QPixmapCache: guard against usage from non-main threads

Depending on the active QPA plugin, QPixmaps may now be created and
used also in non-main threads. But QPixmapCache is not designed to be
used from such threads, so add guards to ignore such access attempts,
both from application code and from Qt library code.

Such unsafe access would often cause a cryptical "~QObject: Timers
cannot be stopped from another thread" warning; that also disappears
with this fix.

[ChangeLog][QtGui][QPixmapCache] Ignore unsafe access from non-main
threads

Task-number: QTBUG-76694
Task-number: QTBUG-72523
Change-Id: Ia2db37e528aec08bfb48808630bdf5e543689039
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/image/qpixmapcache.cpp

@@ -42,6 +42,8 @@
 #include "qobject.h"
 #include "qdebug.h"
 #include "qpixmapcache_p.h"
+#include "qthread.h"
+#include "qcoreapplication.h"
 
 QT_BEGIN_NAMESPACE
 
@@ -83,6 +85,9 @@ QT_BEGIN_NAMESPACE
     with QPixmapCache} explains how to use QPixmapCache to speed up
     applications by caching the results of painting.
 
+    \note QPixmapCache is only usable from the application's main thread.
+    Access from other threads will be ignored and return failure.
+
     \sa QCache, QPixmap
 */
 
@@ -98,6 +103,14 @@ static inline int cost(const QPixmap &pixmap)
     return static_cast<int>(qBound(1LL, costKb, costMax));
 }
 
+static inline bool qt_pixmapcache_thread_test()
+{
+    if (Q_LIKELY(QCoreApplication::instance() && QThread::currentThread() == QCoreApplication::instance()->thread()))
+        return true;
+
+    return false;
+}
+
 /*!
     \class QPixmapCache::Key
     \brief The QPixmapCache::Key class can be used for efficient access
@@ -487,6 +500,8 @@ QPixmapCacheEntry::~QPixmapCacheEntry()
 
 QPixmap *QPixmapCache::find(const QString &key)
 {
+    if (!qt_pixmapcache_thread_test())
+        return nullptr;
     return pm_cache()->object(key);
 }
 
@@ -515,6 +530,8 @@ bool QPixmapCache::find(const QString &key, QPixmap& pixmap)
 
 bool QPixmapCache::find(const QString &key, QPixmap* pixmap)
 {
+    if (!qt_pixmapcache_thread_test())
+        return false;
     QPixmap *ptr = pm_cache()->object(key);
     if (ptr && pixmap)
         *pixmap = *ptr;
@@ -532,6 +549,8 @@ bool QPixmapCache::find(const QString &key, QPixmap* pixmap)
 */
 bool QPixmapCache::find(const Key &key, QPixmap* pixmap)
 {
+    if (!qt_pixmapcache_thread_test())
+        return false;
     //The key is not valid anymore, a flush happened before probably
     if (!key.d || !key.d->isValid)
         return false;
@@ -563,6 +582,8 @@ bool QPixmapCache::find(const Key &key, QPixmap* pixmap)
 
 bool QPixmapCache::insert(const QString &key, const QPixmap &pixmap)
 {
+    if (!qt_pixmapcache_thread_test())
+        return false;
     return pm_cache()->insert(key, pixmap, cost(pixmap));
 }
 
@@ -583,6 +604,8 @@ bool QPixmapCache::insert(const QString &key, const QPixmap &pixmap)
 */
 QPixmapCache::Key QPixmapCache::insert(const QPixmap &pixmap)
 {
+    if (!qt_pixmapcache_thread_test())
+        return QPixmapCache::Key();
     return pm_cache()->insert(pixmap, cost(pixmap));
 }
 
@@ -597,6 +620,8 @@ QPixmapCache::Key QPixmapCache::insert(const QPixmap &pixmap)
 */
 bool QPixmapCache::replace(const Key &key, const QPixmap &pixmap)
 {
+    if (!qt_pixmapcache_thread_test())
+        return false;
     //The key is not valid anymore, a flush happened before probably
     if (!key.d || !key.d->isValid)
         return false;
@@ -626,6 +651,8 @@ int QPixmapCache::cacheLimit()
 
 void QPixmapCache::setCacheLimit(int n)
 {
+    if (!qt_pixmapcache_thread_test())
+        return;
     pm_cache()->setMaxCost(n);
 }
 
@@ -634,6 +661,8 @@ void QPixmapCache::setCacheLimit(int n)
 */
 void QPixmapCache::remove(const QString &key)
 {
+    if (!qt_pixmapcache_thread_test())
+        return;
     pm_cache()->remove(key);
 }
 
@@ -645,6 +674,8 @@ void QPixmapCache::remove(const QString &key)
 */
 void QPixmapCache::remove(const Key &key)
 {
+    if (!qt_pixmapcache_thread_test())
+        return;
     //The key is not valid anymore, a flush happened before probably
     if (!key.d || !key.d->isValid)
         return;
@@ -657,6 +688,8 @@ void QPixmapCache::remove(const Key &key)
 
 void QPixmapCache::clear()
 {
+    if (!QCoreApplication::closingDown() && !qt_pixmapcache_thread_test())
+        return;
     QT_TRY {
         if (pm_cache.exists())
             pm_cache->clear();