Occasionally trim the type cache

As loaded components are kept in a cache, they are never removed by the
garbage collector. So, if you periodically create new components, they
leak. This change adds a floating threshold for the number of
components. When that threshold is surpassed trimCache() is called and
unneeded components are removed.

Task-number: QTBUG-42055
Change-Id: I30e3e4ee287f6d34376713668009c67614a50e0c
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

src/qml/qml/qqmltypeloader.cpp

@@ -1113,6 +1113,7 @@ void QQmlTypeLoader::loadThread(QQmlDataBlob *blob)
 }
 
 #define DATALOADER_MAXIMUM_REDIRECT_RECURSION 16
+#define TYPELOADER_MINIMUM_TRIM_THRESHOLD 64
 
 void QQmlTypeLoader::networkReplyFinished(QNetworkReply *reply)
 {
@@ -1592,7 +1593,8 @@ bool QQmlTypeLoader::QmldirContent::designerSupported() const
 Constructs a new type loader that uses the given \a engine.
 */
 QQmlTypeLoader::QQmlTypeLoader(QQmlEngine *engine)
-    : m_engine(engine), m_thread(new QQmlTypeLoaderThread(this))
+    : m_engine(engine), m_thread(new QQmlTypeLoaderThread(this)),
+      m_typeCacheTrimThreshold(TYPELOADER_MINIMUM_TRIM_THRESHOLD)
 {
 }
 
@@ -1629,6 +1631,10 @@ QQmlTypeData *QQmlTypeLoader::getType(const QUrl &url, Mode mode)
     QQmlTypeData *typeData = m_typeCache.value(url);
 
     if (!typeData) {
+        // Trim before adding the new type, so that we don't immediately trim it away
+        if (m_typeCache.size() >= m_typeCacheTrimThreshold)
+            trimCache();
+
         typeData = new QQmlTypeData(url, this);
         // TODO: if (compiledData == 0), is it safe to omit this insertion?
         m_typeCache.insert(url, typeData);
@@ -1933,12 +1939,22 @@ void QQmlTypeLoader::clearCache()
     qDeleteAll(m_importQmlDirCache);
 
     m_typeCache.clear();
+    m_typeCacheTrimThreshold = TYPELOADER_MINIMUM_TRIM_THRESHOLD;
     m_scriptCache.clear();
     m_qmldirCache.clear();
     m_importDirCache.clear();
     m_importQmlDirCache.clear();
 }
 
+void QQmlTypeLoader::updateTypeCacheTrimThreshold()
+{
+    int size = m_typeCache.size();
+    if (size > m_typeCacheTrimThreshold)
+        m_typeCacheTrimThreshold = size * 2;
+    if (size < m_typeCacheTrimThreshold / 2)
+        m_typeCacheTrimThreshold = qMax(size * 2, TYPELOADER_MINIMUM_TRIM_THRESHOLD);
+}
+
 void QQmlTypeLoader::trimCache()
 {
     while (true) {
@@ -1963,6 +1979,8 @@ void QQmlTypeLoader::trimCache()
         }
     }
 
+    updateTypeCacheTrimThreshold();
+
     // TODO: release any scripts which are no longer referenced by any types
 }
 

src/qml/qml/qqmltypeloader_p.h

@@ -358,6 +358,7 @@ private:
     QQmlTypeLoaderThread *m_thread;
     NetworkReplies m_networkReplies;
     TypeCache m_typeCache;
+    int m_typeCacheTrimThreshold;
     ScriptCache m_scriptCache;
     QmldirCache m_qmldirCache;
     ImportDirCache m_importDirCache;
@@ -365,6 +366,7 @@ private:
 
     template<typename Loader>
     void doLoad(const Loader &loader, QQmlDataBlob *blob, Mode mode);
+    void updateTypeCacheTrimThreshold();
 
     friend struct PlainLoader;
     friend struct CachedLoader;

tests/auto/qml/qqmltypeloader/data/trim_cache.qml

@@ -0,0 +1,8 @@
+import QtQuick 2.2
+Rectangle
+{
+    objectName: "dings"
+    width: 100
+    height: 100
+    color: "blue"
+}

tests/auto/qml/qqmltypeloader/tst_qqmltypeloader.cpp

@@ -35,6 +35,9 @@
 #include <QtQml/qqmlengine.h>
 #include <QtQuick/qquickview.h>
 #include <QtQuick/qquickitem.h>
+#include <QtQml/private/qqmlengine_p.h>
+#include <QtQml/private/qqmltypeloader_p.h>
+#include <QtQml/private/qqmlcompiler_p.h>
 #include "../../shared/util.h"
 
 class tst_QQMLTypeLoader : public QQmlDataTest
@@ -44,6 +47,7 @@ class tst_QQMLTypeLoader : public QQmlDataTest
 private slots:
     void testLoadComplete();
     void loadComponentSynchronously();
+    void trimCache();
 };
 
 void tst_QQMLTypeLoader::testLoadComplete()
@@ -73,6 +77,32 @@ void tst_QQMLTypeLoader::loadComponentSynchronously()
     QVERIFY(o);
 }
 
+void tst_QQMLTypeLoader::trimCache()
+{
+    QQmlEngine engine;
+    QQmlTypeLoader &loader = QQmlEnginePrivate::get(&engine)->typeLoader;
+    for (int i = 0; i < 256; ++i) {
+        QUrl url = testFileUrl("trim_cache.qml");
+        url.setQuery(QString::number(i));
+
+        QQmlTypeData *data = loader.getType(url);
+        if (i % 5 == 0) // keep references to some of them so that they aren't trimmed
+            data->compiledData()->addref();
+
+        data->release();
+    }
+
+    for (int i = 0; i < 256; ++i) {
+        QUrl url = testFileUrl("trim_cache.qml");
+        url.setQuery(QString::number(i));
+        if (i % 5 == 0)
+            QVERIFY(loader.isTypeLoaded(url));
+        else if (i < 128)
+            QVERIFY(!loader.isTypeLoaded(url));
+        // The cache is free to keep the others.
+    }
+}
+
 QTEST_MAIN(tst_QQMLTypeLoader)
 
 #include "tst_qqmltypeloader.moc"