Fix off-by-1 OOB array access in task tracker.

Bug: 978888
Change-Id: Ib16b2e35c2f88e7f554611b21bda48857127c77c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678937Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672696}

base/task/task_traits.h

@@ -66,6 +66,8 @@ enum class TaskPriority : uint8_t {
   HIGHEST = USER_BLOCKING
 };
 
+using TaskPriorityType = std::underlying_type<TaskPriority>::type;
+
 // Valid shutdown behaviors supported by the thread pool.
 enum class TaskShutdownBehavior : uint8_t {
   // Tasks posted with this mode which have not started executing before

base/task/thread_pool/task_tracker.cc

@@ -306,9 +306,11 @@ TaskTracker::TaskTracker(StringPiece histogram_label)
                              "UserBlockingTaskPriority_MayBlock")}},
       tracked_ref_factory_(this) {
   // Confirm that all |task_latency_histograms_| have been initialized above.
-  DCHECK(*(&task_latency_histograms_[static_cast<int>(TaskPriority::HIGHEST) +
-                                     1][0] -
-           1));
+  for (TaskPriorityType i = 0; i < kNumTaskPriorities; ++i) {
+    for (TaskPriorityType j = 0; j < kNumBlockingModes; ++j) {
+      DCHECK(task_latency_histograms_[i][j]);
+    }
+  }
 }
 
 TaskTracker::~TaskTracker() = default;

base/task/thread_pool/task_tracker.h

@@ -281,12 +281,16 @@ class BASE_EXPORT TaskTracker {
   // blocking tasks. Intentionally leaked.
   // TODO(scheduler-dev): Consider using STATIC_HISTOGRAM_POINTER_GROUP for
   // these.
-  static constexpr int kNumTaskPriorities =
-      static_cast<int>(TaskPriority::HIGHEST) + 1;
-  HistogramBase* const task_latency_histograms_[kNumTaskPriorities][2];
-  HistogramBase* const heartbeat_latency_histograms_[kNumTaskPriorities][2];
+  static constexpr auto kNumTaskPriorities =
+      static_cast<TaskPriorityType>(TaskPriority::HIGHEST) + 1;
+  static constexpr TaskPriorityType kNumBlockingModes = 2;
+  HistogramBase* const task_latency_histograms_[kNumTaskPriorities]
+                                               [kNumBlockingModes];
+  HistogramBase* const heartbeat_latency_histograms_[kNumTaskPriorities]
+                                                    [kNumBlockingModes];
   HistogramBase* const
-      num_tasks_run_while_queuing_histograms_[kNumTaskPriorities][2];
+      num_tasks_run_while_queuing_histograms_[kNumTaskPriorities]
+                                             [kNumBlockingModes];
 
   // Ensures all state (e.g. dangling cleaned up workers) is coalesced before
   // destroying the TaskTracker (e.g. in test environments).