refs #545 CAtomicFile used in CValueCache to avoid corruption in case serialization is interrupted.

src/blackmisc/atomicfile.h

@@ -0,0 +1,61 @@
+/* Copyright (C) 2015
+ * swift project Community / Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution and at http://www.swift-project.org/license.html. No part of swift project,
+ * including this file, may be copied, modified, propagated, or distributed except according to the terms
+ * contained in the LICENSE file.
+ */
+
+//! \file
+
+#ifndef BLACKMISC_ATOMICFILE_H
+#define BLACKMISC_ATOMICFILE_H
+
+#include <QFile>
+#include "blackmisc/blackmiscexport.h"
+
+namespace BlackMisc
+{
+    /*!
+     * A subclass of QFile which writes to a temporary file while it is open, then renames the file
+     * when it is closed, so that it overwrites the target file as a single, atomic transaction.
+     *
+     * If the application crashes while data is still being written, the original file is unchanged.
+     */
+    class BLACKMISC_EXPORT CAtomicFile : public QFile
+    {
+    public:
+        //! \copydoc QFile::QFile(const QString &)
+        CAtomicFile(const QString &filename) : QFile(filename) {}
+
+        //! \copydoc QFile::~QFile
+        ~CAtomicFile() { close(); }
+
+        //! \copydoc QFile::open
+        //! Just before opening the file, the filename is changed so we actually write to a temporary file.
+        virtual bool open(OpenMode mode) override;
+
+        //! \copydoc QFileDevice::close
+        //! After closing the file, it is renamed so that it overwrites the target file.
+        virtual void close() override;
+
+        //! Calls close() and returns false if there was an error at any stage.
+        bool checkedClose();
+
+        //! \copydoc QFileDevice::error
+        FileError error() const;
+
+        //! \copydoc QFileDevice::unsetError
+        void unsetError();
+
+    private:
+        static QString randomSuffix();
+        void replaceOriginal();
+
+        QString m_originalFilename;
+        bool m_renameError = false;
+    };
+}
+
+#endif