d3d11va: Link directly to dxgi.dll and d3d11.dll functions if LoadLibrary is unavailable

When targeting the UWP API subset, the LoadLibrary function is not
available (and the fallback, LoadPackagedLibrary, can't be used to
load system DLLs). In these cases, link directly to the functions
in the DLLs instead of trying to load them dynamically at runtime.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -4876,6 +4876,10 @@ if enabled libxcb; then
         check_pkg_config libxcb_xfixes xcb-xfixes xcb/xfixes.h xcb_xfixes_get_cursor_image
 fi
 
+# d3d11va requires linking directly to dxgi and d3d11 if not building for
+# the desktop api partition
+enabled LoadLibrary || d3d11va_extralibs="-ldxgi -ld3d11"
+
 enabled vaapi && require vaapi va/va.h vaInitialize -lva
 
 enabled vaapi &&

libavutil/hwcontext_d3d11va.c

@@ -18,6 +18,10 @@
 
 #include <windows.h>
 
+// Include thread.h before redefining _WIN32_WINNT, to get
+// the right implementation for AVOnce
+#include "thread.h"
+
 #if !defined(_WIN32_WINNT) || _WIN32_WINNT < 0x0600
 #undef _WIN32_WINNT
 #define _WIN32_WINNT 0x0600
@@ -39,6 +43,34 @@
 
 typedef HRESULT(WINAPI *PFN_CREATE_DXGI_FACTORY)(REFIID riid, void **ppFactory);
 
+static AVOnce functions_loaded = AV_ONCE_INIT;
+
+static PFN_CREATE_DXGI_FACTORY mCreateDXGIFactory;
+static PFN_D3D11_CREATE_DEVICE mD3D11CreateDevice;
+
+static av_cold void load_functions(void)
+{
+#if HAVE_LOADLIBRARY
+    // We let these "leak" - this is fine, as unloading has no great benefit, and
+    // Windows will mark a DLL as loaded forever if its internal refcount overflows
+    // from too many LoadLibrary calls.
+    HANDLE d3dlib, dxgilib;
+
+    d3dlib  = LoadLibrary("d3d11.dll");
+    dxgilib = LoadLibrary("dxgi.dll");
+    if (!d3dlib || !dxgilib)
+        return;
+
+    mD3D11CreateDevice = (PFN_D3D11_CREATE_DEVICE) GetProcAddress(d3dlib, "D3D11CreateDevice");
+    mCreateDXGIFactory = (PFN_CREATE_DXGI_FACTORY) GetProcAddress(dxgilib, "CreateDXGIFactory");
+#else
+    // In UWP (which lacks LoadLibrary), CreateDXGIFactory isn't available,
+    // only CreateDXGIFactory1
+    mD3D11CreateDevice = (PFN_D3D11_CREATE_DEVICE) D3D11CreateDevice;
+    mCreateDXGIFactory = (PFN_CREATE_DXGI_FACTORY) CreateDXGIFactory1;
+#endif
+}
+
 typedef struct D3D11VAFramesContext {
     int nb_surfaces_used;
 
@@ -407,22 +439,21 @@ static int d3d11va_device_create(AVHWDeviceContext *ctx, const char *device,
                                  AVDictionary *opts, int flags)
 {
     AVD3D11VADeviceContext *device_hwctx = ctx->hwctx;
-    HANDLE d3dlib;
 
     HRESULT hr;
-    PFN_D3D11_CREATE_DEVICE createD3D;
     IDXGIAdapter           *pAdapter = NULL;
     ID3D10Multithread      *pMultithread;
     UINT creationFlags = D3D11_CREATE_DEVICE_VIDEO_SUPPORT;
+    int ret;
 
-    if (device) {
-        PFN_CREATE_DXGI_FACTORY mCreateDXGIFactory;
-        HMODULE dxgilib = LoadLibrary("dxgi.dll");
-        if (!dxgilib)
+    if ((ret = ff_thread_once(&functions_loaded, load_functions)) != 0)
         return AVERROR_UNKNOWN;
+    if (!mD3D11CreateDevice || !mCreateDXGIFactory) {
+        av_log(ctx, AV_LOG_ERROR, "Failed to load D3D11 library or its functions\n");
+        return AVERROR_UNKNOWN;
+    }
 
-        mCreateDXGIFactory = (PFN_CREATE_DXGI_FACTORY) GetProcAddress(dxgilib, "CreateDXGIFactory");
-        if (mCreateDXGIFactory) {
+    if (device) {
         IDXGIFactory2 *pDXGIFactory;
         hr = mCreateDXGIFactory(&IID_IDXGIFactory2, (void **)&pDXGIFactory);
         if (SUCCEEDED(hr)) {
@@ -432,25 +463,8 @@ static int d3d11va_device_create(AVHWDeviceContext *ctx, const char *device,
             IDXGIFactory2_Release(pDXGIFactory);
         }
     }
-        FreeLibrary(dxgilib);
-    }
-
-    // We let this "leak" - this is fine, as unloading has no great benefit, and
-    // Windows will mark a DLL as loaded forever if its internal refcount overflows
-    // from too many LoadLibrary calls.
-    d3dlib = LoadLibrary("d3d11.dll");
-    if (!d3dlib) {
-        av_log(ctx, AV_LOG_ERROR, "Failed to load D3D11 library\n");
-        return AVERROR_UNKNOWN;
-    }
-
-    createD3D = (PFN_D3D11_CREATE_DEVICE) GetProcAddress(d3dlib, "D3D11CreateDevice");
-    if (!createD3D) {
-        av_log(ctx, AV_LOG_ERROR, "Failed to locate D3D11CreateDevice\n");
-        return AVERROR_UNKNOWN;
-    }
 
-    hr = createD3D(pAdapter, pAdapter ? D3D_DRIVER_TYPE_UNKNOWN : D3D_DRIVER_TYPE_HARDWARE, NULL, creationFlags, NULL, 0,
+    hr = mD3D11CreateDevice(pAdapter, pAdapter ? D3D_DRIVER_TYPE_UNKNOWN : D3D_DRIVER_TYPE_HARDWARE, NULL, creationFlags, NULL, 0,
                    D3D11_SDK_VERSION, &device_hwctx->device, NULL, NULL);
     if (pAdapter)
         IDXGIAdapter_Release(pAdapter);