Make the site engagement service more robust to clock changes and time inconsistencies.

chrome/browser/engagement/site_engagement_service.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/engagement/site_engagement_service.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <utility>
@@ skipping 281 matching lines @@
   // This method should not be called with |update_last_engagement_time| = true
   // if the last engagement time isn't stale.
   DCHECK(!update_last_engagement_time || IsLastEngagementStale());
 
   HostContentSettingsMap* settings_map =
       HostContentSettingsMapFactory::GetForProfile(profile_);
   std::unique_ptr<ContentSettingsForOneType> engagement_settings =
       GetEngagementContentSettings(settings_map);
 
   // We want to rebase last engagement times relative to MaxDecaysPerScore
-  // periods of decay in the past.
+  // periods of decay in the past. If |update_last_engagement_time| = true, we
+  // should have last_engagement_time < rebase_time < now.
   base::Time now = clock_->Now();
   base::Time last_engagement_time = GetLastEngagementTime();
   base::Time rebase_time = now - GetMaxDecayPeriod();
   base::Time new_last_engagement_time;
+  DCHECK(!update_last_engagement_time ||
+         (last_engagement_time < rebase_time && rebase_time < now));
+
   for (const auto& site : *engagement_settings) {
     GURL origin(site.primary_pattern.ToString());
 
     if (origin.is_valid()) {
       SiteEngagementScore score = CreateEngagementScore(origin);
       if (update_last_engagement_time) {
-        // Work out the offset between this score's last engagement time and the
-        // last time the service recorded any engagement. Set the score's last
-        // engagement time to rebase_time - offset to preserve its state,
-        // relative to the rebase date. This ensures that the score will decay
-        // the next time it is used, but will not decay too much.
-        DCHECK_LE(score.last_engagement_time(), rebase_time);
-        base::TimeDelta offset =
-            last_engagement_time - score.last_engagement_time();
-        base::Time rebase_score_time = rebase_time - offset;
-        score.set_last_engagement_time(rebase_score_time);
-        if (rebase_score_time > new_last_engagement_time)
-          new_last_engagement_time = rebase_score_time;
+        // Catch cases of users moving their clocks, or a potential race where
+        // a score content setting is written out to prefs, but the updated
+        // |last_engagement_time| was not written, as both are lossy
+        // preferences.
+        if (score.last_engagement_time() > rebase_time) {
+          score.set_last_engagement_time(now);
+        } else if (score.last_engagement_time() > last_engagement_time) {
+          // This score is newer than |last_engagement_time|, but older than
+          // |rebase_time|. It should still be rebased with no offset as we
+          // don't accurately know what the offset should be.
+          score.set_last_engagement_time(rebase_time);
+        } else {
+          // Work out the offset between this score's last engagement time and
+          // the last time the service recorded any engagement. Set the score's
+          // last engagement time to rebase_time - offset to preserve its state,
+          // relative to the rebase date. This ensures that the score will decay
+          // the next time it is used, but will not decay too much.
+          base::TimeDelta offset =
+              last_engagement_time - score.last_engagement_time();
+          base::Time rebase_score_time = rebase_time - offset;
+          score.set_last_engagement_time(rebase_score_time);
+        }
 
+        if (score.last_engagement_time() > new_last_engagement_time)
+          new_last_engagement_time = score.last_engagement_time();
         score.Commit();
       }
 
       if (score.GetScore() > SiteEngagementScore::GetScoreCleanupThreshold())
         continue;
     }
 
     // This origin has a score of 0. Wipe it from content settings.
     settings_map->SetWebsiteSettingDefaultScope(
         origin, GURL(), CONTENT_SETTINGS_TYPE_SITE_ENGAGEMENT, std::string(),
@@ skipping 261 matching lines @@
     if (!engagement_score.last_shortcut_launch_time().is_null() &&
         engagement_score.last_shortcut_launch_time() > last_visit) {
       engagement_score.set_last_shortcut_launch_time(last_visit);
     }
 
     engagement_score.Commit();
   }
 
   SetLastEngagementTime(now);
 }