SessionManager: Fix fallback to default session if session file is invalid

Creating available sessions metadata is now also faster as it doesn't
need to parse the entire session file, only check for validity.

See #2331

src/lib/session/restoremanager.cpp

@@ -100,6 +100,32 @@ static void loadVersion3(QDataStream &stream, RestoreData &data)
     }
 }
 
+// static
+bool RestoreManager::validateFile(const QString &file)
+{
+    if (!QFile::exists(file)) {
+        return false;
+    }
+
+    QFile recoveryFile(file);
+    if (!recoveryFile.open(QIODevice::ReadOnly)) {
+        return false;
+    }
+
+    QDataStream stream(&recoveryFile);
+
+    int version;
+    stream >> version;
+
+    if (version == Qz::sessionVersion || version == 0x0003 || version == (0x0003 | 0x050000)) {
+        int windowCount;
+        stream >> windowCount;
+        return windowCount > 0;
+    }
+
+    return false;
+}
+
 // static
 void RestoreManager::createFromFile(const QString &file, RestoreData &data)
 {
@@ -108,7 +134,10 @@ void RestoreManager::createFromFile(const QString &file, RestoreData &data)
     }
 
     QFile recoveryFile(file);
-    recoveryFile.open(QIODevice::ReadOnly);
+    if (!recoveryFile.open(QIODevice::ReadOnly)) {
+        return;
+    }
+
     QDataStream stream(&recoveryFile);
 
     int version;

src/lib/session/restoremanager.h

@@ -38,6 +38,7 @@ public:
 
     QObject *recoveryObject(WebPage *page);
 
+    static bool validateFile(const QString &file);
     static void createFromFile(const QString &file, RestoreData &data);
 
 private:

src/lib/session/sessionmanager.cpp

@@ -279,10 +279,8 @@ void SessionManager::fillSessionsMetaDataListIfNeeded()
 
     for (int i = 0; i < sessionFiles.size(); ++i) {
         const QFileInfo &fileInfo = sessionFiles.at(i);
-        RestoreData data;
-        RestoreManager::createFromFile(fileInfo.absoluteFilePath(), data);
 
-        if (data.isEmpty())
+        if (!RestoreManager::validateFile(fileInfo.absoluteFilePath()))
             continue;
 
         SessionMetaData metaData;
@@ -321,8 +319,8 @@ void SessionManager::loadSettings()
         m_lastActiveSessionPath = sessionsDir.absoluteFilePath(m_lastActiveSessionPath);
     }
 
-    // fallback to default session
-    if (!QFile::exists(m_lastActiveSessionPath))
+    // Fallback to default session
+    if (!RestoreManager::validateFile(m_lastActiveSessionPath))
         m_lastActiveSessionPath = defaultSessionPath();
 }