Use a single libusb context

libusb on Windows is limited to only a single context. Trying to open
more than one can cause device enumerations to fail randomly.

libusb is thread-safe and we don't use the manual polling support (with
`poll()`) so this should be safe.

Source/Core/Common/CMakeLists.txt

@@ -34,6 +34,11 @@ set(SRCS Analytics.cpp
          Crypto/ec.cpp
          Logging/LogManager.cpp)
 
+if(LIBUSB_FOUND)
+	set(LIBS ${LIBS} ${LIBUSB_LIBRARIES})
+	set(SRCS ${SRCS} LibusbContext.cpp)
+endif(LIBUSB_FOUND)
+
 if(ANDROID)
 	set(SRCS ${SRCS}
 	         Logging/ConsoleListenerDroid.cpp)

Source/Core/Common/Common.vcxproj

@@ -113,6 +113,7 @@
     <ClInclude Include="Hash.h" />
     <ClInclude Include="IniFile.h" />
     <ClInclude Include="JitRegister.h" />
+    <ClInclude Include="LibusbContext.h" />
     <ClInclude Include="LinearDiskCache.h" />
     <ClInclude Include="MathUtil.h" />
     <ClInclude Include="MD5.h" />
@@ -159,6 +160,9 @@
     <ClCompile Include="Hash.cpp" />
     <ClCompile Include="IniFile.cpp" />
     <ClCompile Include="JitRegister.cpp" />
+    <ClCompile Include="LibusbContext.cpp">
+      <DisableSpecificWarnings>4200;%(DisableSpecificWarnings)</DisableSpecificWarnings>
+    </ClCompile>
     <ClCompile Include="Logging\ConsoleListenerWin.cpp" />
     <ClCompile Include="MathUtil.cpp" />
     <ClCompile Include="MD5.cpp" />

Source/Core/Common/Common.vcxproj.filters

@@ -45,6 +45,7 @@
     <ClInclude Include="FPURoundMode.h" />
     <ClInclude Include="Hash.h" />
     <ClInclude Include="IniFile.h" />
+    <ClInclude Include="LibusbContext.h" />
     <ClInclude Include="LinearDiskCache.h" />
     <ClInclude Include="MathUtil.h" />
     <ClInclude Include="MemArena.h" />
@@ -236,6 +237,7 @@
     <ClCompile Include="FileUtil.cpp" />
     <ClCompile Include="Hash.cpp" />
     <ClCompile Include="IniFile.cpp" />
+    <ClCompile Include="LibusbContext.cpp" />
     <ClCompile Include="MathUtil.cpp" />
     <ClCompile Include="MemArena.cpp" />
     <ClCompile Include="MemoryUtil.cpp" />

Source/Core/Common/LibusbContext.cpp

@@ -0,0 +1,46 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <libusb.h>
+#include <memory>
+#include <mutex>
+
+#include "Common/LibusbContext.h"
+#include "Common/MsgHandler.h"
+
+namespace LibusbContext
+{
+static std::shared_ptr<libusb_context> s_libusb_context;
+static std::once_flag s_context_initialized;
+
+static libusb_context* Create()
+{
+  libusb_context* context;
+  const int ret = libusb_init(&context);
+  if (ret < LIBUSB_SUCCESS)
+  {
+    bool is_windows = false;
+#ifdef _WIN32
+    is_windows = true;
+#endif
+    if (is_windows && ret == LIBUSB_ERROR_NOT_FOUND)
+      PanicAlertT("Failed to initialize libusb because usbdk is not installed.");
+    else
+      PanicAlertT("Failed to initialize libusb: %s", libusb_error_name(ret));
+    return nullptr;
+  }
+  return context;
+}
+
+std::shared_ptr<libusb_context> Get()
+{
+  std::call_once(s_context_initialized, []() {
+    s_libusb_context.reset(Create(), [](auto* context) {
+      if (context != nullptr)
+        libusb_exit(context);
+    });
+  });
+  return s_libusb_context;
+}
+}

Source/Core/Common/LibusbContext.h

@@ -0,0 +1,15 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <memory>
+
+struct libusb_context;
+
+namespace LibusbContext
+{
+// libusb on Windows is limited to only a single context. Trying to open more
+// than one can cause issues with device enumerations.
+// libusb is thread-safe so this context can be safely used from different threads.
+std::shared_ptr<libusb_context> Get();
+}