Add support for priorities to TaskQueue.

BUG=webrtc:7216

Review-Url: https://codereview.webrtc.org/2708353003
Cr-Commit-Position: refs/heads/master@{#16834}

webrtc/base/task_queue.h

@@ -161,8 +161,16 @@ static std::unique_ptr<QueuedTask> NewClosure(const Closure& closure,
 // so assumptions about lifetimes of pending tasks should not be made.
 class LOCKABLE TaskQueue {
  public:
-  explicit TaskQueue(const char* queue_name);
-  // TODO(tommi): Implement move semantics?
+  // TaskQueue priority levels. On some platforms these will map to thread
+  // priorities, on others such as Mac and iOS, GCD queue priorities.
+  enum class Priority {
+    NORMAL = 0,
+    HIGH,
+    LOW,
+  };
+
+  explicit TaskQueue(const char* queue_name,
+                     Priority priority = Priority::NORMAL);
   ~TaskQueue();
 
   static TaskQueue* Current();
@@ -275,8 +283,11 @@ class LOCKABLE TaskQueue {
 
   class WorkerThread : public PlatformThread {
    public:
-    WorkerThread(ThreadRunFunction func, void* obj, const char* thread_name)
-        : PlatformThread(func, obj, thread_name) {}
+    WorkerThread(ThreadRunFunction func,
+                 void* obj,
+                 const char* thread_name,
+                 ThreadPriority priority)
+        : PlatformThread(func, obj, thread_name, priority) {}
 
     bool QueueAPC(PAPCFUNC apc_function, ULONG_PTR data) {
       return PlatformThread::QueueAPC(apc_function, data);

webrtc/base/task_queue_gcd.cc

@@ -21,6 +21,22 @@
 #include "webrtc/base/task_queue_posix.h"
 
 namespace rtc {
+namespace {
+
+using Priority = TaskQueue::Priority;
+
+int TaskQueuePriorityToGCD(Priority priority) {
+  switch (priority) {
+    case Priority::NORMAL:
+      return DISPATCH_QUEUE_PRIORITY_DEFAULT;
+    case Priority::HIGH:
+      return DISPATCH_QUEUE_PRIORITY_HIGH;
+    case Priority::LOW:
+      return DISPATCH_QUEUE_PRIORITY_LOW;
+  }
+}
+}
+
 using internal::GetQueuePtrTls;
 using internal::AutoSetCurrentQueuePtr;
 
@@ -94,7 +110,7 @@ struct TaskQueue::PostTaskAndReplyContext : public TaskQueue::TaskContext {
   dispatch_queue_t reply_queue_;
 };
 
-TaskQueue::TaskQueue(const char* queue_name)
+TaskQueue::TaskQueue(const char* queue_name, Priority priority /*= NORMAL*/)
     : queue_(dispatch_queue_create(queue_name, DISPATCH_QUEUE_SERIAL)),
       context_(new QueueContext(this)) {
   RTC_DCHECK(queue_name);
@@ -104,6 +120,9 @@ TaskQueue::TaskQueue(const char* queue_name)
   // to the queue is released.  This may run after the TaskQueue object has
   // been deleted.
   dispatch_set_finalizer_f(queue_, &QueueContext::DeleteContext);
+
+  dispatch_set_target_queue(
+      queue_, dispatch_get_global_queue(TaskQueuePriorityToGCD(priority), 0));
 }
 
 TaskQueue::~TaskQueue() {

webrtc/base/task_queue_libevent.cc

@@ -30,6 +30,8 @@ static const char kQuit = 1;
 static const char kRunTask = 2;
 static const char kRunReplyTask = 3;
 
+using Priority = TaskQueue::Priority;
+
 // This ignores the SIGPIPE signal on the calling thread.
 // This signal can be fired when trying to write() to a pipe that's being
 // closed or while closing a pipe that's being written to.
@@ -84,6 +86,21 @@ void EventAssign(struct event* ev,
   RTC_CHECK_EQ(0, event_base_set(base, ev));
 #endif
 }
+
+ThreadPriority TaskQueuePriorityToThreadPriority(Priority priority) {
+  switch (priority) {
+    case Priority::HIGH:
+      return kRealtimePriority;
+    case Priority::LOW:
+      return kLowPriority;
+    case Priority::NORMAL:
+      return kNormalPriority;
+    default:
+      RTC_NOTREACHED();
+      break;
+  }
+  return kNormalPriority;
+}
 }  // namespace
 
 struct TaskQueue::QueueContext {
@@ -201,10 +218,13 @@ class TaskQueue::SetTimerTask : public QueuedTask {
   const uint32_t posted_;
 };
 
-TaskQueue::TaskQueue(const char* queue_name)
+TaskQueue::TaskQueue(const char* queue_name, Priority priority /*= NORMAL*/)
     : event_base_(event_base_new()),
       wakeup_event_(new event()),
-      thread_(&TaskQueue::ThreadMain, this, queue_name) {
+      thread_(&TaskQueue::ThreadMain,
+              this,
+              queue_name,
+              TaskQueuePriorityToThreadPriority(priority)) {
   RTC_DCHECK(queue_name);
   int fds[2];
   RTC_CHECK(pipe(fds) == 0);

webrtc/base/task_queue_win.cc

@@ -24,6 +24,8 @@ namespace {
 #define WM_RUN_TASK WM_USER + 1
 #define WM_QUEUE_DELAYED_TASK WM_USER + 2
 
+using Priority = TaskQueue::Priority;
+
 DWORD g_queue_ptr_tls = 0;
 
 BOOL CALLBACK InitializeTls(PINIT_ONCE init_once, void* param, void** context) {
@@ -49,6 +51,21 @@ void CALLBACK InitializeQueueThread(ULONG_PTR param) {
   ::TlsSetValue(GetQueuePtrTls(), data->thread_context);
   data->started->Set();
 }
+
+ThreadPriority TaskQueuePriorityToThreadPriority(Priority priority) {
+  switch (priority) {
+    case Priority::HIGH:
+      return kRealtimePriority;
+    case Priority::LOW:
+      return kLowPriority;
+    case Priority::NORMAL:
+      return kNormalPriority;
+    default:
+      RTC_NOTREACHED();
+      break;
+  }
+  return kNormalPriority;
+}
 }  // namespace
 
 class TaskQueue::MultimediaTimer {
@@ -145,8 +162,11 @@ class TaskQueue::MultimediaTimer {
   RTC_DISALLOW_COPY_AND_ASSIGN(MultimediaTimer);
 };
 
-TaskQueue::TaskQueue(const char* queue_name)
-    : thread_(&TaskQueue::ThreadMain, this, queue_name) {
+TaskQueue::TaskQueue(const char* queue_name, Priority priority /*= NORMAL*/)
+    : thread_(&TaskQueue::ThreadMain,
+              this,
+              queue_name,
+              TaskQueuePriorityToThreadPriority(priority)) {
   RTC_DCHECK(queue_name);
   thread_.Start();
   Event event(false, false);