Windows QPA: Do not use QSettings to access the registry

When commenting out the warning in createOrOpenKey()
(src\corelib\io\qsettings_win.cpp:157), applications produce warnings:
QSettings: Failed to create subkey "Software\Microsoft\Windows NT\CurrentVersion\FontSubstitutes": Access is denied.
indicating that an attempt to open the registry in read/write mode fails.
Add a utility function to read out registry strings in read-only mode to
the font database and use that instead.

Change-Id: I4187344cac7ec2ba27f15b51e237575efc171853
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

3 files changed, 29 insertions, 4 deletions

diff --git a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase.cpp b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase.cpp
index ad653ab95d..434aa16d16 100644
--- a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase.cpp
+++ b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase.cpp
@@ -1981,4 +1981,26 @@ int QWindowsFontDatabase::defaultVerticalDPI()
     return vDPI;
 }
 
+QString QWindowsFontDatabase::readRegistryString(HKEY parentHandle, const wchar_t *keyPath, const wchar_t *keyName)
+{
+    QString result;
+    HKEY handle = 0;
+    if (RegOpenKeyEx(parentHandle, keyPath, 0, KEY_READ, &handle) == ERROR_SUCCESS) {
+        // get the size and type of the value
+        DWORD dataType;
+        DWORD dataSize;
+        if (RegQueryValueEx(handle, keyName, 0, &dataType, 0, &dataSize) == ERROR_SUCCESS) {
+            if (dataType == REG_SZ || dataType == REG_EXPAND_SZ) {
+                dataSize += 2; // '\0' missing?
+                QVarLengthArray<unsigned char> data(dataSize);
+                data[dataSize - 2] = data[dataSize - 1] = '\0';
+                if (RegQueryValueEx(handle, keyName, 0, 0, data.data(), &dataSize) == ERROR_SUCCESS)
+                    result = QString::fromWCharArray(reinterpret_cast<const wchar_t *>(data.data()));
+            }
+        }
+        RegCloseKey(handle);
+    }
+    return result;
+}
+
 QT_END_NAMESPACE
diff --git a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase_p.h b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase_p.h
index d240f77a66..b7ebfc033f 100644
--- a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase_p.h
+++ b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabase_p.h
@@ -130,6 +130,8 @@ public:
     static void setFontOptions(unsigned options);
     static unsigned fontOptions();
 
+    static QString readRegistryString(HKEY parentHandle, const wchar_t *keyPath, const wchar_t *keyName);
+
 private:
     void populateFamily(const QString &familyName, bool registerAlias);
     void removeApplicationFonts();
diff --git a/src/platformsupport/fontdatabases/windows/qwindowsfontenginedirectwrite.cpp b/src/platformsupport/fontdatabases/windows/qwindowsfontenginedirectwrite.cpp
index a1d8c2b9e8..f266e85126 100644
--- a/src/platformsupport/fontdatabases/windows/qwindowsfontenginedirectwrite.cpp
+++ b/src/platformsupport/fontdatabases/windows/qwindowsfontenginedirectwrite.cpp
@@ -42,7 +42,6 @@
 #include "qwindowsfontenginedirectwrite_p.h"
 #include "qwindowsfontdatabase_p.h"
 
-#include <QtCore/QSettings>
 #include <QtCore/QtEndian>
 #include <QtCore/QVarLengthArray>
 #include <QtCore/QFile>
@@ -915,9 +914,11 @@ void QWindowsFontEngineDirectWrite::initFontInfo(const QFontDef &request,
 
 QString QWindowsFontEngineDirectWrite::fontNameSubstitute(const QString &familyName)
 {
-    static const char keyC[] = "HKEY_LOCAL_MACHINE\\Software\\Microsoft\\Windows NT\\CurrentVersion\\"
-                               "FontSubstitutes";
-    return QSettings(QLatin1String(keyC), QSettings::NativeFormat).value(familyName, familyName).toString();
+    const wchar_t key[] = L"Software\\Microsoft\\Windows NT\\CurrentVersion\\FontSubstitutes";
+    const QString substitute =
+        QWindowsFontDatabase::readRegistryString(HKEY_LOCAL_MACHINE, key,
+                                                 reinterpret_cast<const wchar_t *>(familyName.utf16()));
+    return substitute.isEmpty() ? familyName : substitute;
 }
 
 glyph_metrics_t QWindowsFontEngineDirectWrite::alphaMapBoundingBox(glyph_t glyph,