Declare TaskTraits for blocking USB tasks in a constexpr

device/usb/usb_service.h

 // Copyright 2014 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.
 
 #ifndef DEVICE_USB_USB_SERVICE_H_
 #define DEVICE_USB_USB_SERVICE_H_
 
 #include <memory>
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
 
 #include "base/bind_helpers.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/observer_list.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
+#include "base/task_scheduler/task_traits.h"
 #include "base/threading/non_thread_safe.h"
 
 namespace device {
 
 class UsbDevice;
 
 // The USB service handles creating and managing an event handler thread that is
 // used to manage and dispatch USB events. It is also responsible for device
 // discovery on the system, which allows it to re-use device handles to prevent
 // competition for the same USB device.
@@ skipping 11 matching lines @@
     virtual void OnDeviceAdded(scoped_refptr<UsbDevice> device);
     virtual void OnDeviceRemoved(scoped_refptr<UsbDevice> device);
     // For observers that need to process device removal after others have run.
     // Should not depend on any other service's knowledge of connected devices.
     virtual void OnDeviceRemovedCleanup(scoped_refptr<UsbDevice> device);
 
     // Notifies the observer that the UsbService it depends on is shutting down.
     virtual void WillDestroyUsbService();
   };
 
+  // These task traits are to be used for posting blocking tasks to the task
+  // scheduler.
+  static constexpr base::TaskTraits kBlockingTaskTraits = {
+      base::MayBlock(), base::TaskPriority::USER_VISIBLE,
+      base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN};
+
   // Returns nullptr when initialization fails.
   static std::unique_ptr<UsbService> Create();
 
-  // Creates a SequencedTaskRunner appropriate for blocking I/O operations.
+  // Creates a SequencedTaskRunner with kBlockingTaskTraits.
   static scoped_refptr<base::SequencedTaskRunner> CreateBlockingTaskRunner();
 
   virtual ~UsbService();
 
   scoped_refptr<UsbDevice> GetDevice(const std::string& guid);
 
   // Shuts down the UsbService. Must be called before destroying the UsbService
   // when tasks can still be posted to the |blocking_task_runner| provided to
   // Create().
   virtual void Shutdown();
@@ skipping 38 matching lines @@
 #if DCHECK_IS_ON()
   bool did_shutdown_ = false;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_SERVICE_H_