directwrite: support WinRT/UWP, which forbid LoadLibrary

Normally, we delay loading Dwrite.dll until runtime to allow building
and running DirectWrite-enabled libass binaries on old Windows versions
that lack DirectWrite. However, this is forbidden in WinRT/UWP.

DirectWrite is present in all versions of Windows that support WinRT/UWP,
so we lose nothing by requiring it.

Older Windows SDKs (Microsoft and MinGW alike) lack <winapifamily.h>,
so include it only if we really need it.

Based on VLC patch for libass:

https://github.com/videolan/vlc/commit/eedb57aa96d2bc0046a6da2e081c75ae9edf8fd5

and on this autoconf code:

https://github.com/lu-zero/mfx_dispatch/commit/c51a54c15f51579804030c70592c0a26065f1242

Note: the VLC patch retained an unconditional call to FreeLibrary
in destroy_provider. However, FreeLibrary does not actually expect
NULL as argument, so this commit adds a guard to that call.
Perhaps FreeLibrary(NULL) simply fails cleanly and that's why
this has not caused VLC problems, but we should not rely on this.

3 files changed, 52 insertions, 9 deletions

diff --git a/configure.ac b/configure.ac
index 5f8cf7b..8500381 100644
--- a/configure.ac
+++ b/configure.ac
@@ -28,7 +28,7 @@ AC_ARG_ENABLE([profile], AS_HELP_STRING([--enable-profile],
 AC_ARG_ENABLE([fontconfig], AS_HELP_STRING([--disable-fontconfig],
     [disable fontconfig support @<:@default=enabled@:>@]))
 AC_ARG_ENABLE([directwrite], AS_HELP_STRING([--disable-directwrite],
-    [disable DirectWrite support (win32 only) @<:@default=check@:>@]))
+    [disable DirectWrite support (Windows only) @<:@default=check@:>@]))
 AC_ARG_ENABLE([coretext], AS_HELP_STRING([--disable-coretext],
     [disable CoreText support (OSX only) @<:@default=check@:>@]))
 AC_ARG_ENABLE([require-system-font-provider], AS_HELP_STRING([--disable-require-system-font-provider],
@@ -147,9 +147,28 @@ AS_IF([test "x$enable_directwrite" != xno], [
     AC_LINK_IFELSE([
         AC_LANG_PROGRAM([[#include <windows.h>]], [[;]])
     ], [
-        AC_DEFINE(CONFIG_DIRECTWRITE, 1, [found DirectWrite])
         directwrite=true
         AC_MSG_RESULT([yes])
+        AC_MSG_CHECKING([for Win32 desktop APIs])
+        AC_COMPILE_IFELSE([
+            AC_LANG_PROGRAM([[
+                #include <winapifamily.h>
+                #if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+                #error Win32 desktop APIs are available
+                #endif
+            ]], [[;]])
+        ], [
+            # WinRT/UWP/app build: LoadLibrary is
+            # unavailable, but DirectWrite is always present
+            LIBS="$LIBS -ldwrite"
+            AC_DEFINE(CONFIG_DIRECTWRITE, 1, [found DirectWrite (WinRT/UWP)])
+            AC_MSG_RESULT([no])
+        ], [
+            # Win32/desktop build:
+            # DirectWrite is optional but can be loaded via LoadLibrary
+            AC_DEFINE(CONFIG_DIRECTWRITE, 1, [found DirectWrite (Win32)])
+            AC_MSG_RESULT([yes])
+        ])
     ], [
         directwrite=false
         AC_MSG_RESULT([no])
diff --git a/libass/ass_directwrite.c b/libass/ass_directwrite.c
index 55fce52..e6ee298 100644
--- a/libass/ass_directwrite.c
+++ b/libass/ass_directwrite.c
@@ -45,7 +45,9 @@ typedef struct {
 } FontPrivate;
 
 typedef struct {
+#if ASS_WINAPI_DESKTOP
     HMODULE directwrite_lib;
+#endif
     IDWriteFactory *factory;
     IDWriteGdiInterop *gdi_interop;
 } ProviderPrivate;
@@ -389,7 +391,9 @@ static void destroy_provider(void *priv)
     ProviderPrivate *provider_priv = (ProviderPrivate *)priv;
     provider_priv->gdi_interop->lpVtbl->Release(provider_priv->gdi_interop);
     provider_priv->factory->lpVtbl->Release(provider_priv->factory);
+#if ASS_WINAPI_DESKTOP
     FreeLibrary(provider_priv->directwrite_lib);
+#endif
     free(provider_priv);
 }
 
@@ -726,7 +730,15 @@ static ASS_FontProviderFuncs directwrite_callbacks = {
     .get_font_index     = get_font_index,
 };
 
-typedef HRESULT (WINAPI *DWriteCreateFactoryFn)(
+#if ASS_WINAPI_DESKTOP
+typedef HRESULT (WINAPI *DWriteCreateFactoryFn)
+#else
+// LoadLibrary is forbidden in WinRT/UWP apps, so use DirectWrite directly.
+// These apps cannot run on older Windows that lacks DirectWrite,
+// so we lose nothing.
+HRESULT WINAPI DWriteCreateFactory
+#endif
+(
     DWRITE_FACTORY_TYPE factoryType,
     REFIID              iid,
     IUnknown            **factory
@@ -747,22 +759,23 @@ ASS_FontProvider *ass_directwrite_add_provider(ASS_Library *lib,
     IDWriteFactory *dwFactory = NULL;
     IDWriteGdiInterop *dwGdiInterop = NULL;
     ASS_FontProvider *provider = NULL;
-    DWriteCreateFactoryFn DWriteCreateFactoryPtr = NULL;
     ProviderPrivate *priv = NULL;
 
+#if ASS_WINAPI_DESKTOP
     HMODULE directwrite_lib = LoadLibraryW(L"Dwrite.dll");
     if (!directwrite_lib)
         goto cleanup;
 
-    DWriteCreateFactoryPtr =
+    DWriteCreateFactoryFn DWriteCreateFactory =
         (DWriteCreateFactoryFn)(void *)GetProcAddress(directwrite_lib,
                                                       "DWriteCreateFactory");
-    if (!DWriteCreateFactoryPtr)
+    if (!DWriteCreateFactory)
         goto cleanup;
+#endif
 
-    hr = DWriteCreateFactoryPtr(DWRITE_FACTORY_TYPE_SHARED,
-                                &IID_IDWriteFactory,
-                                (IUnknown **) (&dwFactory));
+    hr = DWriteCreateFactory(DWRITE_FACTORY_TYPE_SHARED,
+                             &IID_IDWriteFactory,
+                             (IUnknown **) (&dwFactory));
     if (FAILED(hr) || !dwFactory) {
         ass_msg(lib, MSGL_WARN, "Failed to initialize directwrite.");
         dwFactory = NULL;
@@ -781,7 +794,9 @@ ASS_FontProvider *ass_directwrite_add_provider(ASS_Library *lib,
     if (!priv)
         goto cleanup;
 
+#if ASS_WINAPI_DESKTOP
     priv->directwrite_lib = directwrite_lib;
+#endif
     priv->factory = dwFactory;
     priv->gdi_interop = dwGdiInterop;
 
@@ -798,8 +813,10 @@ cleanup:
         dwGdiInterop->lpVtbl->Release(dwGdiInterop);
     if (dwFactory)
         dwFactory->lpVtbl->Release(dwFactory);
+#if ASS_WINAPI_DESKTOP
     if (directwrite_lib)
         FreeLibrary(directwrite_lib);
+#endif
 
     return NULL;
 }
diff --git a/libass/ass_directwrite.h b/libass/ass_directwrite.h
index bb1159c..a14b951 100644
--- a/libass/ass_directwrite.h
+++ b/libass/ass_directwrite.h
@@ -22,6 +22,13 @@
 #ifndef ASS_DIRECTWRITE_H
 #define ASS_DIRECTWRITE_H
 
+#ifdef WINAPI_FAMILY
+#include <winapifamily.h>
+#define ASS_WINAPI_DESKTOP WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+#else
+#define ASS_WINAPI_DESKTOP 1
+#endif
+
 ASS_FontProvider *
 ass_directwrite_add_provider(ASS_Library *lib, ASS_FontSelector *selector,
                              const char *config, FT_Library ftlib);