Cache null icons and remove them upon system icon theme change

Since Qt 6, qIconCache does not store null icons. In case an icon name
lookup is unsuccessful, the (expensive) lookup is repeated each time.

This patch reverts 9e7c5670509ac81efdf78b691e70e5ce3d408a09, which
removes a null icon from the cache once it has been found in the cache.
In fact, that could no longer happen due to
4dc7102c8410f5699af93b6c55ba1c7df7755bc2, which prevented null icons
from being cached at all. Therefore, it is also reverted by this patch.

The cache will be cleared when
- the system icon theme name changes or
- QGuiApplicationPrivate::processThemeChanged is called (e.g. due to
a change of the system's color scheme)

Fixes: QTBUG-112257
Pick-to: 6.5
Change-Id: I80cd21fa39dc31c4bae60a8e66e78d9da20e9b4b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

5 files changed, 24 insertions, 8 deletions

diff --git a/src/gui/image/qicon.cpp b/src/gui/image/qicon.cpp
index a46c0ad009..d783c7e84f 100644
--- a/src/gui/image/qicon.cpp
+++ b/src/gui/image/qicon.cpp
@@ -98,6 +98,11 @@ QIconPrivate::QIconPrivate(QIconEngine *e)
 {
 }
 
+void QIconPrivate::clearIconCache()
+{
+    qt_cleanup_icon_cache();
+}
+
 /*! \internal
     Computes the displayDevicePixelRatio for a pixmap.
 
@@ -1277,11 +1282,8 @@ void QIcon::setFallbackThemeName(const QString &name)
 QIcon QIcon::fromTheme(const QString &name)
 {
 
-    if (QIcon *cachedIcon = qtIconCache()->object(name)) {
-        if (!cachedIcon->isNull())
-            return *cachedIcon;
-        qtIconCache()->remove(name);
-    }
+    if (QIcon *cachedIcon = qtIconCache()->object(name))
+        return *cachedIcon;
 
     QIcon icon;
     if (QDir::isAbsolutePath(name)) {
@@ -1292,8 +1294,7 @@ QIcon QIcon::fromTheme(const QString &name)
         QIconEngine * const engine = (platformTheme && !hasUserTheme) ? platformTheme->createIconEngine(name)
                                                    : new QIconLoaderEngine(name);
         icon = QIcon(engine);
-        if (!icon.isNull())
-            qtIconCache()->insert(name, new QIcon(icon));
+        qtIconCache()->insert(name, new QIcon(icon));
     }
 
     return icon;
diff --git a/src/gui/image/qicon_p.h b/src/gui/image/qicon_p.h
index ab144e61ff..c8d228259e 100644
--- a/src/gui/image/qicon_p.h
+++ b/src/gui/image/qicon_p.h
@@ -42,6 +42,8 @@ public:
     int serialNum;
     int detach_no;
     bool is_mask;
+
+    static void clearIconCache();
 };
 
 
diff --git a/src/gui/image/qiconloader.cpp b/src/gui/image/qiconloader.cpp
index f5ae864746..92bb5e65db 100644
--- a/src/gui/image/qiconloader.cpp
+++ b/src/gui/image/qiconloader.cpp
@@ -125,8 +125,18 @@ void QIconLoader::updateSystemTheme()
     }
 }
 
+void QIconLoader::invalidateKey()
+{
+    m_themeKey++;
+
+    QIconPrivate::clearIconCache();
+}
+
 void QIconLoader::setThemeName(const QString &themeName)
 {
+    if (m_userTheme == themeName)
+        return;
+
     m_userTheme = themeName;
     invalidateKey();
 }
diff --git a/src/gui/image/qiconloader_p.h b/src/gui/image/qiconloader_p.h
index cd3227c207..fbb21073ce 100644
--- a/src/gui/image/qiconloader_p.h
+++ b/src/gui/image/qiconloader_p.h
@@ -158,7 +158,7 @@ public:
     QIconDirInfo dirInfo(int dirindex);
     static QIconLoader *instance();
     void updateSystemTheme();
-    void invalidateKey() { m_themeKey++; }
+    void invalidateKey();
     void ensureInitialized();
     bool hasUserTheme() const { return !m_userTheme.isEmpty(); }
 
diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index febef13c98..d26de0b07c 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -50,6 +50,7 @@
 #include <qpa/qwindowsysteminterface.h>
 #include <qpa/qwindowsysteminterface_p.h>
 #include "private/qwindow_p.h"
+#include "private/qicon_p.h"
 #include "private/qcursor_p.h"
 #if QT_CONFIG(opengl)
 #  include "private/qopenglcontext_p.h"
@@ -2616,6 +2617,8 @@ void QGuiApplicationPrivate::processThemeChanged(QWindowSystemInterfacePrivate::
     if (self)
         self->handleThemeChanged();
 
+    QIconPrivate::clearIconCache();
+
     QEvent themeChangeEvent(QEvent::ThemeChange);
     const QWindowList windows = tce->window ? QWindowList{tce->window} : window_list;
     for (auto *window : windows)