Windows: Add helper function to check if the app has a package identity

Having a package identity is required to use many modern
Windows APIs.

https://docs.microsoft.com/en-us/windows/apps/desktop/modernize/modernize-packaged-apps

Change-Id: Ib08dbdce97cb082fa1664df815457dbee82dd3b9
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/kernel/qfunctions_win.cpp

@@ -3,9 +3,16 @@
 
 #include "qfunctions_win_p.h"
 
+#include <QtCore/qdebug.h>
+
 #include <combaseapi.h>
 #include <objbase.h>
 
+#if __has_include(<appmodel.h>)
+#  include <appmodel.h>
+#  define HAS_APPMODEL
+#endif
+
 QT_BEGIN_NAMESPACE
 
 QComHelper::QComHelper(COINIT concurrencyModel)
@@ -25,4 +32,33 @@ QComHelper::~QComHelper()
         CoUninitialize();
 }
 
+/*!
+    Checks if the application has a \e{package identity}
+
+    Having a \e{package identity} is required to use many modern
+    Windows APIs.
+
+    https://docs.microsoft.com/en-us/windows/apps/desktop/modernize/modernize-packaged-apps
+*/
+bool qt_win_hasPackageIdentity()
+{
+#if defined(HAS_APPMODEL)
+    static const bool hasPackageIdentity = []() {
+        UINT32 length = 0;
+        switch (const auto result = GetCurrentPackageFullName(&length, nullptr)) {
+        case ERROR_INSUFFICIENT_BUFFER:
+            return true;
+        case APPMODEL_ERROR_NO_PACKAGE:
+            return false;
+        default:
+            qWarning("Failed to resolve package identity (error code %ld)", result);
+            return false;
+        }
+    }();
+    return hasPackageIdentity;
+#else
+    return false;
+#endif
+}
+
 QT_END_NAMESPACE

src/corelib/kernel/qfunctions_win_p.h

@@ -17,7 +17,7 @@
 
 #include <QtCore/private/qglobal_p.h>
 
-#if defined(Q_OS_WIN)
+#if defined(Q_OS_WIN) || defined(Q_QDOC)
 
 #if !defined(QT_BOOTSTRAPPED)
 #include <QtCore/private/qfunctions_winrt_p.h>
@@ -41,6 +41,8 @@ private:
     HRESULT m_initResult = E_FAIL;
 };
 
+Q_CORE_EXPORT bool qt_win_hasPackageIdentity();
+
 QT_END_NAMESPACE
 
 #endif // Q_OS_WIN