try harder to avoid creating svg renderers

now is actually possible to obtain a startup with zero svg renderers
    * svg::isValid will create a renderer only if really nevessary
    * the rects cache is ensured to be written on disk
    * fixed the check on the theme metadata age
    * rect cache is shared between all applications

src/plasma/private/theme_p.cpp

@@ -173,7 +173,7 @@ bool ThemePrivate::useCache()
             }
 
             // now we check for, and remove if necessary, old caches
-            foreach (const QString &file, QStandardPaths::locateAll(QStandardPaths::CacheLocation, cacheFileBase)) {
+            foreach (const QString &file, QStandardPaths::locateAll(QStandardPaths::GenericCacheLocation, cacheFileBase)) {
                 if (currentCacheFileName.isEmpty() ||
                         !file.endsWith(currentCacheFileName)) {
                     QFile::remove(file);
@@ -192,11 +192,11 @@ bool ThemePrivate::useCache()
             // the cache should be dropped; we need a way to detect system color change when the
             // application is not running.
             // check for expired cache
-            const QString cacheFilePath = QStandardPaths::writableLocation(QStandardPaths::CacheLocation) + '/' + cacheFile;
+            const QString cacheFilePath = QStandardPaths::writableLocation(QStandardPaths::GenericCacheLocation) + '/' + cacheFile + ".kcache";
             if (!cacheFilePath.isEmpty()) {
                 const QFileInfo cacheFileInfo(cacheFilePath);
                 const QFileInfo metadataFileInfo(themeMetadataPath);
-                cachesTooOld = cacheFileInfo.lastModified().toTime_t() > metadataFileInfo.lastModified().toTime_t();
+                cachesTooOld = cacheFileInfo.lastModified().toTime_t() < metadataFileInfo.lastModified().toTime_t();
             }
         }
 
@@ -216,13 +216,13 @@ bool ThemePrivate::useCache()
         }
 
         // now we check for (and remove) old caches
-        foreach (const QString &file, QStandardPaths::locateAll(QStandardPaths::CacheLocation, svgElementsFileNameBase + QLatin1Char('*'))) {
+        foreach (const QString &file, QStandardPaths::locateAll(QStandardPaths::GenericCacheLocation, svgElementsFileNameBase + QLatin1Char('*'))) {
             if (cachesTooOld || !file.endsWith(svgElementsFileName)) {
                 QFile::remove(file);
             }
         }
 
-        const QString svgElementsFile = QStandardPaths::writableLocation(QStandardPaths::CacheLocation) + '/' + svgElementsFileName;
+        const QString svgElementsFile = QStandardPaths::writableLocation(QStandardPaths::GenericCacheLocation) + '/' + svgElementsFileName;
         svgElementsCache = KSharedConfig::openConfig(svgElementsFile);
     }
 
@@ -668,8 +668,9 @@ void ThemePrivate::setThemeName(const QString &tempThemeName, bool writeSettings
         cg.sync();
     }
 
-    if(emitChanged)
+    if(emitChanged) {
         scheduleThemeChangeNotification(SvgElementsCache);
+    }
 }
 
 bool ThemePrivate::eventFilter(QObject *watched, QEvent *event)

src/plasma/svg.cpp

@@ -204,7 +204,8 @@ bool SvgPrivate::setImagePath(const QString &imagePath)
 
     if (themed) {
         themePath = actualPath;
-        themeFailed = false;
+        path = actualTheme()->imagePath(themePath);
+        themeFailed = path.isEmpty();
         QObject::connect(actualTheme(), SIGNAL(themeChanged()), q, SLOT(themeChanged()));
     } else if (QFile::exists(actualPath)) {
         QObject::connect(cacheAndColorsTheme(), SIGNAL(themeChanged()), q, SLOT(themeChanged()), Qt::UniqueConnection);
@@ -220,8 +221,9 @@ bool SvgPrivate::setImagePath(const QString &imagePath)
 
     // also images with absolute path needs to have a natural size initialized,
     // even if looks a bit weird using Theme to store non-themed stuff
-    if (themed || QFile::exists(actualPath)) {
+    if ((themed && QFile::exists(path)) || QFile::exists(actualPath)) {
         QRectF rect;
+
         if (cacheAndColorsTheme()->findInRectsCache(path, "_Natural", rect)) {
             naturalSize = rect.size();
         } else {
@@ -482,6 +484,10 @@ QRectF SvgPrivate::elementRect(const QString &elementId)
         }
     }
 
+    if (path.isEmpty()) {
+        return QRectF();
+    }
+
     QString id = cacheId(elementId);
 
     if (localRectCache.contains(id)) {
@@ -489,7 +495,12 @@ QRectF SvgPrivate::elementRect(const QString &elementId)
     }
 
     QRectF rect;
-    if (cacheAndColorsTheme()->findInRectsCache(path, id, rect)) {
+    bool found = cacheAndColorsTheme()->findInRectsCache(path, id, rect);
+
+    //This is a corner case where we are *sure* the element is not valid
+    if (found && rect == QRectF()) {
+        return rect;
+    } else if (found) {
         localRectCache.insert(id, rect);
     } else {
         rect = findAndCacheElementRect(elementId);
@@ -519,6 +530,7 @@ QRectF SvgPrivate::findAndCacheElementRect(const QString &elementId)
                          elementRect.width() * dx * devicePixelRatio, elementRect.height() * dy * devicePixelRatio);
 
     cacheAndColorsTheme()->insertIntoRectsCache(path, id, elementRect);
+
     return elementRect;
 }
 
@@ -780,6 +792,16 @@ bool Svg::isValid() const
         return false;
     }
 
+    //try very hard to avoid creation of a parser
+    QRectF rect;
+    if (d->cacheAndColorsTheme()->findInRectsCache(d->path, "_Natural", rect)) {
+        return true;
+    }
+
+    if (!QFile::exists(d->path)) {
+        return false;
+    }
+
     d->createRenderer();
     return d->renderer->isValid();
 }

src/plasma/theme.cpp

@@ -98,6 +98,9 @@ Theme::~Theme()
             KConfigGroup imageGroup(d->svgElementsCache, it.key());
             imageGroup.writeEntry("invalidElements", it.value().toList()); //FIXME: add QSet support to KConfig
         }
+
+        //The application is probably dying, last occasion to write to disk
+        d->svgElementsCache->sync();
     }
 
     if (d == ThemePrivate::globalTheme) {