Set database to WAL mode on startup

Using WAL mode avoids having to lock the database for certain
transactions.  Therefore, deadlocks between read/writes from different
threads should not happen anymore.  These were rare, but happened
sometimes on slower hardware or slow storage devices.

BUG: 465110

src/database.cpp

@@ -47,6 +47,8 @@ void Database::closeDatabase(const QString &connectionName)
 
 bool Database::migrate()
 {
+    setWalMode();
+
     int dbversion = version();
     if (dbversion < 1)
         TRUE_OR_RETURN(migrateTo1());
@@ -200,6 +202,26 @@ int Database::version()
     return -1;
 }
 
+void Database::setWalMode()
+{
+    bool ok = false;
+    QSqlQuery query;
+    query.prepare(QStringLiteral("PRAGMA journal_mode;"));
+    execute(query);
+    if (query.next()) {
+        ok = (query.value(0).toString() == QStringLiteral("wal"));
+    }
+
+    if (!ok) {
+        query.prepare(QStringLiteral("PRAGMA journal_mode=WAL;"));
+        execute(query);
+        if (query.next()) {
+            ok = (query.value(0).toString() == QStringLiteral("wal"));
+        }
+        qDebug() << "Activating WAL mode on database:" << (ok ? "ok" : "not ok!");
+    }
+}
+
 void Database::cleanup()
 {
     // TODO: create database sanity checks, or, alternatively, create database scrub routine

src/database.h

@@ -43,6 +43,7 @@ private:
     bool migrateTo5();
     bool migrateTo6();
     void cleanup();
+    void setWalMode();
 
     inline static const QString m_dbName = QStringLiteral("database.db3");
 };