QWindowsFontDatabase: fix a data race

QRawFont::loadFromData can be called from any thread. It calls into
QPlatformFontDatabase::fontEngine. The code path may end up creating a
UniqueFontData and storing it a QWindowsFontDatabase member (a QMap).

Two threads calling loadFromData (on different QRawFont objects)
will therefore race on the mutation of the map.

Add the missing mutex protection.

Change-Id: Ib38c2b3539679c820d997a6548e4b397cbc2bb73
Pick-to: 6.2 6.5 6.6
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

2 files changed, 10 insertions, 1 deletions

diff --git a/src/gui/text/windows/qwindowsfontdatabase.cpp b/src/gui/text/windows/qwindowsfontdatabase.cpp
index 832107d827..40f7020d38 100644
--- a/src/gui/text/windows/qwindowsfontdatabase.cpp
+++ b/src/gui/text/windows/qwindowsfontdatabase.cpp
@@ -32,6 +32,8 @@
 #  include "qwindowsfontenginedirectwrite_p.h"
 #endif
 
+#include <mutex>
+
 QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
@@ -847,7 +849,10 @@ QT_WARNING_POP
                     UniqueFontData uniqueData;
                     uniqueData.handle = fontHandle;
                     uniqueData.refCount.ref();
-                    m_uniqueFontData[uniqueFamilyName] = uniqueData;
+                    {
+                        const std::scoped_lock lock(m_uniqueFontDataMutex);
+                        m_uniqueFontData[uniqueFamilyName] = uniqueData;
+                    }
                 }
             } else {
                 RemoveFontMemResourceEx(fontHandle);
@@ -1107,6 +1112,7 @@ bool QWindowsFontDatabase::fontsAlwaysScalable() const
 
 void QWindowsFontDatabase::derefUniqueFont(const QString &uniqueFont)
 {
+    const std::scoped_lock lock(m_uniqueFontDataMutex);
     const auto it = m_uniqueFontData.find(uniqueFont);
     if (it != m_uniqueFontData.end()) {
         if (!it->refCount.deref()) {
@@ -1118,6 +1124,7 @@ void QWindowsFontDatabase::derefUniqueFont(const QString &uniqueFont)
 
 void QWindowsFontDatabase::refUniqueFont(const QString &uniqueFont)
 {
+    const std::scoped_lock lock(m_uniqueFontDataMutex);
     const auto it = m_uniqueFontData.find(uniqueFont);
     if (it != m_uniqueFontData.end())
         it->refCount.ref();
diff --git a/src/gui/text/windows/qwindowsfontdatabase_p.h b/src/gui/text/windows/qwindowsfontdatabase_p.h
index 923f875336..6d6ce4cda4 100644
--- a/src/gui/text/windows/qwindowsfontdatabase_p.h
+++ b/src/gui/text/windows/qwindowsfontdatabase_p.h
@@ -21,6 +21,7 @@
 #include <QtCore/QSharedPointer>
 #include <QtCore/QLoggingCategory>
 #include <QtCore/qhashfunctions.h>
+#include <QtCore/qmutex.h>
 #include <QtCore/qt_windows.h>
 
 QT_BEGIN_NAMESPACE
@@ -89,6 +90,7 @@ private:
         QAtomicInt refCount;
     };
 
+    QMutex m_uniqueFontDataMutex; // protects m_uniqueFontData
     QMap<QString, UniqueFontData> m_uniqueFontData;
 
     static unsigned m_fontOptions;