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

src/blackmisc/atomicfile.cpp

@@ -0,0 +1,91 @@
+/* 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.
+ */
+
+#include "blackmisc/atomicfile.h"
+#include "blackmisc/algorithm.h"
+#include <QFileInfo>
+#include <QDir>
+#if defined(Q_OS_POSIX)
+#include <stdio.h>
+#elif defined(Q_OS_WIN32)
+#include <windows.h>
+#endif
+
+namespace BlackMisc
+{
+
+    bool CAtomicFile::open(CAtomicFile::OpenMode mode)
+    {
+        Q_ASSERT_X(!(mode & (ReadOnly | Append)), Q_FUNC_INFO, "ReadOnly and Append flags are incompatible with CAtomicFile");
+
+        m_originalFilename = fileName();
+        QFileInfo fileInfo(fileName());
+        setFileName(QFileInfo(fileInfo.dir(), ".tmp." + fileInfo.fileName() + "." + randomSuffix()).filePath());
+        if (exists()) { remove(); }
+
+        bool ok = QFile::open(mode);
+        if (! ok) { setFileName(m_originalFilename); }
+        return ok;
+    }
+
+    void CAtomicFile::close()
+    {
+        if (! isOpen()) { return; }
+
+        QFile::close();
+
+        if (error() == NoError) { replaceOriginal(); }
+        setFileName(m_originalFilename);
+    }
+
+    bool CAtomicFile::checkedClose()
+    {
+        close();
+        return error() == NoError;
+    }
+
+    CAtomicFile::FileError CAtomicFile::error() const
+    {
+        if (m_renameError) { return RenameError; }
+        return QFile::error();
+    }
+
+    void CAtomicFile::unsetError()
+    {
+        m_renameError = false;
+        QFile::unsetError();
+    }
+
+    QString CAtomicFile::randomSuffix()
+    {
+        Q_CONSTEXPR auto max = 2176782335;
+        return QString::number(std::uniform_int_distribution<std::decay<decltype(max)>::type>(0, max)(Private::defaultRandomGenerator()), 36);
+    }
+
+#if defined(Q_OS_POSIX)
+    void CAtomicFile::replaceOriginal()
+    {
+        auto result = ::rename(qPrintable(fileName()), qPrintable(m_originalFilename));
+        if (result < 0) { m_renameError = true; }
+    }
+#elif defined(Q_OS_WIN32)
+    void CAtomicFile::replaceOriginal()
+    {
+        auto result = MoveFileExA(qPrintable(fileName()), qPrintable(m_originalFilename), MOVEFILE_REPLACE_EXISTING);
+        if (! result) { m_renameError = true; }
+    }
+#else
+    void CAtomicFile::replaceOriginal()
+    {
+        if (exists(m_originalFilename)) { remove(m_originalFilename); }
+        rename(m_originalFilename);
+    }
+#endif
+
+}

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

src/blackmisc/valuecache.cpp

@@ -12,6 +12,7 @@
 #include "blackmisc/logmessage.h"
 #include "blackmisc/algorithm.h"
 #include "blackmisc/lockfree.h"
+#include "blackmisc/atomicfile.h"
 #include <QThread>
 #include <QJsonDocument>
 
@@ -230,23 +231,25 @@ namespace BlackMisc
         }
         for (auto it = namespaces.cbegin(); it != namespaces.cend(); ++it)
         {
-            QFile file(dir + "/" + it.key() + ".json");
-            if (! file.open(QFile::ReadWrite | QFile::Text))
+            QFile readFile(dir + "/" + it.key() + ".json");
+            if (! readFile.open(QFile::ReadWrite | QFile::Text))
             {
-                return CLogMessage(this).error("Failed to open %1: %2") << file.fileName() << file.errorString();
+                return CLogMessage(this).error("Failed to open %1: %2") << readFile.fileName() << readFile.errorString();
             }
-            auto json = QJsonDocument::fromJson(file.readAll());
+            auto json = QJsonDocument::fromJson(readFile.readAll());
             if (json.isArray() || (json.isNull() && ! json.isEmpty()))
             {
-                return CLogMessage(this).error("Invalid JSON format in %1") << file.fileName();
+                return CLogMessage(this).error("Invalid JSON format in %1") << readFile.fileName();
             }
             CVariantMap storedValues;
             storedValues.convertFromJson(json.object());
             storedValues.insert(*it);
             json.setObject(storedValues.toJson());
-            if (! (file.seek(0) && file.resize(0) && file.write(json.toJson()) > 0))
+            readFile.close();
+            CAtomicFile writeFile(readFile.fileName());
+            if (! (writeFile.open(QFile::WriteOnly | QFile::Text) && writeFile.write(json.toJson()) > 0 && writeFile.checkedClose()))
             {
-                return CLogMessage(this).error("Failed to write to %1: %2") << file.fileName() << file.errorString();
+                return CLogMessage(this).error("Failed to write to %1: %2") << writeFile.fileName() << writeFile.errorString();
             }
         }
         return {};