Call libusb_init on the FILE thread.

device/usb/usb_service_impl.h

 // Copyright 2015 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.
 
 #include "device/usb/usb_service.h"
 
 #include <stddef.h>
 
 #include <map>
 #include <queue>
@@ skipping 38 matching lines @@
   void GetDevices(const GetDevicesCallback& callback) override;
 
 #if defined(OS_WIN)
   // device::DeviceMonitorWin::Observer implementation
   void OnDeviceAdded(const GUID& class_guid,
                      const std::string& device_path) override;
   void OnDeviceRemoved(const GUID& class_guid,
                        const std::string& device_path) override;
 #endif  // OS_WIN
 
+  void OnUsbContext(scoped_refptr<UsbContext> context);
+
   // Enumerate USB devices from OS and update devices_ map.
   void RefreshDevices();
   void OnDeviceList(libusb_device** platform_devices, size_t device_count);
   void RefreshDevicesComplete();
 
   // Creates a new UsbDevice based on the given libusb device.
   void EnumerateDevice(PlatformUsbDevice platform_device,
                        const base::Closure& refresh_complete);
 
   void AddDevice(const base::Closure& refresh_complete,
                  scoped_refptr<UsbDeviceImpl> device);
   void RemoveDevice(scoped_refptr<UsbDeviceImpl> device);
 
   // Handle hotplug events from libusb.
   static int LIBUSB_CALL HotplugCallback(libusb_context* context,
                                          PlatformUsbDevice device,
                                          libusb_hotplug_event event,
                                          void* user_data);
   // These functions release a reference to the provided platform device.
   void OnPlatformDeviceAdded(PlatformUsbDevice platform_device);
   void OnPlatformDeviceRemoved(PlatformUsbDevice platform_device);
 
   // Add |platform_device| to the |ignored_devices_| and
   // run |refresh_complete|.
   void EnumerationFailed(PlatformUsbDevice platform_device,
                          const base::Closure& refresh_complete);
 
   scoped_refptr<UsbContext> context_;
+  bool usb_unavailable_ = false;

[juncai, 2016/12/08 20:00:39]

nit: maybe change the variable name to be:
bool usb_available_ = true;

[Reilly Grant (use Gerrit), 2016/12/08 22:44:39]

The reason I named this usb_unavailable_ is because until we see if we can
initialize context_ we don't know if USB will be available or not, so it seemed
less surprising to have a variable that becomes true only when we've given up on
initializing USB than a variable that it true before we've even tried.

[juncai, 2016/12/08 23:02:10]

I see. Thanks for the explanation.

 
   // When available the device list will be updated when new devices are
   // connected instead of only when a full enumeration is requested.
   // TODO(reillyg): Support this on all platforms. crbug.com/411715
   bool hotplug_enabled_ = false;
   libusb_hotplug_callback_handle hotplug_handle_;
 
   // Enumeration callbacks are queued until an enumeration completes.
   bool enumeration_ready_ = false;
   bool enumeration_in_progress_ = false;
@@ skipping 16 matching lines @@
 #if defined(OS_WIN)
   ScopedObserver<DeviceMonitorWin, DeviceMonitorWin::Observer> device_observer_;
 #endif  // OS_WIN
 
   base::WeakPtrFactory<UsbServiceImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UsbServiceImpl);
 };
 
 }  // namespace device