QSqlDatabase: add moveToThread()/currentThread()

Add QSqlDatabase::moveToThread() to be able to move the driver instance
to another thread.

[ChangeLog][Sql][QSqLDatabase] QSqlDatabase gained two new functions
moveToThread() and currentThread() to be able to use it in another
thread than the one it was created in.

Fixes: QTBUG-39957
Change-Id: I9cb51358f73a3a2fa72813bfdbe059279d388bd7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/sql/kernel/qsqldatabase.cpp

@@ -7,6 +7,7 @@
 #include "qcoreapplication.h"
 #include "qreadwritelock.h"
 #include "qsqldriver.h"
+#include "qsqldriver_p.h"
 #include "qsqldriverplugin.h"
 #include "qsqlindex.h"
 #include "QtCore/qapplicationstatic.h"
@@ -282,6 +283,10 @@ void QSqlDatabasePrivate::disable()
     QSqlDriver.  Alternatively, you can subclass your own database
     driver from QSqlDriver. See \l{How to Write Your Own Database
     Driver} for more information.
+    A QSqlDatabase instance must only be accessed by the thread it
+    was created in. Therefore you have to make sure to create them
+    in the correct context. Alternatively you can change the context
+    with QSqlDatabase::moveToThread().
 
     Create a connection (i.e., an instance of QSqlDatabase) by calling
     one of the static addDatabase() functions, where you specify
@@ -1333,6 +1338,50 @@ QSql::NumericalPrecisionPolicy QSqlDatabase::numericalPrecisionPolicy() const
         return d->precisionPolicy;
 }
 
+/*!
+    \since 6.8
+
+    Changes the thread affinity for QSqlDatabase and its associated driver.
+    This function returns \c true when the function succeeds. Event processing
+    will continue in the \a targetThread.
+
+    During this operation you have to make sure that there is no QSqlQuery
+    bound to this instance otherwise the QSqlDatabase will not be moved to
+    the given thread and the function returns \c false.
+
+    Since the associated driver is derived from QObject, all constraints for
+    moving a QObject to another thread also apply to this function.
+
+    \sa QObject::moveToThread(), {Threads and the SQL Module}
+*/
+bool QSqlDatabase::moveToThread(QThread *targetThread)
+{
+    if (auto drv = driver()) {
+        if (drv != QSqlDatabasePrivate::shared_null()->driver) {
+            // two instances are alive - the one here and the one in dbDict()
+            if (d->ref.loadRelaxed() > 2) {
+                qWarning("QSqlDatabasePrivate::moveToThread: connection '%ls' is still in use "
+                         "in the current thread.", qUtf16Printable(d->connName));
+                return false;
+            }
+            return drv->moveToThread(targetThread);
+        }
+    }
+    return false;
+}
+
+/*!
+    \since 6.8
+
+    Returns a pointer to the associated QThread instance.
+*/
+QThread *QSqlDatabase::currentThread() const
+{
+    if (auto drv = driver())
+        return drv->thread();
+    return nullptr;
+}
+
 
 #ifndef QT_NO_DEBUG_STREAM
 QDebug operator<<(QDebug dbg, const QSqlDatabase &d)

src/sql/kernel/qsqldatabase.h

@@ -18,6 +18,7 @@ class QSqlIndex;
 class QSqlRecord;
 class QSqlQuery;
 class QSqlDatabasePrivate;
+class QThread;
 
 class Q_SQL_EXPORT QSqlDriverCreatorBase
 {
@@ -80,6 +81,8 @@ public:
     QString connectionName() const;
     void setNumericalPrecisionPolicy(QSql::NumericalPrecisionPolicy precisionPolicy);
     QSql::NumericalPrecisionPolicy numericalPrecisionPolicy() const;
+    bool moveToThread(QThread *targetThread);
+    QThread *currentThread() const;
 
     QSqlDriver* driver() const;
 

tests/auto/sql/kernel/qsqldatabase/tst_qsqldatabase.cpp

@@ -104,6 +104,8 @@ private slots:
     void infinityAndNan();
     void multipleThreads_data() { generic_data(); }
     void multipleThreads();
+    void moveToThread_data() { generic_data(); }
+    void moveToThread();
 
     void db2_valueCacheUpdate_data() { generic_data("QDB2"); }
     void db2_valueCacheUpdate();
@@ -2335,5 +2337,32 @@ void tst_QSqlDatabase::multipleThreads()
     QTRY_VERIFY(t.isFinished());
 }
 
+void tst_QSqlDatabase::moveToThread()
+{
+    QFETCH(QString, dbName);
+    QSqlDatabase db = QSqlDatabase::database(dbName);
+    auto clonedDb = QSqlDatabase::cloneDatabase(db, "clonedDb");
+    auto mainThread = QThread::currentThread();
+    CHECK_DATABASE(db);
+    QCOMPARE(db.currentThread(), mainThread);
+    QCOMPARE(clonedDb.currentThread(), mainThread);
+    std::unique_ptr<QThread> t(QThread::create([&] {
+        db.moveToThread(mainThread);
+        QThread::currentThread()->exit();
+    }));
+    db.moveToThread(t.get());
+    QCOMPARE(db.currentThread(), t.get());
+    QCOMPARE(clonedDb.currentThread(), mainThread);
+    t->start();
+    QTRY_VERIFY(t->isRunning());
+    QTRY_VERIFY(t->wait(30000));
+    QCOMPARE(db.currentThread(), mainThread);
+    QCOMPARE(clonedDb.currentThread(), mainThread);
+    db = QSqlDatabase();
+    clonedDb = QSqlDatabase();
+    QSqlDatabase::removeDatabase("clonedDb");
+}
+
+
 QTEST_MAIN(tst_QSqlDatabase)
 #include "tst_qsqldatabase.moc"