Fail gracefully when libusb_init fails

chrome/browser/usb/usb_service.h

 // Copyright (c) 2012 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 CHROME_BROWSER_USB_USB_SERVICE_H_
 #define CHROME_BROWSER_USB_USB_SERVICE_H_
 
 #include <map>
 #include <utility>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/singleton.h"
 
 namespace base {
 
 template <class T> class DeleteHelper;
 
 }  // namespace base
 
+struct InitUsbContextTraits;
 template <typename T> struct DefaultSingletonTraits;
 
 typedef struct libusb_device* PlatformUsbDevice;
+typedef struct libusb_context* PlatformUsbContext;
 
 class UsbContext;
 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.
 class UsbService {
  public:
   typedef scoped_ptr<std::vector<scoped_refptr<UsbDevice> > >
       ScopedDeviceVector;
 
   // Must be called on FILE thread.
+  // Returns NULL when failed to initialized.
   static UsbService* GetInstance();
 
   scoped_refptr<UsbDevice> GetDeviceById(uint32 unique_id);
 
   // Get all of the devices attached to the system, inserting them into
   // |devices|. Clears |devices| before use. The result will be sorted by id
   // in increasing order. Must be called on FILE thread.
   void GetDevices(std::vector<scoped_refptr<UsbDevice> >* devices);
 
  private:
+  friend struct InitUsbContextTraits;
   friend struct DefaultSingletonTraits<UsbService>;
   friend class base::DeleteHelper<UsbService>;
 
-  UsbService();
+  explicit UsbService(PlatformUsbContext context);
   virtual ~UsbService();
 
   // Return true if |device|'s vendor and product identifiers match |vendor_id|
   // and |product_id|.
   static bool DeviceMatches(scoped_refptr<UsbDevice> device,
                             const uint16 vendor_id,
                             const uint16 product_id);
 
   // Enumerate USB devices from OS and Update devices_ map.
   void RefreshDevices();
 
   scoped_refptr<UsbContext> context_;
 
   // TODO(ikarienator): Figure out a better solution.
   uint32 next_unique_id_;
 
   // The map from PlatformUsbDevices to UsbDevices.
   typedef std::map<PlatformUsbDevice, scoped_refptr<UsbDevice> > DeviceMap;
   DeviceMap devices_;
 
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 #endif  // CHROME_BROWSER_USB_USB_SERVICE_H_