Refactor Database to keep sql operations in single thread

src/database.cpp

@@ -18,60 +18,47 @@
 
 #include "database.h"
 #include "models/playlistmodel.h"
+#include "mainwindow.h"
 #include <QtSql>
 #include <QStandardPaths>
 #include <QDir>
-#include <QMutexLocker>
 #include <QtDebug>
 
+struct DatabaseJob {
+    enum Type {
+        PutThumbnail,
+        GetThumbnail
+    } type;
+
+    QImage image;
+    QString hash;
+    bool result;
+    bool completed;
+    DatabaseJob()
+        : result(false)
+        , completed(false)
+    {}
+};
+
 static Database* instance = 0;
 
 Database::Database(QObject *parent) :
-    QObject(parent)
+    QThread(parent)
+    , m_commitTimer(0)
 {
-    QDir dir(QStandardPaths::standardLocations(QStandardPaths::DataLocation).first());
-    if (!dir.exists())
-        dir.mkpath(dir.path());
-
-    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
-    db.setDatabaseName(dir.filePath("db.sqlite3"));
-    db.open();
-
-    // Initialize version table, if needed.
-    int version = 0;
-    QSqlQuery query;
-    if (query.exec("CREATE TABLE version (version INTEGER);")) {
-        if (!query.exec("INSERT INTO version VALUES (0);"))
-            qCritical() << __PRETTY_FUNCTION__ << "Failed to create version table.";
-    } else if (query.exec("SELECT version FROM version")) {
-        query.next();
-        version = query.value(0).toInt();
-    } else {
-        qCritical() << __PRETTY_FUNCTION__ << "Failed to get version.";
-    }
-    if (version < 1 && upgradeVersion1())
-        version = 1;
-    qDebug() << "Database version is" << version;
 }
 
 Database &Database::singleton(QWidget *parent)
 {
-    if (!instance)
+    if (!instance) {
         instance = new Database(parent);
+        instance->start();
+    }
     return *instance;
 }
 
-Database::~Database()
-{
-    QString connection = QSqlDatabase::database().connectionName();
-    QSqlDatabase::database().close();
-    QSqlDatabase::removeDatabase(connection);
-    instance = 0;
-}
-
 bool Database::upgradeVersion1()
 {
-    QMutexLocker lock(&m_mutex);
     bool success = false;
     QSqlQuery query;
     if (query.exec("CREATE TABLE thumbnails (hash TEXT PRIMARY KEY NOT NULL, accessed DATETIME NOT NULL, image BLOB);")) {
@@ -84,46 +71,94 @@ bool Database::upgradeVersion1()
     return success;
 }
 
-bool Database::putThumbnail(const QString& hash, const QImage& image)
+void Database::doJob(DatabaseJob * job)
 {
-    QMutexLocker lock(&m_mutex);
-    QByteArray ba;
-    QBuffer buffer(&ba);
-    buffer.open(QIODevice::WriteOnly);
-    image.save(&buffer, "PNG");
+    if (!m_commitTimer->isActive())
+        QSqlDatabase::database().transaction();
+    m_commitTimer->start();
 
-    QSqlQuery query;
-    query.prepare("DELETE FROM thumbnails WHERE hash = :hash;");
-    query.bindValue(":hash", hash);
-    query.exec();
-    query.prepare("INSERT INTO thumbnails VALUES (:hash, datetime('now'), :image);");
-    query.bindValue(":hash", hash);
-    query.bindValue(":image", ba);
-    bool result = query.exec();
-    if (!result)
-        qCritical() << __FUNCTION__ << query.lastError();
+    if (job->type == DatabaseJob::PutThumbnail) {
+        QByteArray ba;
+        QBuffer buffer(&ba);
+        buffer.open(QIODevice::WriteOnly);
+        job->image.save(&buffer, "PNG");
+
+        QSqlQuery query;
+        query.prepare("DELETE FROM thumbnails WHERE hash = :hash;");
+        query.bindValue(":hash", job->hash);
+        query.exec();
+        query.prepare("INSERT INTO thumbnails VALUES (:hash, datetime('now'), :image);");
+        query.bindValue(":hash", job->hash);
+        query.bindValue(":image", ba);
+        job->result = query.exec();
+        if (!job->result)
+            qCritical() << __FUNCTION__ << query.lastError();
+    } else if (job->type == DatabaseJob::GetThumbnail) {
+        QImage result;
+        QSqlQuery query;
+        query.prepare("SELECT image FROM thumbnails WHERE hash = :hash;");
+        query.bindValue(":hash", job->hash);
+        if (query.exec() && query.first()) {
+            result.loadFromData(query.value(0).toByteArray(), "PNG");
+            QSqlQuery update;
+            update.prepare("UPDATE thumbnails SET accessed = datetime('now') WHERE hash = :hash ;");
+            update.bindValue(":hash", job->hash);
+            if (!update.exec())
+                qCritical() << __FUNCTION__ << update.lastError();
+        }
+        job->image = result;
+    }
     deleteOldThumbnails();
-    return result;
+    job->completed = true;
 }
 
-QImage Database::getThumbnail(const QString &hash)
+void Database::commitTransaction()
 {
-    QMutexLocker lock(&m_mutex);
-    QImage result;
-    QSqlQuery query;
-    query.prepare("SELECT image FROM thumbnails WHERE hash = :hash;");
-    query.bindValue(":hash", hash);
-    if (query.exec() && query.first()) {
-        result.loadFromData(query.value(0).toByteArray(), "PNG");
-        QSqlQuery update;
-        update.prepare("UPDATE thumbnails SET accessed = datetime('now') WHERE hash = :hash ;");
-        update.bindValue(":hash", hash);
-        if (!update.exec())
-            qCritical() << __FUNCTION__ << update.lastError();
+    QSqlDatabase::database().commit();
+}
+
+bool Database::putThumbnail(const QString& hash, const QImage& image)
+{
+    DatabaseJob job;
+    job.type = DatabaseJob::PutThumbnail;
+    job.hash = hash;
+    job.image = image;
+    submitAndWaitForJob(&job);
+    return job.result;
+}
+
+void Database::submitAndWaitForJob(DatabaseJob * job)
+{
+    job->completed = false;
+    m_mutex.lock();
+    m_jobs.append(job);
+    if (m_jobs.size() == 1) {
+        //worker was idle until now
+        m_waitForNewJob.wakeAll();
     }
-//    qDebug() << __FUNCTION__ << result.byteCount();
-    deleteOldThumbnails();
-    return result;
+    while (!job->completed) {
+        m_waitForFinished.wait(&m_mutex);
+    }
+    m_mutex.unlock();
+}
+
+QImage Database::getThumbnail(const QString &hash)
+{
+    DatabaseJob job;
+    job.type = DatabaseJob::GetThumbnail;
+    job.hash = hash;
+    submitAndWaitForJob(&job);
+    return job.image;
+}
+
+void Database::shutdown()
+{
+    requestInterruption();
+    wait();
+    QString connection = QSqlDatabase::database().connectionName();
+    QSqlDatabase::database().close();
+    QSqlDatabase::removeDatabase(connection);
+    instance = 0;
 }
 
 void Database::deleteOldThumbnails()
@@ -134,3 +169,59 @@ void Database::deleteOldThumbnails()
         qCritical() << __FUNCTION__ << query.lastError();
 }
 
+void Database::run()
+{
+    connect(&MAIN, SIGNAL(aboutToShutDown()),
+            this, SLOT(shutdown()), Qt::DirectConnection);
+
+    QDir dir(QStandardPaths::standardLocations(QStandardPaths::DataLocation).first());
+    if (!dir.exists())
+        dir.mkpath(dir.path());
+
+    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
+    db.setDatabaseName(dir.filePath("db.sqlite3"));
+    db.open();
+
+    m_commitTimer = new QTimer();
+    m_commitTimer->setSingleShot(true);
+    m_commitTimer->setInterval(5000);
+    connect(m_commitTimer, SIGNAL(timeout()),
+            this, SLOT(commitTransaction()));
+
+    // Initialize version table, if needed.
+    int version = 0;
+    QSqlQuery query;
+    if (query.exec("CREATE TABLE version (version INTEGER);")) {
+        if (!query.exec("INSERT INTO version VALUES (0);"))
+            qCritical() << __PRETTY_FUNCTION__ << "Failed to create version table.";
+    } else if (query.exec("SELECT version FROM version")) {
+        query.next();
+        version = query.value(0).toInt();
+    } else {
+        qCritical() << __PRETTY_FUNCTION__ << "Failed to get version.";
+    }
+    if (version < 1 && upgradeVersion1())
+        version = 1;
+    qDebug() << "Database version is" << version;
+
+    while (true) {
+        DatabaseJob * newJob = 0;
+        m_mutex.lock();
+        if (m_jobs.isEmpty())
+            m_waitForNewJob.wait(&m_mutex, 1000);
+        else
+            newJob = m_jobs.takeFirst();
+        m_mutex.unlock();
+        QCoreApplication::processEvents();
+        if (newJob) {
+            doJob(newJob);
+            m_waitForFinished.wakeAll();
+        }
+        if (isInterruptionRequested())
+            break;
+    }
+    if (m_commitTimer->isActive())
+        commitTransaction();
+    delete m_commitTimer;
+}
+

src/database.h

@@ -19,26 +19,42 @@
 #ifndef DATABASE_H
 #define DATABASE_H
 
-#include <QObject>
+#include <QThread>
 #include <QImage>
 #include <QMutex>
+#include <QWaitCondition>
 
-class Database : public QObject
+struct DatabaseJob;
+class QTimer;
+class Database : public QThread
 {
     Q_OBJECT
     explicit Database(QObject *parent = 0);
 
 public:
     static Database& singleton(QWidget* parent = 0);
-    ~Database();
 
     bool upgradeVersion1();
     bool putThumbnail(const QString& hash, const QImage& image);
     QImage getThumbnail(const QString& hash);
 
+private slots:
+    void commitTransaction();
+
+private slots:
+    void shutdown();
+
 private:
+    void doJob(DatabaseJob * job);
+    void submitAndWaitForJob(DatabaseJob * job);
     void deleteOldThumbnails();
+    void run();
+
+    QList<DatabaseJob*> m_jobs;
     QMutex m_mutex;
+    QWaitCondition m_waitForFinished;
+    QWaitCondition m_waitForNewJob;
+    QTimer * m_commitTimer;
 };
 
 #define DB Database::singleton()

src/mainwindow.cpp

@@ -1599,6 +1599,7 @@ void MainWindow::closeEvent(QCloseEvent* event)
         if (!m_htmlEditor || m_htmlEditor->close()) {
             writeSettings();
             event->accept();
+            emit aboutToShutDown();
             QApplication::exit(m_exitCode);
             return;
         }

src/mainwindow.h

@@ -77,6 +77,7 @@ class MainWindow : public QMainWindow
     void producerOpened();
     void profileChanged();
     void openFailed(QString);
+    void aboutToShutDown();
 
 protected:
     MainWindow();