Sqlite: Add locking for sqlite transactions

If you use the database in a multi-threaded environment, you must always
your statements in transactions.

For read-only statements, you use DeferredTransaction and write statements
you use ImmediateTransaction. If you mix read and write statements you have
to use ImmediateTransaction. Don't use DeferredTransaction because it leads
to undefined behavior.

Change-Id: Ida298a20f33423c8da09e768a7b658dd8e918f46
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/libs/sqlite/sqlitedatabase.h

@@ -31,6 +31,7 @@
 
 #include <utils/smallstring.h>
 
+#include <mutex>
 #include <vector>
 
 namespace Sqlite {
@@ -38,11 +39,13 @@ namespace Sqlite {
 class SQLITE_EXPORT Database
 {
     template <typename Database>
-    friend class SqliteAbstractTransaction;
-    friend class SqliteStatement;
-    friend class SqliteBackend;
+    friend class AbstractTransaction;
+    friend class Statement;
+    friend class Backend;
 
 public:
+    using MutexType = std::mutex;
+
     Database();
     Database(Utils::PathString &&databaseFilePath);
 
@@ -79,12 +82,13 @@ public:
 
 private:
     void initializeTables();
-
+    std::mutex &databaseMutex() { return m_databaseMutex; }
 
 private:
+    Utils::PathString m_databaseFilePath;
     DatabaseBackend m_databaseBackend;
     std::vector<Table> m_sqliteTables;
-    Utils::PathString m_databaseFilePath;
+    std::mutex m_databaseMutex;
     JournalMode m_journalMode = JournalMode::Wal;
     OpenMode m_openMode = OpenMode::ReadWrite;
     bool m_isOpen = false;