In TabActivityWatcher, a label_id_ of a tab is updated by:

label_id_ = label_id_ ? label_id_ + 1 : NewInt64ForLabelIdOrQueryId();

If a foregrounded event is logged, then label_id_ is set to 0 and a new
one is assigned the next time by
    (++internal_id_for_logging) << kIdShiftBits;

If no foregrounded event is logged between two discarding events,
the label_id_ will increase by 1.

So if it increases 8192 times, it will collide with another label_id_
generated with (++internal_id_for_logging) << kIdShiftBits;

We have seen this occasionally in the logging, and the simplest way to
fix this is increase kIdShiftBits a bit (from 13 to 16).

Bug: 1049896

Change-Id: Ibbc56ee24ba502433283f53c38946b98a25ac37c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041837Reviewed-by: Tony Yeoman <tby@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740096}

chrome/browser/resource_coordinator/tab_activity_watcher.cc

@@ -53,13 +53,14 @@ static int32_t reactivation_index = 0;
 int64_t internal_id_for_logging = 0;
 // Returns an int64_t number as label_id or query_id.
 int64_t NewInt64ForLabelIdOrQueryId() {
-  // The id is shifted 13 bits so that the lower bits are reserved for counting
+  // The id is shifted 16 bits so that the lower bits are reserved for counting
   // multiple queries.
-  // We choose 13 so that the lower bits for counting multiple queries and
+  // We choose 16 so that the lower bits for counting multiple queries and
   // higher bits for labeling queries are both unlikely to overflow. (lower bits
-  // only overflows when we have more than 8192 queries without labeling events;
-  // higher bits only overflow when we have more than 100 billion discards.
-  constexpr int kIdShiftBits = 13;
+  // only overflows when we have more than 65536 queries without labeling
+  // events; higher bits only overflow when we have more than 100 billion
+  // discards.
+  constexpr int kIdShiftBits = 16;
   return (++internal_id_for_logging) << kIdShiftBits;
 }
 

chrome/browser/resource_coordinator/tab_activity_watcher_browsertest.cc

@@ -56,7 +56,7 @@ const UkmMetricMap kBasicMetricValues({
 
 // These parameters don't affect logging.
 const bool kCheckNavigationSuccess = true;
-const int64_t kIdShift = 1 << 13;
+const int64_t kIdShift = 1 << 16;
 
 }  // namespace
 

chrome/browser/resource_coordinator/tab_activity_watcher_unittest.cc

@@ -48,7 +48,7 @@ namespace {
 
 const char* kTabMetricsEntryName = TabManager_TabMetrics::kEntryName;
 
-const int64_t kIdShift = 1 << 13;
+const int64_t kIdShift = 1 << 16;
 
 // Test URLs need to be from different origins to test site engagement score.
 const std::vector<GURL>& TestUrls() {