Move device/usb classes from the FILE thread to UI thread.

device/usb/usb_service_impl.cc

Index: device/usb/usb_service_impl.cc
diff --git a/device/usb/usb_service_impl.cc b/device/usb/usb_service_impl.cc
index 6bdac2bfdc9744212da07fd7aa5f58871223b60a..ce5b51a4f8be15011f4d937a71120646fa118b9e 100644
--- a/device/usb/usb_service_impl.cc
+++ b/device/usb/usb_service_impl.cc
@@ -4,6 +4,7 @@
 
 #include "device/usb/usb_service_impl.h"
 
+#include <algorithm>
 #include <set>
 
 #include "base/bind.h"
@@ -11,25 +12,160 @@
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/strings/utf_string_conversions.h"
 #include "base/thread_task_runner_handle.h"
 #include "components/device_event_log/device_event_log.h"
 #include "device/usb/usb_error.h"
+#include "third_party/libusb/src/libusb/libusb.h"
 
 #if defined(OS_WIN)
 #include <setupapi.h>
 #include <usbiodef.h>
 
-#include "base/scoped_observer.h"
 #include "base/strings/string_util.h"
-#include "device/core/device_monitor_win.h"
 #endif  // OS_WIN
 
-namespace device {
+#if defined(USE_UDEV)
+#include "device/udev_linux/scoped_udev.h"
+#endif  // USE_UDEV
 
-#if defined(OS_WIN)
+namespace device {
 
 namespace {
 
+#if defined(USE_UDEV)
+
+void ReadDeviceStrings(PlatformUsbDevice platform_device,
+                       libusb_device_descriptor* descriptor,
+                       base::string16* manufacturer_string,
+                       base::string16* product_string,
+                       base::string16* serial_number,
+                       std::string* device_node) {
+  ScopedUdevPtr udev(udev_new());
+  ScopedUdevEnumeratePtr enumerate(udev_enumerate_new(udev.get()));
+
+  udev_enumerate_add_match_subsystem(enumerate.get(), "usb");
+  if (udev_enumerate_scan_devices(enumerate.get()) != 0) {
+    return;
+  }
+  std::string bus_number =
+      base::IntToString(libusb_get_bus_number(platform_device));
+  std::string device_address =
+      base::IntToString(libusb_get_device_address(platform_device));
+  udev_list_entry* devices = udev_enumerate_get_list_entry(enumerate.get());
+  for (udev_list_entry* i = devices; i != NULL;
+       i = udev_list_entry_get_next(i)) {
+    ScopedUdevDevicePtr device(
+        udev_device_new_from_syspath(udev.get(), udev_list_entry_get_name(i)));
+    if (device) {
+      const char* value = udev_device_get_sysattr_value(device.get(), "busnum");
+      if (!value || bus_number != value) {
+        continue;
+      }
+      value = udev_device_get_sysattr_value(device.get(), "devnum");
+      if (!value || device_address != value) {
+        continue;
+      }
+
+      value = udev_device_get_devnode(device.get());
+      if (value) {
+        *device_node = value;
+      }
+      value = udev_device_get_sysattr_value(device.get(), "manufacturer");
+      if (value) {
+        *manufacturer_string = base::UTF8ToUTF16(value);
+      }
+      value = udev_device_get_sysattr_value(device.get(), "product");
+      if (value) {
+        *product_string = base::UTF8ToUTF16(value);
+      }
+      value = udev_device_get_sysattr_value(device.get(), "serial");
+      if (value) {
+        *serial_number = base::UTF8ToUTF16(value);
+      }
+      break;
+    }
+  }
+}
+
+#else
+
+uint16 ReadDeviceLanguage(PlatformUsbDeviceHandle handle) {
+  uint16 language_id = 0x0409;
+  uint8 buffer[256];
+  int size =
+      libusb_get_string_descriptor(handle, 0, 0, &buffer[0], sizeof(buffer));
+  if (size < 0) {
+    USB_LOG(EVENT) << "Failed to get supported string languages: "
+                   << ConvertPlatformUsbErrorToString(size);
+  } else if (size >= 4) {
+    // Just pick the first supported language.
+    language_id = buffer[2] | (buffer[3] << 8);
+  } else {
+    USB_LOG(EVENT) << "List of available string languages invalid.";
+  }
+
+  return language_id;
+}
+
+void ReadDeviceString(PlatformUsbDeviceHandle handle,
+                      uint8 string_id,
+                      uint16 language_id,
+                      base::string16* string) {
+  if (string_id == 0) {
+    return;
+  }
+
+  uint8 buffer[256];
+  int size = libusb_get_string_descriptor(handle, string_id, language_id,
+                                          &buffer[0], sizeof(buffer));
+  if (size < 0) {
+    USB_LOG(EVENT) << "Failed to read string " << (int)string_id
+                   << " from the device: "
+                   << ConvertPlatformUsbErrorToString(size);
+  } else if (size > 2) {
+    *string = base::string16(reinterpret_cast<base::char16*>(&buffer[2]),
+                             size / 2 - 1);
+  } else {
+    USB_LOG(EVENT) << "String descriptor " << string_id << " is invalid.";
+  }
+}
+
+void ReadDeviceStrings(PlatformUsbDevice platform_device,
+                       libusb_device_descriptor* descriptor,
+                       base::string16* manufacturer_string,
+                       base::string16* product_string,
+                       base::string16* serial_number,
+                       std::string* device_node) {
+  if (descriptor->iManufacturer == 0 && descriptor->iProduct == 0 &&
+      descriptor->iSerialNumber == 0) {
+    // Don't bother distrubing the device if it doesn't have any string
+    // descriptors we care about.
+    return;
+  }
+
+  PlatformUsbDeviceHandle handle;
+  int rv = libusb_open(platform_device, &handle);
+  if (rv != LIBUSB_SUCCESS) {
+    USB_LOG(EVENT) << "Failed to open device to read string descriptors: "
+                   << ConvertPlatformUsbErrorToString(rv);
+    return;
+  }
+
+  uint16 language_id = ReadDeviceLanguage(handle);
+  ReadDeviceString(handle, descriptor->iManufacturer, language_id,
+                   manufacturer_string);
+  ReadDeviceString(handle, descriptor->iProduct, language_id, product_string);
+  ReadDeviceString(handle, descriptor->iSerialNumber, language_id,
+                   serial_number);
+  libusb_close(handle);
+}
+
+#endif  // USE_UDEV
+
+#if defined(OS_WIN)
+
 // Wrapper around a HDEVINFO that automatically destroys it.
 class ScopedDeviceInfoList {
  public:
@@ -81,62 +217,57 @@ class ScopedDeviceInfo {
   SP_DEVINFO_DATA dev_info_data_;
 };
 
-}  // namespace
-
-// This class lives on the application main thread so that it can listen for
-// device change notification window messages. It registers for notifications
-// that may indicate new devices that the UsbService will enumerate.
-class UsbServiceImpl::UIThreadHelper final
-    : private DeviceMonitorWin::Observer {
- public:
-  UIThreadHelper(base::WeakPtr<UsbServiceImpl> usb_service)
-      : task_runner_(base::ThreadTaskRunnerHandle::Get()),
-        usb_service_(usb_service),
-        device_observer_(this) {}
-
-  ~UIThreadHelper() {}
+bool IsWinUsbInterface(const std::string& device_path) {
+  ScopedDeviceInfoList dev_info_list(SetupDiCreateDeviceInfoList(NULL, NULL));
+  if (!dev_info_list.valid()) {
+    USB_PLOG(ERROR) << "Failed to create a device information set";
+    return false;
+  }
 
-  void Start() {
-    DeviceMonitorWin* device_monitor = DeviceMonitorWin::GetForAllInterfaces();
-    if (device_monitor) {
-      device_observer_.Add(device_monitor);
-    }
+  // This will add the device to |dev_info_list| so we can query driver info.
+  if (!SetupDiOpenDeviceInterfaceA(dev_info_list.get(), device_path.c_str(), 0,
+                                   NULL)) {
+    USB_PLOG(ERROR) << "Failed to get device interface data for "
+                    << device_path;
+    return false;
   }
 
- private:
-  void OnDeviceAdded(const GUID& class_guid,
-                     const std::string& device_path) override {
-    // Only the root node of a composite USB device has the class GUID
-    // GUID_DEVINTERFACE_USB_DEVICE but we want to wait until WinUSB is loaded.
-    // This first pass filter will catch anything that's sitting on the USB bus
-    // (including devices on 3rd party USB controllers) to avoid the more
-    // expensive driver check that needs to be done on the FILE thread.
-    if (device_path.find("usb") != std::string::npos) {
-      task_runner_->PostTask(
-          FROM_HERE, base::Bind(&UsbServiceImpl::RefreshDevicesIfWinUsbDevice,
-                                usb_service_, device_path));
-    }
+  ScopedDeviceInfo dev_info;
+  if (!SetupDiEnumDeviceInfo(dev_info_list.get(), 0, dev_info.get())) {
+    USB_PLOG(ERROR) << "Failed to get device info for " << device_path;
+    return false;
   }
+  dev_info.set_valid(dev_info_list.get());
 
-  void OnDeviceRemoved(const GUID& class_guid,
-                       const std::string& device_path) override {
-    // The root USB device node is removed last
-    if (class_guid == GUID_DEVINTERFACE_USB_DEVICE) {
-      task_runner_->PostTask(
-          FROM_HERE, base::Bind(&UsbServiceImpl::RefreshDevices, usb_service_));
-    }
+  DWORD reg_data_type;
+  BYTE buffer[256];
+  if (!SetupDiGetDeviceRegistryPropertyA(dev_info_list.get(), dev_info.get(),
+                                         SPDRP_SERVICE, &reg_data_type,
+                                         &buffer[0], sizeof buffer, NULL)) {
+    USB_PLOG(ERROR) << "Failed to get device service property";
+    return false;
+  }
+  if (reg_data_type != REG_SZ) {
+    USB_LOG(ERROR) << "Unexpected data type for driver service: "
+                   << reg_data_type;
+    return false;
   }
 
-  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
-  base::WeakPtr<UsbServiceImpl> usb_service_;
-  ScopedObserver<DeviceMonitorWin, DeviceMonitorWin::Observer> device_observer_;
-};
+  USB_LOG(DEBUG) << "Driver for " << device_path << " is " << buffer << ".";
+  if (base::strncasecmp("WinUSB", (const char*)&buffer[0], sizeof "WinUSB") ==
+      0) {
+    return true;
+  }
+  return false;
+}
 
 #endif  // OS_WIN
 
+}  // namespace
+
 // static
 UsbService* UsbServiceImpl::Create(
-    scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner) {
+    scoped_refptr<base::SequencedTaskRunner> blocking_task_runner) {
   PlatformUsbContext context = NULL;
   const int rv = libusb_init(&context);
   if (rv != LIBUSB_SUCCESS) {
@@ -148,42 +279,21 @@ UsbService* UsbServiceImpl::Create(
     return nullptr;
   }
 
-  return new UsbServiceImpl(context, ui_task_runner);
-}
-
-scoped_refptr<UsbDevice> UsbServiceImpl::GetDeviceById(uint32 unique_id) {
-  DCHECK(CalledOnValidThread());
-  RefreshDevices();
-  DeviceMap::iterator it = devices_.find(unique_id);
-  if (it != devices_.end()) {
-    return it->second;
-  }
-  return NULL;
-}
-
-void UsbServiceImpl::GetDevices(
-    std::vector<scoped_refptr<UsbDevice> >* devices) {
-  DCHECK(CalledOnValidThread());
-  STLClearObject(devices);
-
-  if (!hotplug_enabled_) {
-    RefreshDevices();
-  }
-
-  for (const auto& map_entry : devices_) {
-    devices->push_back(map_entry.second);
-  }
+  return new UsbServiceImpl(context, blocking_task_runner);
 }
 
 UsbServiceImpl::UsbServiceImpl(
     PlatformUsbContext context,
-    scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner)
+    scoped_refptr<base::SequencedTaskRunner> blocking_task_runner)
     : context_(new UsbContext(context)),
-      ui_task_runner_(ui_task_runner),
-      next_unique_id_(0),
-      hotplug_enabled_(false),
+      task_runner_(base::ThreadTaskRunnerHandle::Get()),
+      blocking_task_runner_(blocking_task_runner),
+#if defined(OS_WIN)
+      device_observer_(this),
+#endif
       weak_factory_(this) {
-  task_runner_ = base::ThreadTaskRunnerHandle::Get();
+  base::MessageLoop::current()->AddDestructionObserver(this);
+
   int rv = libusb_hotplug_register_callback(
       context_->context(),
       static_cast<libusb_hotplug_event>(LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED |
@@ -193,149 +303,269 @@ UsbServiceImpl::UsbServiceImpl(
       &UsbServiceImpl::HotplugCallback, this, &hotplug_handle_);
   if (rv == LIBUSB_SUCCESS) {
     hotplug_enabled_ = true;
+
+    // libusb will call the hotplug callback for each device currently
+    // enumerated. Once this is complete enumeration_ready_ can be set to true
+    // but we must first wait for any tasks posted to blocking_task_runner_ to
+    // complete.
+    blocking_task_runner_->PostTaskAndReply(
+        FROM_HERE, base::Bind(&base::DoNothing),
+        base::Bind(&UsbServiceImpl::RefreshDevicesComplete,
+                   weak_factory_.GetWeakPtr(), nullptr, 0));
   } else {
+    RefreshDevices("");
 #if defined(OS_WIN)
-    ui_thread_helper_ = new UIThreadHelper(weak_factory_.GetWeakPtr());
-    ui_task_runner_->PostTask(FROM_HERE,
-                              base::Bind(&UIThreadHelper::Start,
-                                         base::Unretained(ui_thread_helper_)));
+    DeviceMonitorWin* device_monitor = DeviceMonitorWin::GetForAllInterfaces();
+    if (device_monitor) {
+      device_observer_.Add(device_monitor);
+    }
 #endif  // OS_WIN
   }
 }
 
 UsbServiceImpl::~UsbServiceImpl() {
+  base::MessageLoop::current()->RemoveDestructionObserver(this);
+
   if (hotplug_enabled_) {
     libusb_hotplug_deregister_callback(context_->context(), hotplug_handle_);
   }
-#if defined(OS_WIN)
-  if (ui_thread_helper_) {
-    ui_task_runner_->DeleteSoon(FROM_HERE, ui_thread_helper_);
-  }
-#endif  // OS_WIN
   for (const auto& map_entry : devices_) {
     map_entry.second->OnDisconnect();
   }
 }
 
-void UsbServiceImpl::RefreshDevices() {
+scoped_refptr<UsbDevice> UsbServiceImpl::GetDeviceById(uint32 unique_id) {
   DCHECK(CalledOnValidThread());
-
-  libusb_device** platform_devices = NULL;
-  const ssize_t device_count =
-      libusb_get_device_list(context_->context(), &platform_devices);
-  if (device_count < 0) {
-    USB_LOG(ERROR) << "Failed to get device list: "
-                   << ConvertPlatformUsbErrorToString(device_count);
+  DeviceMap::iterator it = devices_.find(unique_id);
+  if (it != devices_.end()) {
+    return it->second;
   }
+  return NULL;
+}
 
-  std::set<UsbDevice*> connected_devices;
-  std::vector<PlatformUsbDevice> disconnected_devices;
+void UsbServiceImpl::GetDevices(const GetDevicesCallback& callback) {
+  DCHECK(CalledOnValidThread());
 
-  // Populates new devices.
-  for (ssize_t i = 0; i < device_count; ++i) {
-    if (!ContainsKey(platform_devices_, platform_devices[i])) {
-      scoped_refptr<UsbDeviceImpl> new_device = AddDevice(platform_devices[i]);
-      if (new_device) {
-        connected_devices.insert(new_device.get());
-      }
-    } else {
-      connected_devices.insert(platform_devices_[platform_devices[i]].get());
+  if (!enumeration_ready_) {
+    // On startup wait for the first enumeration,
+    pending_enumerations_.push_back(callback);
+  } else if (hotplug_enabled_) {
+    // The device list is updated live when hotplug events are supported.
+    std::vector<scoped_refptr<UsbDevice>> devices;
+    for (const auto& map_entry : devices_) {
+      devices.push_back(map_entry.second);
+    }
+    callback.Run(devices);
+  } else {
+    // Only post one re-enumeration task at a time.
+    if (pending_enumerations_.empty()) {
+      RefreshDevices("");
     }
+    pending_enumerations_.push_back(callback);
   }
+}
 
-  // Find disconnected devices.
-  for (const auto& map_entry : platform_devices_) {
-    PlatformUsbDevice platform_device = map_entry.first;
-    scoped_refptr<UsbDeviceImpl> device = map_entry.second;
-    if (!ContainsKey(connected_devices, device.get())) {
-      disconnected_devices.push_back(platform_device);
-      devices_.erase(device->unique_id());
-
-      NotifyDeviceRemoved(device);
-      device->OnDisconnect();
-    }
+#if defined(OS_WIN)
+
+void UsbServiceImpl::OnDeviceAdded(const GUID& class_guid,
+                                   const std::string& device_path) {
+  // Only the root node of a composite USB device has the class GUID
+  // GUID_DEVINTERFACE_USB_DEVICE but we want to wait until WinUSB is loaded.
+  // This first pass filter will catch anything that's sitting on the USB bus
+  // (including devices on 3rd party USB controllers) to avoid the more
+  // expensive driver check that needs to be done on the FILE thread.
+  if (device_path.find("usb") != std::string::npos) {
+    RefreshDevices(device_path);
   }
+}
 
-  // Remove disconnected devices from platform_devices_.
-  for (const PlatformUsbDevice& platform_device : disconnected_devices) {
-    // UsbDevice will be destroyed after this. The corresponding
-    // PlatformUsbDevice will be unref'ed during this process.
-    platform_devices_.erase(platform_device);
+void UsbServiceImpl::OnDeviceRemoved(const GUID& class_guid,
+                                     const std::string& device_path) {
+  // The root USB device node is removed last
+  if (class_guid == GUID_DEVINTERFACE_USB_DEVICE) {
+    RefreshDevices("");
   }
+}
 
-  libusb_free_device_list(platform_devices, true);
+#endif  // OS_WIN
+
+void UsbServiceImpl::WillDestroyCurrentMessageLoop() {
+  DCHECK(CalledOnValidThread());
+  delete this;
 }
 
-#if defined(OS_WIN)
-void UsbServiceImpl::RefreshDevicesIfWinUsbDevice(
-    const std::string& device_path) {
-  ScopedDeviceInfoList dev_info_list(SetupDiCreateDeviceInfoList(NULL, NULL));
-  if (!dev_info_list.valid()) {
-    USB_PLOG(ERROR) << "Failed to create a device information set";
-    return;
-  }
+void UsbServiceImpl::RefreshDevices(const std::string& new_device_path) {
+  DCHECK(CalledOnValidThread());
 
-  // This will add the device to |dev_info_list| so we can query driver info.
-  if (!SetupDiOpenDeviceInterfaceA(dev_info_list.get(), device_path.c_str(), 0,
-                                   NULL)) {
-    USB_PLOG(ERROR) << "Failed to get device interface data for "
-                    << device_path;
-    return;
+  std::set<PlatformUsbDevice> current_devices;
+  for (const auto& map_entry : platform_devices_) {
+    current_devices.insert(map_entry.first);
   }
+  blocking_task_runner_->PostTask(
+      FROM_HERE, base::Bind(&UsbServiceImpl::RefreshDevicesOnBlockingThread,
+                            weak_factory_.GetWeakPtr(), new_device_path,
+                            task_runner_, context_, current_devices));
+}
 
-  ScopedDeviceInfo dev_info;
-  if (!SetupDiEnumDeviceInfo(dev_info_list.get(), 0, dev_info.get())) {
-    USB_PLOG(ERROR) << "Failed to get device info for " << device_path;
-    return;
+// static
+void UsbServiceImpl::RefreshDevicesOnBlockingThread(
+    base::WeakPtr<UsbServiceImpl> usb_service,
+    const std::string& new_device_path,
+    scoped_refptr<base::SequencedTaskRunner> task_runner,
+    scoped_refptr<UsbContext> usb_context,
+    const std::set<PlatformUsbDevice>& previous_devices) {
+  if (!new_device_path.empty()) {
+#if defined(OS_WIN)
+    if (!IsWinUsbInterface(new_device_path)) {
+      // Wait to call libusb_get_device_list until libusb will be able to find
+      // a WinUSB interface for the device.
+      return;
+    }
+#endif  // defined(OS_WIN)
   }
-  dev_info.set_valid(dev_info_list.get());
 
-  DWORD reg_data_type;
-  BYTE buffer[256];
-  if (!SetupDiGetDeviceRegistryPropertyA(dev_info_list.get(), dev_info.get(),
-                                         SPDRP_SERVICE, &reg_data_type,
-                                         &buffer[0], sizeof buffer, NULL)) {
-    USB_PLOG(ERROR) << "Failed to get device service property";
-    return;
-  }
-  if (reg_data_type != REG_SZ) {
-    USB_LOG(ERROR) << "Unexpected data type for driver service: "
-                   << reg_data_type;
+  libusb_device** platform_devices = NULL;
+  const ssize_t device_count =
+      libusb_get_device_list(usb_context->context(), &platform_devices);
+  if (device_count < 0) {
+    USB_LOG(ERROR) << "Failed to get device list: "
+                   << ConvertPlatformUsbErrorToString(device_count);
+    task_runner->PostTask(FROM_HERE,
+                          base::Bind(&UsbServiceImpl::RefreshDevicesComplete,
+                                     usb_service, nullptr, 0));
     return;
   }
 
-  USB_LOG(DEBUG) << "Driver for " << device_path << " is " << buffer << ".";
-  if (base::strncasecmp("WinUSB", (const char*)&buffer[0], sizeof "WinUSB") ==
-      0) {
-    RefreshDevices();
+  // Find new devices.
+  for (ssize_t i = 0; i < device_count; ++i) {
+    PlatformUsbDevice platform_device = platform_devices[i];
+    if (previous_devices.find(platform_device) == previous_devices.end()) {
+      libusb_ref_device(platform_device);
+      AddDeviceOnBlockingThread(usb_service, task_runner, platform_device);
+    }
   }
+
+  // |platform_devices| will be freed in this callback.
+  task_runner->PostTask(
+      FROM_HERE, base::Bind(&UsbServiceImpl::RefreshDevicesComplete,
+                            usb_service, platform_devices, device_count));
 }
-#endif  // OS_WIN
 
-scoped_refptr<UsbDeviceImpl> UsbServiceImpl::AddDevice(
+// static
+void UsbServiceImpl::AddDeviceOnBlockingThread(
+    base::WeakPtr<UsbServiceImpl> usb_service,
+    scoped_refptr<base::SequencedTaskRunner> task_runner,
     PlatformUsbDevice platform_device) {
   libusb_device_descriptor descriptor;
   int rv = libusb_get_device_descriptor(platform_device, &descriptor);
   if (rv == LIBUSB_SUCCESS) {
-    uint32 unique_id;
-    do {
-      unique_id = ++next_unique_id_;
-    } while (devices_.find(unique_id) != devices_.end());
-
-    scoped_refptr<UsbDeviceImpl> new_device(new UsbDeviceImpl(
-        context_, ui_task_runner_, platform_device, descriptor.idVendor,
-        descriptor.idProduct, unique_id));
-    platform_devices_[platform_device] = new_device;
-    devices_[unique_id] = new_device;
-    NotifyDeviceAdded(new_device);
-    return new_device;
+    base::string16 manufacturer_string;
+    base::string16 product_string;
+    base::string16 serial_number;
+    std::string device_node;
+    ReadDeviceStrings(platform_device, &descriptor, &manufacturer_string,
+                      &product_string, &serial_number, &device_node);
+
+    task_runner->PostTask(
+        FROM_HERE, base::Bind(&UsbServiceImpl::AddDevice, usb_service,
+                              platform_device, descriptor.idVendor,
+                              descriptor.idProduct, manufacturer_string,
+                              product_string, serial_number, device_node));
   } else {
     USB_LOG(EVENT) << "Failed to get device descriptor: "
                    << ConvertPlatformUsbErrorToString(rv);
-    return nullptr;
+    libusb_unref_device(platform_device);
   }
 }
 
+void UsbServiceImpl::RefreshDevicesComplete(libusb_device** platform_devices,
+                                            ssize_t device_count) {
+  if (platform_devices) {
+    // Mark devices seen in this enumeration.
+    for (ssize_t i = 0; i < device_count; ++i) {
+      const PlatformDeviceMap::iterator it =
+          platform_devices_.find(platform_devices[i]);
+      if (it != platform_devices_.end()) {
+        it->second->set_visited(true);
+      }
+    }
+
+    // Remove devices not seen in this enumeration.
+    for (PlatformDeviceMap::iterator it = platform_devices_.begin();
+         it != platform_devices_.end();
+         /* incremented internally */) {
+      PlatformDeviceMap::iterator current = it++;
+      const scoped_refptr<UsbDeviceImpl>& device = current->second;
+      if (device->was_visited()) {
+        device->set_visited(false);
+      } else {
+        RemoveDevice(device);
+      }
+    }
+
+    libusb_free_device_list(platform_devices, true);
+  }
+
+  enumeration_ready_ = true;
+
+  if (!pending_enumerations_.empty()) {
+    std::vector<scoped_refptr<UsbDevice>> devices;
+    for (const auto& map_entry : devices_) {
+      devices.push_back(map_entry.second);
+    }
+
+    std::vector<GetDevicesCallback> pending_enumerations;
+    pending_enumerations.swap(pending_enumerations_);
+    for (const GetDevicesCallback& callback : pending_enumerations) {
+      callback.Run(devices);
+    }
+  }
+}
+
+void UsbServiceImpl::AddDevice(PlatformUsbDevice platform_device,
+                               uint16 vendor_id,
+                               uint16 product_id,
+                               base::string16 manufacturer_string,
+                               base::string16 product_string,
+                               base::string16 serial_number,
+                               std::string device_node) {
+  uint32 unique_id;
+  do {
+    unique_id = ++next_unique_id_;
+  } while (devices_.find(unique_id) != devices_.end());
+
+  scoped_refptr<UsbDeviceImpl> device(
+      new UsbDeviceImpl(context_, platform_device, vendor_id, product_id,
+                        unique_id, manufacturer_string, product_string,
+                        serial_number, blocking_task_runner_));
+
+  platform_devices_[platform_device] = device;
+  devices_[unique_id] = device;
+
+  USB_LOG(USER) << "USB device added: vendor=" << device->vendor_id() << " \""
+                << device->manufacturer_string()
+                << "\", product=" << device->product_id() << " \""
+                << device->product_string() << "\", serial=\""
+                << device->serial_number()
+                << "\", uniqueId=" << device->unique_id();
+
+  if (enumeration_ready_) {
+    NotifyDeviceAdded(device);
+  }
+
+  libusb_unref_device(platform_device);
+}
+
+void UsbServiceImpl::RemoveDevice(scoped_refptr<UsbDeviceImpl> device) {
+  platform_devices_.erase(device->platform_device());
+  devices_.erase(device->unique_id());
+
+  USB_LOG(USER) << "USB device removed: uniqueId=" << device->unique_id();
+
+  NotifyDeviceRemoved(device);
+  device->OnDisconnect();
+}
+
 // static
 int LIBUSB_CALL UsbServiceImpl::HotplugCallback(libusb_context* context,
                                                 PlatformUsbDevice device,
@@ -348,22 +578,22 @@ int LIBUSB_CALL UsbServiceImpl::HotplugCallback(libusb_context* context,
   UsbServiceImpl* self = reinterpret_cast<UsbServiceImpl*>(user_data);
   switch (event) {
     case LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED:
-      libusb_ref_device(device);  // Released in OnDeviceAdded.
+      libusb_ref_device(device);  // Released in OnPlatformDeviceAdded.
       if (self->task_runner_->BelongsToCurrentThread()) {
-        self->OnDeviceAdded(device);
+        self->OnPlatformDeviceAdded(device);
       } else {
         self->task_runner_->PostTask(
-            FROM_HERE, base::Bind(&UsbServiceImpl::OnDeviceAdded,
+            FROM_HERE, base::Bind(&UsbServiceImpl::OnPlatformDeviceAdded,
                                   base::Unretained(self), device));
       }
       break;
     case LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT:
-      libusb_ref_device(device);  // Released in OnDeviceRemoved.
+      libusb_ref_device(device);  // Released in OnPlatformDeviceRemoved.
       if (self->task_runner_->BelongsToCurrentThread()) {
-        self->OnDeviceRemoved(device);
+        self->OnPlatformDeviceRemoved(device);
       } else {
         self->task_runner_->PostTask(
-            FROM_HERE, base::Bind(&UsbServiceImpl::OnDeviceRemoved,
+            FROM_HERE, base::Bind(&UsbServiceImpl::OnPlatformDeviceRemoved,
                                   base::Unretained(self), device));
       }
       break;
@@ -374,30 +604,28 @@ int LIBUSB_CALL UsbServiceImpl::HotplugCallback(libusb_context* context,
   return 0;
 }
 
-void UsbServiceImpl::OnDeviceAdded(PlatformUsbDevice platform_device) {
+void UsbServiceImpl::OnPlatformDeviceAdded(PlatformUsbDevice platform_device) {
   DCHECK(CalledOnValidThread());
   DCHECK(!ContainsKey(platform_devices_, platform_device));
+  blocking_task_runner_->PostTask(
+      FROM_HERE,
+      base::Bind(&UsbServiceImpl::AddDeviceOnBlockingThread,
+                 weak_factory_.GetWeakPtr(), task_runner_, platform_device));
 
-  AddDevice(platform_device);
-  libusb_unref_device(platform_device);
+  // libusb_unref_device(platform_device) is called by the task above.
 }
 
-void UsbServiceImpl::OnDeviceRemoved(PlatformUsbDevice platform_device) {
+void UsbServiceImpl::OnPlatformDeviceRemoved(
+    PlatformUsbDevice platform_device) {
   DCHECK(CalledOnValidThread());
-
   PlatformDeviceMap::iterator it = platform_devices_.find(platform_device);
   if (it != platform_devices_.end()) {
     scoped_refptr<UsbDeviceImpl> device = it->second;
-    DeviceMap::iterator dev_it = devices_.find(device->unique_id());
-    if (dev_it != devices_.end()) {
-      devices_.erase(dev_it);
-    } else {
-      NOTREACHED();
-    }
-    platform_devices_.erase(it);
-
-    NotifyDeviceRemoved(device);
-    device->OnDisconnect();
+    // Serialize with calls to AddDeviceOnBlockingThread.
+    blocking_task_runner_->PostTaskAndReply(
+        FROM_HERE, base::Bind(&base::DoNothing),
+        base::Bind(&UsbServiceImpl::RemoveDevice, weak_factory_.GetWeakPtr(),
+                   device));
   } else {
     NOTREACHED();
   }