Complete TaskPriority::BACKGROUND->BEST_EFFORT mass rename

Survey base/task_scheduler for remaining unprefixed "BACKGROUND" strings
referring to task priority (i.e. not referring to
"background thread priority").

Best-effort survey of the codebase for "BACKGROUND priority" left behind
in comments.

Update docs and remove TaskPriority::BACKGROUND for good

TBR=robliao@chromium.org

Bug: 863341
Change-Id: I055ac623767c1d77895d78dbf544a2d781a6637d
Reviewed-on: https://chromium-review.googlesource.com/1153461
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578802}

base/task_scheduler/post_task.h

@@ -77,7 +77,7 @@ BASE_EXPORT void PostTask(const Location& from_here, OnceClosure task);
 // expires. Calling this is equivalent to calling PostDelayedTaskWithTraits with
 // plain TaskTraits.
 //
-// Use PostDelayedTaskWithTraits to specify a BACKGROUND priority if the task
+// Use PostDelayedTaskWithTraits to specify a BEST_EFFORT priority if the task
 // doesn't have to run as soon as |delay| expires.
 BASE_EXPORT void PostDelayedTask(const Location& from_here,
                                  OnceClosure task,
@@ -127,8 +127,8 @@ BASE_EXPORT void PostTaskWithTraits(const Location& from_here,
 // Posts |task| with specific |traits| to the TaskScheduler. |task| will not run
 // before |delay| expires.
 //
-// Specify a BACKGROUND priority via |traits| if the task doesn't have to run as
-// soon as |delay| expires.
+// Specify a BEST_EFFORT priority via |traits| if the task doesn't have to run
+// as soon as |delay| expires.
 BASE_EXPORT void PostDelayedTaskWithTraits(const Location& from_here,
                                            const TaskTraits& traits,
                                            OnceClosure task,

base/task_scheduler/sequence_unittest.cc

@@ -94,9 +94,9 @@ TEST(TaskSchedulerSequenceTest, PushTakeRemove) {
   EXPECT_TRUE(sequence->Pop());
 }
 
-// Verifies the sort key of a sequence that contains one BACKGROUND task.
+// Verifies the sort key of a sequence that contains one BEST_EFFORT task.
 TEST(TaskSchedulerSequenceTest, GetSortKeyBackground) {
-  // Create a sequence with a BACKGROUND task.
+  // Create a sequence with a BEST_EFFORT task.
   Task background_task(FROM_HERE, DoNothing(), {TaskPriority::BEST_EFFORT},
                        TimeDelta());
   scoped_refptr<Sequence> background_sequence = MakeRefCounted<Sequence>();

base/task_scheduler/task_scheduler_impl.cc

@@ -228,15 +228,14 @@ std::vector<const HistogramBase*> TaskSchedulerImpl::GetHistograms() const {
 
 int TaskSchedulerImpl::GetMaxConcurrentNonBlockedTasksWithTraitsDeprecated(
     const TaskTraits& traits) const {
-  // This method does not support getting the maximum number of BACKGROUND tasks
-  // that can run concurrently in a pool.
+  // This method does not support getting the maximum number of BEST_EFFORT
+  // tasks that can run concurrently in a pool.
   DCHECK_NE(traits.priority(), TaskPriority::BEST_EFFORT);
   return GetWorkerPoolForTraits(traits)
       ->GetMaxConcurrentNonBlockedTasksDeprecated();
 }
 
 void TaskSchedulerImpl::Shutdown() {
-  // TODO(fdoray): Increase the priority of BACKGROUND tasks blocking shutdown.
   task_tracker_->Shutdown();
 }
 

base/task_scheduler/task_traits.cc

@@ -15,7 +15,7 @@ namespace base {
 const char* TaskPriorityToString(TaskPriority task_priority) {
   switch (task_priority) {
     case TaskPriority::BEST_EFFORT:
-      return "BACKGROUND";
+      return "BEST_EFFORT";
     case TaskPriority::USER_VISIBLE:
       return "USER_VISIBLE";
     case TaskPriority::USER_BLOCKING:

base/task_scheduler/task_traits.h

@@ -23,13 +23,10 @@ namespace base {
 enum class TaskPriority {
   // This will always be equal to the lowest priority available.
   LOWEST = 0,
-  // User won't notice if this task takes an arbitrarily long time to complete.
-  // TODO(gab): Eliminate BACKGROUND in favor of BEST_EFFORT.
-  BACKGROUND = LOWEST,
   // This task will only be scheduled when machine resources are available. Once
   // running, it may be descheduled if higher priority work arrives (in this
   // process or another) and its running on a non-critical thread.
-  BEST_EFFORT = BACKGROUND,
+  BEST_EFFORT = LOWEST,
   // This task affects UI or responsiveness of future user interactions. It is
   // not an immediate response to a user interaction.
   // Examples:
@@ -77,8 +74,9 @@ enum class TaskShutdownBehavior {
   // until they're executed. Generally, this should be used only to save
   // critical user data.
   //
-  // Note: Tasks with BACKGROUND priority that block shutdown will be promoted
-  // to USER_VISIBLE priority during shutdown.
+  // Note: Background threads will be promoted to normal threads at shutdown
+  // (i.e. TaskPriority::BEST_EFFORT + TaskShutdownBehavior::BLOCK_SHUTDOWN will
+  // resolve without a priority inversion).
   BLOCK_SHUTDOWN,
 };
 

chrome/browser/chromeos/platform_keys/platform_keys.cc

@@ -68,7 +68,7 @@ void IntersectCertificates(
 
   // This is triggered by a call to the
   // chrome.platformKeys.selectClientCertificates extensions API. Completion
-  // does not affect browser responsiveness, hence the BACKGROUND priority.
+  // does not affect browser responsiveness, hence the BEST_EFFORT priority.
   base::PostTaskWithTraitsAndReply(
       FROM_HERE,
       {base::TaskPriority::BEST_EFFORT,

chrome/browser/conflicts/module_inspector_win.h

@@ -68,7 +68,7 @@ class ModuleInspector {
   base::queue<ModuleInfoKey> queue_;
 
   // The task runner where module inspections takes place. It originally starts
-  // at BACKGROUND priority, but is changed to USER_VISIBLE when
+  // at BEST_EFFORT priority, but is changed to USER_VISIBLE when
   // IncreaseInspectionPriority() is called.
   scoped_refptr<base::SequencedTaskRunner> task_runner_;
 

chrome/browser/profiles/profile_impl_io_data.cc

@@ -123,7 +123,7 @@ void MaybeDeleteMediaCache(const base::FilePath& media_cache_path) {
   }
   base::PostTaskWithTraits(
       FROM_HERE,
-      {base::TaskPriority::BACKGROUND, base::MayBlock(),
+      {base::TaskPriority::BEST_EFFORT, base::MayBlock(),
        base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN},
       base::BindOnce(base::IgnoreResult(&base::DeleteFile), media_cache_path,
                      true /* recursive */));

chrome/browser/web_applications/web_app.h

@@ -187,7 +187,7 @@ namespace internals {
 
 // Run an IO task on a worker thread. Ownership of |shortcut_info| transfers
 // to a closure that deletes it on the UI thread when the task is complete.
-// Tasks posted here run with BACKGROUND priority and block shutdown.
+// Tasks posted here run with BEST_EFFORT priority and block shutdown.
 void PostShortcutIOTask(base::OnceCallback<void(const ShortcutInfo&)> task,
                         std::unique_ptr<ShortcutInfo> shortcut_info);
 void PostShortcutIOTaskAndReply(

components/optimization_guide/optimization_guide_service.h

@@ -59,7 +59,7 @@ class OptimizationGuideService {
   void SetLatestProcessedVersionForTesting(const base::Version& version);
 
  private:
-  // Always called as part of a background priority task.
+  // Always called as part of a BEST_EFFORT priority task.
   void ProcessHintsInBackground(const ComponentInfo& component_info);
 
   // Adds the observer on IO thread.

docs/threading_and_tasks.md

@@ -380,12 +380,12 @@ of how to specify `TaskTraits`.
 ```cpp
 // This task has no explicit TaskTraits. It cannot block. Its priority
 // is inherited from the calling context (e.g. if it is posted from
-// a BACKGROUND task, it will have a BACKGROUND priority). It will either
+// a BEST_EFFORT task, it will have a BEST_EFFORT priority). It will either
 // block shutdown or be skipped on shutdown.
 base::PostTask(FROM_HERE, base::BindOnce(...));
 
 // This task has the highest priority. The task scheduler will try to
-// run it before USER_VISIBLE and BACKGROUND tasks.
+// run it before USER_VISIBLE and BEST_EFFORT tasks.
 base::PostTaskWithTraits(
     FROM_HERE, {base::TaskPriority::USER_BLOCKING},
     base::BindOnce(...));

third_party/webrtc_overrides/rtc_base/task_queue.cc

@@ -43,7 +43,7 @@ base::TaskTraits TaskQueuePriority2Traits(TaskQueue::Priority priority) {
 #endif
       break;
     case TaskQueue::Priority::LOW:
-      return {base::MayBlock(), base::TaskPriority::BACKGROUND};
+      return {base::MayBlock(), base::TaskPriority::BEST_EFFORT};
     case TaskQueue::Priority::NORMAL:
     default:
 #if defined(OS_ANDROID)