Use separate libusb contexts to avoid thread safety issues

Unfortunately, it appears that using libusb's synchronous transfer API
from several threads causes nasty race conditions in event handling and
can lead to deadlocks, despite the fact that libusb's synchronous API
is documented to be perfectly fine to use from several threads (only
the manual polling functionality is supposed to require special
precautions).

Since usbdk was the only real reason for using a single libusb context
and since usbdk (currently) has so many issues with Dolphin, I think
dropping support for it in order to fix other backends is acceptable.

Source/Core/Core/IOS/USB/Bluetooth/BTReal.cpp

@@ -28,7 +28,6 @@
 #include "Core/Core.h"
 #include "Core/HW/Memmap.h"
 #include "Core/IOS/Device.h"
-#include "Core/LibusbUtils.h"
 #include "VideoCommon/OnScreenDisplay.h"
 
 namespace IOS::HLE::Device
@@ -75,11 +74,10 @@ BluetoothReal::~BluetoothReal()
 
 IPCCommandResult BluetoothReal::Open(const OpenRequest& request)
 {
-  auto& context = LibusbUtils::GetContext();
-  if (!context.IsValid())
+  if (!m_context.IsValid())
     return GetDefaultReply(IPC_EACCES);
 
-  context.GetDeviceList([this](libusb_device* device) {
+  m_context.GetDeviceList([this](libusb_device* device) {
     libusb_device_descriptor device_descriptor;
     libusb_get_device_descriptor(device, &device_descriptor);
     auto config_descriptor = LibusbUtils::MakeConfigDescriptor(device);

Source/Core/Core/IOS/USB/Bluetooth/BTReal.h

@@ -19,6 +19,7 @@
 #include "Core/IOS/USB/Bluetooth/BTBase.h"
 #include "Core/IOS/USB/Bluetooth/hci.h"
 #include "Core/IOS/USB/USBV0.h"
+#include "Core/LibusbUtils.h"
 
 class PointerWrap;
 struct libusb_device;
@@ -70,6 +71,7 @@ private:
   std::atomic<SyncButtonState> m_sync_button_state{SyncButtonState::Unpressed};
   Common::Timer m_sync_button_held_timer;
 
+  LibusbUtils::Context m_context;
   libusb_device* m_device = nullptr;
   libusb_device_handle* m_handle = nullptr;
 

Source/Core/Core/IOS/USB/Host.cpp

@@ -24,7 +24,6 @@
 #include "Core/Core.h"
 #include "Core/IOS/USB/Common.h"
 #include "Core/IOS/USB/LibusbDevice.h"
-#include "Core/LibusbUtils.h"
 
 namespace IOS::HLE::Device
 {
@@ -121,10 +120,9 @@ bool USBHost::AddNewDevices(std::set<u64>& new_devices, DeviceChangeHooks& hooks
   if (SConfig::GetInstance().m_usb_passthrough_devices.empty())
     return true;
 
-  auto& context = LibusbUtils::GetContext();
-  if (context.IsValid())
+  if (m_context.IsValid())
   {
-    context.GetDeviceList([&](libusb_device* device) {
+    m_context.GetDeviceList([&](libusb_device* device) {
       libusb_device_descriptor descriptor;
       libusb_get_device_descriptor(device, &descriptor);
       const std::pair<u16, u16> vid_pid = {descriptor.idVendor, descriptor.idProduct};

Source/Core/Core/IOS/USB/Host.h

@@ -20,6 +20,7 @@
 #include "Core/IOS/Device.h"
 #include "Core/IOS/IOS.h"
 #include "Core/IOS/USB/Common.h"
+#include "Core/LibusbUtils.h"
 
 class PointerWrap;
 
@@ -71,5 +72,6 @@ private:
   std::thread m_scan_thread;
   Common::Event m_first_scan_complete_event;
   bool m_has_initialised = false;
+  LibusbUtils::Context m_context;
 };
 }  // namespace IOS::HLE::Device

Source/Core/Core/LibusbUtils.cpp

@@ -109,12 +109,6 @@ bool Context::GetDeviceList(GetDeviceListCallback callback)
   return m_impl->GetDeviceList(std::move(callback));
 }
 
-Context& GetContext()
-{
-  static Context s_context;
-  return s_context;
-}
-
 ConfigDescriptor MakeConfigDescriptor(libusb_device* device, u8 config_num)
 {
 #if defined(__LIBUSB__)

Source/Core/Core/LibusbUtils.h

@@ -38,11 +38,6 @@ private:
   std::unique_ptr<Impl> m_impl;
 };
 
-// Use this to get a libusb context. Do *not* use any other context
-// because some libusb backends such as UsbDk only work properly with a single context.
-// Additionally, device lists must be retrieved using GetDeviceList for thread safety reasons.
-Context& GetContext();
-
 using ConfigDescriptor = UniquePtr<libusb_config_descriptor>;
 ConfigDescriptor MakeConfigDescriptor(libusb_device* device, u8 config_num = 0);
 }  // namespace LibusbUtils

Source/Core/InputCommon/GCAdapter.cpp

@@ -72,6 +72,8 @@ static bool s_libusb_hotplug_enabled = false;
 static libusb_hotplug_callback_handle s_hotplug_handle;
 #endif
 
+static LibusbUtils::Context s_libusb_context;
+
 static u8 s_endpoint_in = 0;
 static u8 s_endpoint_out = 0;
 
@@ -147,7 +149,6 @@ static int HotplugCallback(libusb_context* ctx, libusb_device* dev, libusb_hotpl
 
 static void ScanThreadFunc()
 {
-  auto& context = LibusbUtils::GetContext();
   Common::SetCurrentThreadName("GC Adapter Scanning Thread");
   NOTICE_LOG(SERIALINTERFACE, "GC Adapter scanning thread started");
 
@@ -158,7 +159,7 @@ static void ScanThreadFunc()
   if (s_libusb_hotplug_enabled)
   {
     if (libusb_hotplug_register_callback(
-            context,
+            s_libusb_context,
             (libusb_hotplug_event)(LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED |
                                    LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT),
             LIBUSB_HOTPLUG_ENUMERATE, 0x057e, 0x0337, LIBUSB_HOTPLUG_MATCH_ANY, HotplugCallback,
@@ -212,7 +213,7 @@ void StartScanThread()
 {
   if (s_adapter_detect_thread_running.IsSet())
     return;
-  if (!LibusbUtils::GetContext().IsValid())
+  if (!s_libusb_context.IsValid())
     return;
   s_adapter_detect_thread_running.Set(true);
   s_adapter_detect_thread = std::thread(ScanThreadFunc);
@@ -240,7 +241,7 @@ static void Setup()
     s_controller_rumble[i] = 0;
   }
 
-  LibusbUtils::GetContext().GetDeviceList([](libusb_device* device) {
+  s_libusb_context.GetDeviceList([](libusb_device* device) {
     if (CheckDeviceAccess(device))
     {
       // Only connect to a single adapter in case the user has multiple connected
@@ -364,8 +365,8 @@ void Shutdown()
 {
   StopScanThread();
 #if defined(LIBUSB_API_VERSION) && LIBUSB_API_VERSION >= 0x01000102
-  if (LibusbUtils::GetContext().IsValid() && s_libusb_hotplug_enabled)
-    libusb_hotplug_deregister_callback(LibusbUtils::GetContext(), s_hotplug_handle);
+  if (s_libusb_context.IsValid() && s_libusb_hotplug_enabled)
+    libusb_hotplug_deregister_callback(s_libusb_context, s_hotplug_handle);
 #endif
   Reset();
 

Source/Core/UICommon/USBUtils.cpp

@@ -35,7 +35,7 @@ std::map<std::pair<u16, u16>, std::string> GetInsertedDevices()
   std::map<std::pair<u16, u16>, std::string> devices;
 
 #ifdef __LIBUSB__
-  auto& context = LibusbUtils::GetContext();
+  LibusbUtils::Context context;
   if (!context.IsValid())
     return devices;