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

device/usb/usb_context.cc

Index: device/usb/usb_context.cc
diff --git a/device/usb/usb_context.cc b/device/usb/usb_context.cc
index cb8214fedb135876bffe2e1ec4aec03e4130e736..7661469d294911994d8fb4b83332fb6fe267b2af 100644
--- a/device/usb/usb_context.cc
+++ b/device/usb/usb_context.cc
@@ -6,7 +6,6 @@
 
 #include "base/atomicops.h"
 #include "base/logging.h"
-#include "base/synchronization/waitable_event.h"
 #include "base/threading/platform_thread.h"
 #include "device/usb/usb_error.h"
 #include "third_party/libusb/src/libusb/interrupt.h"
@@ -25,35 +24,30 @@ class UsbContext::UsbEventHandler : public base::PlatformThread::Delegate {
   // base::PlatformThread::Delegate
   void ThreadMain() override;
 
+  void Stop();
+
  private:
   base::subtle::Atomic32 running_;
   libusb_context* context_;
   base::PlatformThreadHandle thread_handle_;
-  base::WaitableEvent start_polling_;
   DISALLOW_COPY_AND_ASSIGN(UsbEventHandler);
 };
 
 UsbContext::UsbEventHandler::UsbEventHandler(libusb_context* context)
-    : context_(context), thread_handle_(0), start_polling_(false, false) {
+    : context_(context), thread_handle_(0) {
   base::subtle::Release_Store(&running_, 1);
   bool success = base::PlatformThread::Create(0, this, &thread_handle_);
   DCHECK(success) << "Failed to create USB IO handling thread.";
-  start_polling_.Wait();
 }
 
 UsbContext::UsbEventHandler::~UsbEventHandler() {
-  base::subtle::Release_Store(&running_, 0);
-  libusb_interrupt_handle_event(context_);
-  base::PlatformThread::Join(thread_handle_);
+  libusb_exit(context_);
 }
 
 void UsbContext::UsbEventHandler::ThreadMain() {
   base::PlatformThread::SetName("UsbEventHandler");
   VLOG(1) << "UsbEventHandler started.";
 
-  if (base::subtle::Acquire_Load(&running_)) {
-    start_polling_.Signal();
-  }
   while (base::subtle::Acquire_Load(&running_)) {
     const int rv = libusb_handle_events(context_);
     if (rv != LIBUSB_SUCCESS) {
@@ -61,20 +55,24 @@ void UsbContext::UsbEventHandler::ThreadMain() {
               << ConvertPlatformUsbErrorToString(rv);
     }
   }
+
   VLOG(1) << "UsbEventHandler shutting down.";
+  delete this;
+}
+
+void UsbContext::UsbEventHandler::Stop() {
+  base::subtle::Release_Store(&running_, 0);
+  libusb_interrupt_handle_event(context_);
 }
 
 UsbContext::UsbContext(PlatformUsbContext context) : context_(context) {
-  DCHECK(thread_checker_.CalledOnValidThread());
+  // Ownership of the PlatformUsbContext is passed to the event handler thread.
   event_handler_ = new UsbEventHandler(context_);
 }
 
 UsbContext::~UsbContext() {
-  // destruction of UsbEventHandler is a blocking operation.
   DCHECK(thread_checker_.CalledOnValidThread());
-  delete event_handler_;
-  event_handler_ = NULL;
-  libusb_exit(context_);
+  event_handler_->Stop();
 }
 
 }  // namespace device