[Sessions] Add detailed logging of SessionRestore events.

chrome/browser/sessions/session_restore_stats_collector.cc

 // Copyright 2015 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/sessions/session_restore_stats_collector.h"
 
 #include <string>
 
 #include "base/metrics/histogram.h"
 #include "base/strings/stringprintf.h"
+#include "base/time/default_tick_clock.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/web_contents.h"
 
+namespace {
+
 using content::NavigationController;
 using content::RenderWidgetHost;
+using content::Source;
 using content::WebContents;
 
-// static
-void SessionRestoreStatsCollector::TrackTabs(
-    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
-    const base::TimeTicks& restore_started) {
-  if (!shared_collector_)
-    shared_collector_ = new SessionRestoreStatsCollector(restore_started);
+// The enumeration values stored in the "SessionRestore.Actions" histogram.
+enum SessionRestoreActionsUma {
+  // Counts the total number of session restores that have occurred.
+  SESSION_RESTORE_ACTIONS_UMA_INITIATED = 0,
+  // Counts the number of session restores that have seen deferred tab loadings
+  // for whatever reason (almost certainly due to memory pressure).
+  SESSION_RESTORE_ACTIONS_UMA_DEFERRED_TABS = 1,
+  // The size of this enum. Must be the last entry.
+  SESSION_RESTORE_ACTIONS_UMA_MAX,
+};
 
-  shared_collector_->AddTabs(tabs);
+// Emits a SessionRestore.Actions UMA event.
+void EmitUmaSessionRestoreActionEvent(SessionRestoreActionsUma action) {
+  UMA_HISTOGRAM_ENUMERATION("SessionRestore.Actions", action,
+                            SESSION_RESTORE_ACTIONS_UMA_MAX);
 }
 
-// static
-void SessionRestoreStatsCollector::TrackActiveTabs(
-    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
-    const base::TimeTicks& restore_started) {
-  if (!shared_collector_)
-    shared_collector_ = new SessionRestoreStatsCollector(restore_started);
+// The enumeration of values stored in the "SessionRestore.TabActions"
+// histogram.
+enum SessionRestoreTabActionsUma {
+  // Incremented for each tab created in a session restore.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_CREATED = 0,
+  // Incremented for each tab that session restore decides not to load.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADING_DEFERRED = 1,
+  // Incremented for each tab that is successfully loaded.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADED = 2,
+  // Incremented for each session-restore-deferred tab that is subsequently
+  // loaded.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_DEFERRED_TAB_LOADED = 3,
+  // The size of this enum. Must be the last entry.
+  SESSION_RESTORE_TAB_ACTIONS_UMA_MAX,
+};
 
-  std::vector<SessionRestoreDelegate::RestoredTab> active_tabs;
-  for (auto tab : tabs) {
-    if (tab.is_active())
-      active_tabs.push_back(tab);
-  }
-  shared_collector_->AddTabs(active_tabs);
+// Emits a SessionRestore.TabActions UMA event.
+void EmitUmaSessionRestoreTabActionEvent(SessionRestoreTabActionsUma action) {
+  UMA_HISTOGRAM_ENUMERATION("SessionRestore.TabActions", action,
+                            SESSION_RESTORE_TAB_ACTIONS_UMA_MAX);
+}
+
+}  // namespace
+
+SessionRestoreStatsCollector::TabLoaderStats::TabLoaderStats()
+    : tab_count(0u), tabs_loaded(0u), parallel_tab_loads(0u) {
+}
+
+SessionRestoreStatsCollector::TabState::TabState(
+    NavigationController* controller)
+    : controller(controller),
+      render_widget_host(nullptr),
+      is_deferred(false),
+      loading_state(TAB_IS_NOT_LOADING) {
 }
 
 SessionRestoreStatsCollector::SessionRestoreStatsCollector(
-    const base::TimeTicks& restore_started)
-    : got_first_foreground_load_(false),
+    const base::TimeTicks& restore_started,
+    scoped_ptr<StatsReportingDelegate> reporting_delegate)
+    : done_tracking_non_deferred_tabs_(false),
+      got_first_foreground_load_(false),
       got_first_paint_(false),
       restore_started_(restore_started),
-      tab_count_(0),
-      max_parallel_tab_loads_(0) {
+      waiting_for_load_tab_count_(0u),
+      loading_tab_count_(0u),
+      deferred_tab_count_(0u),
+      tick_clock_(new base::DefaultTickClock()),
+      reporting_delegate_(reporting_delegate.Pass()) {
   this_retainer_ = this;
-  registrar_.Add(
-      this, content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
-      content::NotificationService::AllSources());
 }
 
 SessionRestoreStatsCollector::~SessionRestoreStatsCollector() {
-  DCHECK((got_first_paint_ || render_widget_hosts_to_paint_.empty()) &&
-         tabs_tracked_.empty() && render_widget_hosts_loading_.empty());
-  DCHECK(shared_collector_ == this);
-  shared_collector_ = nullptr;
+}
+
+void SessionRestoreStatsCollector::TrackTabs(
+    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs) {
+  DCHECK(!done_tracking_non_deferred_tabs_);
+
+  // If this is the first call to TrackTabs then start observing events.
+  if (tab_loader_stats_.tab_count == 0) {
+    registrar_.Add(
+        this, content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
+        content::NotificationService::AllSources());
+  }
+
+  tab_loader_stats_.tab_count += tabs.size();
+  waiting_for_load_tab_count_ += tabs.size();
+  for (const auto& tab : tabs) {
+    TabState* tab_state =
+        RegisterForNotifications(&tab.contents()->GetController());
+    // Active tabs have already started loading.
+    if (tab.is_active())
+      MarkTabAsLoading(tab_state);
+  }
+}
+
+void SessionRestoreStatsCollector::DeferTab(NavigationController* tab) {
+  TabState* tab_state = GetTabState(tab);
+
+  // If the tab is no longer being tracked it has already finished loading.
+  // This can occur if the user forces the tab to load before the entire session
+  // restore is over, and the TabLoader then decides it would defer loading of
+  // that tab.
+  if (!tab_state)
+    return;
+
+  // Mark this tab as deferred, if its still being tracked. A tab should not be
+  // marked as deferred twice.
+  DCHECK(!tab_state->is_deferred);
+  tab_state->is_deferred = true;
+  ++deferred_tab_count_;
+
+  // A tab that didn't start loading before it was deferred is not to be
+  // actively monitored for loading.
+  if (tab_state->loading_state == TAB_IS_NOT_LOADING) {
+    DCHECK_LT(0u, waiting_for_load_tab_count_);
+    if (--waiting_for_load_tab_count_ == 0)
+      ReleaseIfDoneTracking();
+  }
+
+  reporting_delegate_->ReportTabDeferred();
 }
 
 void SessionRestoreStatsCollector::Observe(
     int type,
     const content::NotificationSource& source,
     const content::NotificationDetails& details) {
   switch (type) {
     case content::NOTIFICATION_LOAD_START: {
-      // Add this render_widget_host to the set of those we're waiting for
-      // paints on. We want to only record stats for paints that occur after
-      // a load has finished.
-      NavigationController* tab =
-          content::Source<NavigationController>(source).ptr();
-      RenderWidgetHost* render_widget_host = GetRenderWidgetHost(tab);
-      DCHECK(render_widget_host);
-      render_widget_hosts_loading_.insert(render_widget_host);
+      // This occurs when a tab has started to load. This can be because of
+      // the tab loader (only for non-deferred tabs) or because the user clicked
+      // on the tab.
+      NavigationController* tab = Source<NavigationController>(source).ptr();
+      TabState* tab_state = GetTabState(tab);
+      MarkTabAsLoading(tab_state);
       break;
     }
     case content::NOTIFICATION_WEB_CONTENTS_DESTROYED: {
-      WebContents* web_contents = content::Source<WebContents>(source).ptr();
-      RemoveTab(&web_contents->GetController());
+      // This happens when a tab has been closed. A tab can be in any state
+      // when this occurs. Simply stop tracking the tab.
+      WebContents* web_contents = Source<WebContents>(source).ptr();
+      NavigationController* tab = &web_contents->GetController();
+      RemoveTab(tab);
       break;
     }
     case content::NOTIFICATION_LOAD_STOP: {
-      NavigationController* tab =
-          content::Source<NavigationController>(source).ptr();
-      RenderWidgetHost* render_widget_host = GetRenderWidgetHost(tab);
-      render_widget_hosts_to_paint_.insert(render_widget_host);
-      RemoveTab(tab);
-      if (!got_first_foreground_load_ && render_widget_host &&
-          render_widget_host->GetView() &&
+      // This occurs to loading tabs when they have finished loading. The tab
+      // may or may not already have painted at this point.
+
+      // Update the tab state and any global state as necessary.
+      NavigationController* tab = Source<NavigationController>(source).ptr();
+      TabState* tab_state = GetTabState(tab);
+      DCHECK(tab_state);
+      tab_state->loading_state = TAB_IS_LOADED;
+      DCHECK_LT(0u, loading_tab_count_);
+      --loading_tab_count_;
+      if (!tab_state->is_deferred) {
+        DCHECK_LT(0u, waiting_for_load_tab_count_);
+        --waiting_for_load_tab_count_;
+      }
+
+      if (tab_state->is_deferred) {
+        reporting_delegate_->ReportDeferredTabLoaded();
+      } else {
+        DCHECK(!done_tracking_non_deferred_tabs_);
+        ++tab_loader_stats_.tabs_loaded;
+      }
+
+      // Update statistics for foreground tabs.
+      base::TimeDelta time_to_load = tick_clock_->NowTicks() - restore_started_;
+      if (!got_first_foreground_load_ && tab_state->render_widget_host &&
+          tab_state->render_widget_host->GetView() &&
+          tab_state->render_widget_host->GetView()->IsShowing()) {
+        got_first_foreground_load_ = true;
+        DCHECK(!done_tracking_non_deferred_tabs_);
+        tab_loader_stats_.foreground_tab_first_loaded = time_to_load;
+      }
+
+      // Update statistics for all tabs, if this wasn't a deferred tab. This is
+      // done here and not in ReleaseIfDoneTracking because it is possible to
+      // wait for a paint long after all loads have completed.
+      if (!done_tracking_non_deferred_tabs_ && !tab_state->is_deferred)
+        tab_loader_stats_.non_deferred_tabs_loaded = time_to_load;
+
+      // By default tabs transition to being tracked for paint events after the
+      // load event has been seen. However, if the first paint event has already
+      // been seen then this is not necessary and the tab can be removed.
+      if (got_first_paint_)
+        RemoveTab(tab);
+
+      break;
+    }
+    case content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE: {
+      // This notification is across all tabs in the browser so notifications
+      // will arrive for tabs that the collector is not explicitly tracking.
+
+      // Only process this event if first paint hasn't been seen and this is a
+      // paint of a visible tab.
+      RenderWidgetHost* render_widget_host =
+          Source<RenderWidgetHost>(source).ptr();
+      if (!got_first_paint_ && render_widget_host->GetView() &&
           render_widget_host->GetView()->IsShowing()) {
-        got_first_foreground_load_ = true;
-        base::TimeDelta time_to_load =
-            base::TimeTicks::Now() - restore_started_;
-        UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstLoaded",
-                                   time_to_load,
-                                   base::TimeDelta::FromMilliseconds(10),
-                                   base::TimeDelta::FromSeconds(100), 100);
-        // Record a time for the number of tabs, to help track down
-        // contention.
-        std::string time_for_count = base::StringPrintf(
-            "SessionRestore.ForegroundTabFirstLoaded_%d", tab_count_);
-        base::HistogramBase* counter_for_count =
-            base::Histogram::FactoryTimeGet(
-                time_for_count, base::TimeDelta::FromMilliseconds(10),
-                base::TimeDelta::FromSeconds(100), 100,
-                base::Histogram::kUmaTargetedHistogramFlag);
-        counter_for_count->AddTime(time_to_load);
+        got_first_paint_ = true;
+        TabState* tab_state = GetTabState(render_widget_host);
+        if (tab_state) {
+          // This is a paint for a tab that is explicitly being tracked so
+          // update the statistics. Otherwise the host is for a tab that's not
+          // being tracked thus some other tab has visibility and has rendered
+          // and there's no point in tracking the time to first paint. This can
+          // happen because the user opened a different tab or restored tabs
+          // to an already existing browser and an existing tab was in the
+          // foreground.
+          base::TimeDelta time_to_paint =
+              tick_clock_->NowTicks() - restore_started_;
+          DCHECK(!done_tracking_non_deferred_tabs_);
+          tab_loader_stats_.foreground_tab_first_paint = time_to_paint;
+        }
+
+        // Once first paint has been observed the entire to-paint tracking
+        // mechanism is no longer needed.
+        registrar_.Remove(
+            this,
+            content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
+            content::NotificationService::AllSources());
+
+        // Remove any tabs that have loaded. These were only being kept around
+        // while waiting for a paint event.
+        std::vector<NavigationController*> loaded_tabs;
+        for (auto& map_entry : tabs_tracked_) {
+          TabState& tab_state = map_entry.second;
+          if (tab_state.loading_state == TAB_IS_LOADED)
+            loaded_tabs.push_back(tab_state.controller);
+        }
+        for (auto& tab : loaded_tabs)
+          RemoveTab(tab);
       }
       break;
     }
-    case content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE: {
-      RenderWidgetHost* render_widget_host =
-          content::Source<RenderWidgetHost>(source).ptr();
-      if (!got_first_paint_ && render_widget_host->GetView() &&
-          render_widget_host->GetView()->IsShowing()) {
-        if (render_widget_hosts_to_paint_.find(render_widget_host) !=
-            render_widget_hosts_to_paint_.end()) {
-          // Got a paint for one of our renderers, so record time.
-          got_first_paint_ = true;
-          base::TimeDelta time_to_paint =
-              base::TimeTicks::Now() - restore_started_;
-          // TODO(danduong): to remove this with 467680, to make sure we
-          // don't forget to clean this up.
-          UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstPaint",
-                                     time_to_paint,
-                                     base::TimeDelta::FromMilliseconds(10),
-                                     base::TimeDelta::FromSeconds(100), 100);
-          // Record a time for the number of tabs, to help track down
-          // contention.
-          std::string time_for_count = base::StringPrintf(
-              "SessionRestore.ForegroundTabFirstPaint_%d", tab_count_);
-          base::HistogramBase* counter_for_count =
-              base::Histogram::FactoryTimeGet(
-                  time_for_count, base::TimeDelta::FromMilliseconds(10),
-                  base::TimeDelta::FromSeconds(100), 100,
-                  base::Histogram::kUmaTargetedHistogramFlag);
-          counter_for_count->AddTime(time_to_paint);
-          UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstPaint2",
-                                     time_to_paint,
-                                     base::TimeDelta::FromMilliseconds(100),
-                                     base::TimeDelta::FromMinutes(16), 50);
-          // Record a time for the number of tabs, to help track down
-          // contention.
-          std::string time_for_count2 = base::StringPrintf(
-              "SessionRestore.ForegroundTabFirstPaint2_%d", tab_count_);
-          base::HistogramBase* counter_for_count2 =
-              base::Histogram::FactoryTimeGet(
-                  time_for_count2, base::TimeDelta::FromMilliseconds(100),
-                  base::TimeDelta::FromMinutes(16), 50,
-                  base::Histogram::kUmaTargetedHistogramFlag);
-          counter_for_count2->AddTime(time_to_paint);
-        } else if (render_widget_hosts_loading_.find(render_widget_host) ==
-                   render_widget_hosts_loading_.end()) {
-          // If this is a host for a tab we're not loading some other tab
-          // has rendered and there's no point tracking the time. This could
-          // happen because the user opened a different tab or restored tabs
-          // to an already existing browser and an existing tab painted.
-          got_first_paint_ = true;
-        }
-      }
-      break;
-    }
     default:
       NOTREACHED() << "Unknown notification received:" << type;
       break;
   }
 
-  // Check if we are done and if so, reset |this_retainer_| as the collector no
-  // longer needs to stay alive.
-  if ((got_first_paint_ || render_widget_hosts_to_paint_.empty()) &&
-      tabs_tracked_.empty() && render_widget_hosts_loading_.empty())
-    this_retainer_ = nullptr;
+  ReleaseIfDoneTracking();
 }
 
-void SessionRestoreStatsCollector::AddTabs(
-    const std::vector<SessionRestoreDelegate::RestoredTab>& tabs) {
-  tab_count_ += tabs.size();
-  for (auto& tab : tabs) {
-    RegisterForNotifications(&tab.contents()->GetController());
-    if (tab.is_active()) {
-      RenderWidgetHost* render_widget_host =
-          GetRenderWidgetHost(&tab.contents()->GetController());
-      render_widget_hosts_loading_.insert(render_widget_host);
-    }
+void SessionRestoreStatsCollector::RemoveTab(NavigationController* tab) {
+  // Stop observing this tab.
+  registrar_.Remove(this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
+                    Source<WebContents>(tab->GetWebContents()));
+  registrar_.Remove(this, content::NOTIFICATION_LOAD_STOP,
+                    Source<NavigationController>(tab));
+  registrar_.Remove(this, content::NOTIFICATION_LOAD_START,
+                    Source<NavigationController>(tab));
+
+  auto tab_it = tabs_tracked_.find(tab);
+  DCHECK(tab_it != tabs_tracked_.end());
+  TabState& tab_state = tab_it->second;
+
+  // If this tab was waiting for a NOTIFICATION_LOAD_STOP event then update
+  // the loading counts.
+  if (tab_state.loading_state == TAB_IS_LOADING) {
+    DCHECK_LT(0u, loading_tab_count_);
+    --loading_tab_count_;
+  }
+
+  // Only non-deferred not-loading/not-loaded tabs are waiting to be loaded.
+  if (tab_state.loading_state != TAB_IS_LOADED && !tab_state.is_deferred) {
+    DCHECK_LT(0u, waiting_for_load_tab_count_);
+    // It's possible for waiting_for_load_tab_count_ to reach zero here. This
+    // function is only called from 'Observe', so the transition will be
+    // noticed there.
+    --waiting_for_load_tab_count_;
+  }
+
+  if (tab_state.is_deferred)
+    --deferred_tab_count_;
+
+  // Remove the tab from the |tracked_tabs_| map.
+  tabs_tracked_.erase(tab_it);
+
+  // It is possible for all restored contents to be destroyed or forcibly
+  // renavigated before a first paint has arrived. This can be detected by
+  // tabs_tracked_ containing only deferred tabs. At this point the paint
+  // mechanism can be disabled and stats collection will stop.
+  if (tabs_tracked_.size() == deferred_tab_count_ && !got_first_paint_) {
+    got_first_paint_ = true;
+    registrar_.Remove(
+        this, content::NOTIFICATION_RENDER_WIDGET_HOST_DID_UPDATE_BACKING_STORE,
+        content::NotificationService::AllSources());
   }
 }
 
-void SessionRestoreStatsCollector::RemoveTab(NavigationController* tab) {
-  registrar_.Remove(this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-                    content::Source<WebContents>(tab->GetWebContents()));
-  registrar_.Remove(this, content::NOTIFICATION_LOAD_STOP,
-                    content::Source<NavigationController>(tab));
-  registrar_.Remove(this, content::NOTIFICATION_LOAD_START,
-                    content::Source<NavigationController>(tab));
-  if (render_widget_hosts_loading_.size() > max_parallel_tab_loads_)
-    max_parallel_tab_loads_ = render_widget_hosts_loading_.size();
-  RenderWidgetHost* render_widget_host = GetRenderWidgetHost(tab);
-  render_widget_hosts_loading_.erase(render_widget_host);
-  tabs_tracked_.erase(tab);
-
-  // If there are no more tabs loading or being tracked, restore is done and
-  // record the time. Note that we are not yet finished, as we might still be
-  // waiting for our first paint, which can happen after all tabs are done
-  // loading.
-  // TODO(georgesak): review behaviour of ForegroundTabFirstPaint.
-  if (tabs_tracked_.empty() && render_widget_hosts_loading_.empty()) {
-    base::TimeDelta time_to_load = base::TimeTicks::Now() - restore_started_;
-    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.AllTabsLoaded", time_to_load,
-                               base::TimeDelta::FromMilliseconds(10),
-                               base::TimeDelta::FromSeconds(100), 100);
-    // Record a time for the number of tabs, to help track down contention.
-    std::string time_for_count =
-        base::StringPrintf("SessionRestore.AllTabsLoaded_%d", tab_count_);
-    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
-        time_for_count, base::TimeDelta::FromMilliseconds(10),
-        base::TimeDelta::FromSeconds(100), 100,
-        base::Histogram::kUmaTargetedHistogramFlag);
-    counter_for_count->AddTime(time_to_load);
-
-    UMA_HISTOGRAM_COUNTS_100("SessionRestore.ParallelTabLoads",
-                             max_parallel_tab_loads_);
-  }
-}
-
-void SessionRestoreStatsCollector::RegisterForNotifications(
+SessionRestoreStatsCollector::TabState*
+SessionRestoreStatsCollector::RegisterForNotifications(
     NavigationController* tab) {
   registrar_.Add(this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
-                 content::Source<WebContents>(tab->GetWebContents()));
+                 Source<WebContents>(tab->GetWebContents()));
   registrar_.Add(this, content::NOTIFICATION_LOAD_STOP,
-                 content::Source<NavigationController>(tab));
+                 Source<NavigationController>(tab));
   registrar_.Add(this, content::NOTIFICATION_LOAD_START,
-                 content::Source<NavigationController>(tab));
-  tabs_tracked_.insert(tab);
+                 Source<NavigationController>(tab));
+  auto result = tabs_tracked_.insert(std::make_pair(tab, TabState(tab)));
+  DCHECK(result.second);
+  TabState* tab_state = &result.first->second;
+  return tab_state;
 }
 
 RenderWidgetHost* SessionRestoreStatsCollector::GetRenderWidgetHost(
     NavigationController* tab) {
   WebContents* web_contents = tab->GetWebContents();
   if (web_contents) {
     content::RenderWidgetHostView* render_widget_host_view =
         web_contents->GetRenderWidgetHostView();
     if (render_widget_host_view)
       return render_widget_host_view->GetRenderWidgetHost();
   }
   return nullptr;
 }
 
-// static
-SessionRestoreStatsCollector* SessionRestoreStatsCollector::shared_collector_ =
-    nullptr;
+SessionRestoreStatsCollector::TabState*
+SessionRestoreStatsCollector::GetTabState(NavigationController* tab) {
+  // This lookup can fail because DeferTab calls can arrive for tabs that have
+  // already loaded (user forced) and are no longer tracked.
+  auto it = tabs_tracked_.find(tab);
+  if (it == tabs_tracked_.end())
+    return nullptr;
+  return &it->second;
+}
+
+SessionRestoreStatsCollector::TabState*
+SessionRestoreStatsCollector::GetTabState(RenderWidgetHost* tab) {
+  for (auto& pair : tabs_tracked_) {
+    if (pair.second.render_widget_host == tab)
+      return &pair.second;
+  }
+  // It's possible for this lookup to fail as paint events can be received for
+  // tabs that aren't being tracked.
+  return nullptr;
+}
+
+void SessionRestoreStatsCollector::MarkTabAsLoading(TabState* tab_state) {
+  // If the tab has already started or finished loading then a user navigation
+  // has caused the tab to be forcibly reloaded. This tab can be removed from
+  // observation.
+  if (tab_state->loading_state == TAB_IS_LOADED) {
+    RemoveTab(tab_state->controller);
+    return;
+  }
+
+  DCHECK_EQ(TAB_IS_NOT_LOADING, tab_state->loading_state);
+  if (tab_state->loading_state != TAB_IS_NOT_LOADING)
+    return;
+  tab_state->loading_state = TAB_IS_LOADING;
+  ++loading_tab_count_;
+
+  if (!done_tracking_non_deferred_tabs_) {
+    tab_loader_stats_.parallel_tab_loads =
+        std::max(tab_loader_stats_.parallel_tab_loads, loading_tab_count_);
+  }
+
+  // Get the RenderWidgetHost for the tab and add it to the secondary index.
+  RenderWidgetHost* render_widget_host =
+      GetRenderWidgetHost(tab_state->controller);
+  DCHECK(render_widget_host);
+  tab_state->render_widget_host = render_widget_host;
+}
+
+void SessionRestoreStatsCollector::ReleaseIfDoneTracking() {
+  // If non-deferred tabs are no longer being tracked then report tab loader
+  // statistics.
+  if (!done_tracking_non_deferred_tabs_ && got_first_paint_ &&
+      waiting_for_load_tab_count_ == 0) {
+    done_tracking_non_deferred_tabs_ = true;
+    reporting_delegate_->ReportTabLoaderStats(tab_loader_stats_);
+  }
+
+  // If tracking is completely finished then emit collected metrics and destroy
+  // this stats collector.
+  if (done_tracking_non_deferred_tabs_ && tabs_tracked_.empty())
+    this_retainer_ = nullptr;
+}
+
+SessionRestoreStatsCollector::UmaStatsReportingDelegate::
+    UmaStatsReportingDelegate()
+    : got_report_tab_deferred_(false) {
+}
+
+void SessionRestoreStatsCollector::UmaStatsReportingDelegate::
+    ReportTabLoaderStats(const TabLoaderStats& tab_loader_stats) {
+  UMA_HISTOGRAM_COUNTS_100("SessionRestore.TabCount",
+                           tab_loader_stats.tab_count);
+
+  EmitUmaSessionRestoreActionEvent(SESSION_RESTORE_ACTIONS_UMA_INITIATED);
+
+  for (size_t i = 0; i < tab_loader_stats.tab_count; ++i) {
+    EmitUmaSessionRestoreTabActionEvent(
+        SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_CREATED);
+  }
+
+  for (size_t i = 0; i < tab_loader_stats.tabs_loaded; ++i) {
+    EmitUmaSessionRestoreTabActionEvent(
+        SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADED);
+  }
+
+  if (!tab_loader_stats.foreground_tab_first_loaded.is_zero()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstLoaded",
+                               tab_loader_stats.foreground_tab_first_loaded,
+                               base::TimeDelta::FromMilliseconds(10),
+                               base::TimeDelta::FromSeconds(100), 100);
+
+    // Record a time for the number of tabs, to help track down contention.
+    std::string time_for_count = base::StringPrintf(
+        "SessionRestore.ForegroundTabFirstLoaded_%u",
+        static_cast<unsigned int>(tab_loader_stats.tab_count));
+    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
+        time_for_count, base::TimeDelta::FromMilliseconds(10),
+        base::TimeDelta::FromSeconds(100), 100,
+        base::Histogram::kUmaTargetedHistogramFlag);
+    counter_for_count->AddTime(tab_loader_stats.foreground_tab_first_loaded);
+  }
+
+  if (!tab_loader_stats.foreground_tab_first_paint.is_zero()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.ForegroundTabFirstPaint3",
+                               tab_loader_stats.foreground_tab_first_paint,
+                               base::TimeDelta::FromMilliseconds(100),
+                               base::TimeDelta::FromMinutes(16), 50);
+
+    std::string time_for_count = base::StringPrintf(
+        "SessionRestore.ForegroundTabFirstPaint3_%u",
+        static_cast<unsigned int>(tab_loader_stats.tab_count));
+    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
+        time_for_count, base::TimeDelta::FromMilliseconds(100),
+        base::TimeDelta::FromMinutes(16), 50,
+        base::Histogram::kUmaTargetedHistogramFlag);
+    counter_for_count->AddTime(tab_loader_stats.foreground_tab_first_paint);
+  }
+
+  if (!tab_loader_stats.non_deferred_tabs_loaded.is_zero()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("SessionRestore.AllTabsLoaded",
+                               tab_loader_stats.non_deferred_tabs_loaded,
+                               base::TimeDelta::FromMilliseconds(10),
+                               base::TimeDelta::FromSeconds(100), 100);
+
+    // Record a time for the number of tabs, to help track down contention.
+    std::string time_for_count = base::StringPrintf(
+        "SessionRestore.AllTabsLoaded_%u",
+        static_cast<unsigned int>(tab_loader_stats.tab_count));
+    base::HistogramBase* counter_for_count = base::Histogram::FactoryTimeGet(
+        time_for_count, base::TimeDelta::FromMilliseconds(10),
+        base::TimeDelta::FromSeconds(100), 100,
+        base::Histogram::kUmaTargetedHistogramFlag);
+    counter_for_count->AddTime(tab_loader_stats.non_deferred_tabs_loaded);
+  }
+
+  UMA_HISTOGRAM_COUNTS_100("SessionRestore.ParallelTabLoads",
+                           tab_loader_stats.parallel_tab_loads);
+}
+
+void SessionRestoreStatsCollector::UmaStatsReportingDelegate::
+    ReportTabDeferred() {
+  if (!got_report_tab_deferred_) {
+    got_report_tab_deferred_ = true;
+    EmitUmaSessionRestoreActionEvent(SESSION_RESTORE_ACTIONS_UMA_DEFERRED_TABS);
+  }
+
+  EmitUmaSessionRestoreTabActionEvent(
+      SESSION_RESTORE_TAB_ACTIONS_UMA_TAB_LOADING_DEFERRED);
+}
+
+void SessionRestoreStatsCollector::UmaStatsReportingDelegate::
+    ReportDeferredTabLoaded() {
+  EmitUmaSessionRestoreTabActionEvent(
+      SESSION_RESTORE_TAB_ACTIONS_UMA_DEFERRED_TAB_LOADED);
+}