Use the task scheduler for blocking tasks in the USB service on Linux

device/udev_linux/udev_watcher.cc

Index: device/udev_linux/udev_watcher.cc
diff --git a/device/udev_linux/udev_watcher.cc b/device/udev_linux/udev_watcher.cc
new file mode 100644
index 0000000000000000000000000000000000000000..d9e490de63a0f6bd2f4d501726bd549b615bda6c
--- /dev/null
+++ b/device/udev_linux/udev_watcher.cc
@@ -0,0 +1,98 @@
+// Copyright 2017 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/udev_linux/udev_watcher.h"
+
+#include "base/bind.h"
+#include "base/memory/ptr_util.h"
+
+namespace device {
+
+UdevWatcher::Observer::~Observer() = default;
+
+void UdevWatcher::Observer::OnDeviceAdded(ScopedUdevDevicePtr device) {}
+
+void UdevWatcher::Observer::OnDeviceRemoved(ScopedUdevDevicePtr device) {}
+
+std::unique_ptr<UdevWatcher> UdevWatcher::StartWatching(Observer* observer) {
+  ScopedUdevPtr udev(udev_new());
+  if (!udev) {
+    LOG(ERROR) << "Failed to initialize udev.";
+    return nullptr;
+  }
+
+  ScopedUdevMonitorPtr udev_monitor(
+      udev_monitor_new_from_netlink(udev.get(), "udev"));
+  if (!udev_monitor) {
+    LOG(ERROR) << "Failed to initialize a udev monitor.";
+    return nullptr;
+  }
+
+  if (udev_monitor_enable_receiving(udev_monitor.get()) != 0) {
+    LOG(ERROR) << "Failed to enable receiving udev events.";
+    return nullptr;
+  }
+
+  int monitor_fd = udev_monitor_get_fd(udev_monitor.get());
+  if (monitor_fd < 0) {
+    LOG(ERROR) << "Udev monitor file descriptor unavailable.";
+    return nullptr;
+  }
+
+  return base::WrapUnique(new UdevWatcher(
+      std::move(udev), std::move(udev_monitor), monitor_fd, observer));
+}
+
+UdevWatcher::~UdevWatcher() {
+  DCHECK(sequence_checker_.CalledOnValidSequence());
+};
+
+void UdevWatcher::EnumerateExistingDevices() {
+  DCHECK(sequence_checker_.CalledOnValidSequence());
+  ScopedUdevEnumeratePtr enumerate(udev_enumerate_new(udev_.get()));
+  if (!enumerate) {
+    LOG(ERROR) << "Failed to initialize a udev enumerator.";
+    return;
+  }
+
+  if (udev_enumerate_scan_devices(enumerate.get()) != 0) {
+    LOG(ERROR) << "Failed to begin udev enumeration.";
+    return;
+  }
+
+  udev_list_entry* devices = udev_enumerate_get_list_entry(enumerate.get());
+  for (udev_list_entry* i = devices; i != nullptr;
+       i = udev_list_entry_get_next(i)) {
+    ScopedUdevDevicePtr device(
+        udev_device_new_from_syspath(udev_.get(), udev_list_entry_get_name(i)));
+    if (device)
+      observer_->OnDeviceAdded(std::move(device));
+  }
+}
+
+UdevWatcher::UdevWatcher(ScopedUdevPtr udev,
+                         ScopedUdevMonitorPtr udev_monitor,
+                         int monitor_fd,
+                         Observer* observer)
+    : udev_(std::move(udev)),
+      udev_monitor_(std::move(udev_monitor)),
+      observer_(observer) {
+  file_watcher_ = base::FileDescriptorWatcher::WatchReadable(
+      monitor_fd,
+      base::Bind(&UdevWatcher::OnMonitorReadable, base::Unretained(this)));
+}
+
+void UdevWatcher::OnMonitorReadable() {
+  ScopedUdevDevicePtr device(udev_monitor_receive_device(udev_monitor_.get()));
+  if (!device)
+    return;
+
+  std::string action(udev_device_get_action(device.get()));
+  if (action == "add")
+    observer_->OnDeviceAdded(std::move(device));
+  else if (action == "remove")
+    observer_->OnDeviceRemoved(std::move(device));
+}
+
+}  // namespace device