QmlPreview: Protect QQmlPreviewFileLoader::load with another mutex

We can concurrently load multiple files from different threads, but the
loader can only handle one such request at a time.

Fixes: QTBUG-95825
Change-Id: I2b94d55f2cf0da6e84dabc47df5699cc57b903a6
Reviewed-by: Maximilian Goldstein <max.goldstein@qt.io>
Reviewed-by: Knud Dollereder <knud.dollereder@qt.io>
(cherry picked from commit a068fdc0b2cf7d4223d9ad15345fe9d345875936)
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

3 files changed, 22 insertions, 11 deletions

diff --git a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp
index 620c1cf1ba..7b53402df9 100644
--- a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp
+++ b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileengine.cpp
@@ -398,6 +398,14 @@ bool QQmlPreviewFileEngine::supportsExtension(Extension extension) const
 
 void QQmlPreviewFileEngine::load() const
 {
+    // We can get here from different threads on different instances of QQmlPreviewFileEngine.
+    // However, there is only one loader per QQmlPreviewFileEngineHandler and it is not thread-safe.
+    // Its content mutex doesn't help us here because we explicitly wait on it in load(), which
+    // causes it to be released. Therefore, lock the load mutex first.
+    // This doesn't cause any deadlocks because the only thread that wakes the loader on the content
+    // mutex never calls load(). It's the QML debug server thread that handles the debug protocol.
+    QMutexLocker loadLocker(m_loader->loadMutex());
+
     m_result = m_loader->load(m_absolute);
     switch (m_result) {
     case QQmlPreviewFileLoader::File:
diff --git a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.cpp b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.cpp
index 72abba6154..f97dcfc767 100644
--- a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.cpp
+++ b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.cpp
@@ -101,7 +101,7 @@ QQmlPreviewFileLoader::~QQmlPreviewFileLoader() {
 
 QQmlPreviewFileLoader::Result QQmlPreviewFileLoader::load(const QString &path)
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     m_path = path;
 
     auto fileIterator = m_fileCache.constFind(path);
@@ -124,19 +124,19 @@ QQmlPreviewFileLoader::Result QQmlPreviewFileLoader::load(const QString &path)
     m_entries.clear();
     m_contents.clear();
     emit request(path);
-    m_waitCondition.wait(&m_mutex);
+    m_waitCondition.wait(&m_contentMutex);
     return m_result;
 }
 
 QByteArray QQmlPreviewFileLoader::contents()
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     return m_contents;
 }
 
 QStringList QQmlPreviewFileLoader::entries()
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     return m_entries;
 }
 
@@ -144,20 +144,20 @@ void QQmlPreviewFileLoader::whitelist(const QUrl &url)
 {
     const QString path = QQmlFile::urlToLocalFileOrQrc(url);
     if (!path.isEmpty()) {
-        QMutexLocker locker(&m_mutex);
+        QMutexLocker locker(&m_contentMutex);
         m_blacklist.whitelist(path);
     }
 }
 
 bool QQmlPreviewFileLoader::isBlacklisted(const QString &path)
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     return m_blacklist.isBlacklisted(path);
 }
 
 void QQmlPreviewFileLoader::file(const QString &path, const QByteArray &contents)
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     m_blacklist.whitelist(path);
     m_fileCache[path] = contents;
     if (path == m_path) {
@@ -169,7 +169,7 @@ void QQmlPreviewFileLoader::file(const QString &path, const QByteArray &contents
 
 void QQmlPreviewFileLoader::directory(const QString &path, const QStringList &entries)
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     m_blacklist.whitelist(path);
     m_directoryCache[path] = entries;
     if (path == m_path) {
@@ -181,7 +181,7 @@ void QQmlPreviewFileLoader::directory(const QString &path, const QStringList &en
 
 void QQmlPreviewFileLoader::error(const QString &path)
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     m_blacklist.blacklist(path);
     if (path == m_path) {
         m_result = Fallback;
@@ -191,7 +191,7 @@ void QQmlPreviewFileLoader::error(const QString &path)
 
 void QQmlPreviewFileLoader::clearCache()
 {
-    QMutexLocker locker(&m_mutex);
+    QMutexLocker locker(&m_contentMutex);
     m_fileCache.clear();
     m_directoryCache.clear();
 }
diff --git a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.h b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.h
index 9c337935e3..e66294cb06 100644
--- a/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.h
+++ b/src/plugins/qmltooling/qmldbg_preview/qqmlpreviewfileloader.h
@@ -79,7 +79,9 @@ public:
     QQmlPreviewFileLoader(QQmlPreviewServiceImpl *service);
     ~QQmlPreviewFileLoader();
 
+    QMutex *loadMutex() { return &m_loadMutex; }
     Result load(const QString &file);
+
     QByteArray contents();
     QStringList entries();
 
@@ -90,7 +92,8 @@ signals:
     void request(const QString &file);
 
 private:
-    QMutex m_mutex;
+    QMutex m_loadMutex;
+    QMutex m_contentMutex;
     QWaitCondition m_waitCondition;
 
     QThread m_thread;