls, UrlInfo::STARTUP_LIST_MOTIVATED);
+  if (kInitialDnsPrefetchListEnabled)
+    DnsPrefetchMotivatedList(startup_urls, UrlInfo::STARTUP_LIST_MOTIVATED);
 
   DeserializeReferrers(*referral_list);
 
@@ -628,8 +642,8 @@ void Predictor::FinalizeInitializationOnIOThread(
 
 void Predictor::LearnAboutInitialNavigation(const GURL& url) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  if (!PredictorEnabled() || nullptr == initial_observer_.get() ||
-      !CanPreresolveAndPreconnect()) {
+  if (!PredictorEnabled() || !kInitialDnsPrefetchListEnabled ||
+      !initial_observer_ || !CanPreresolveAndPreconnect()) {
     return;
   }
   initial_observer_->Append(url, this);
@@ -683,8 +697,8 @@ void Predictor::SaveStateForNextStartup() {
   if (!CanPreresolveAndPreconnect())
     return;
 
-  std::unique_ptr<base::ListValue> startup_list(new base::ListValue);
-  std::unique_ptr<base::ListValue> referral_list(new base::ListValue);
+  auto startup_list = base::MakeUnique<base::ListValue>();
+  auto referral_list = base::MakeUnique<base::ListValue>();
 
   // Get raw pointers to pass to the first task. Ownership of the unique_ptrs
   // will be passed to the reply task.
@@ -708,13 +722,15 @@ void Predictor::UpdatePrefsOnUIThread(
     std::unique_ptr<base::ListValue> referral_list) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   user_prefs_->Set(prefs::kDnsPrefetchingStartupList, *startup_list);
+  // May be empty if kInitialDnsPrefetchListEnabled is false. Still update the
+  // prefs to clear the state.
   user_prefs_->Set(prefs::kDnsPrefetchingHostReferralList, *referral_list);
 }
 
 void Predictor::WriteDnsPrefetchState(base::ListValue* startup_list,
                                       base::ListValue* referral_list) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  if (initial_observer_.get())
+  if (initial_observer_)
     initial_observer_->GetInitialDnsResolutionList(startup_list);
 
   SerializeReferrers(referral_list);