create interface QSqlDriver::cancelQuery()

This is useful for canceling a long running sql query.
Note that it needs support from the individual drivers.

Change-Id: Ia170a70487ff4ee13c85f12bc13e62fb198617fe
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>

src/sql/kernel/qsqldriver.cpp

@@ -789,4 +789,27 @@ QSql::NumericalPrecisionPolicy QSqlDriver::numericalPrecisionPolicy() const
     return d_func()->precisionPolicy;
 }
 
+/*!
+    \since 5.0
+    \internal
+
+    Tries to cancel the running query, if the underlying driver has the
+    capability to cancel queries. Returns true on success, otherwise false.
+
+    This function can be called from a different thread.
+
+    If you use this function as a slot, you need to use a Qt::DirectConnection
+    from a different thread.
+
+    Reimplement this function to support canceling running queries in
+    your own QSqlDriver subclass. It must be implemented in a thread-safe
+    manner.
+
+    \sa QSqlDriver::hasFeature()
+*/
+bool QSqlDriver::cancelQuery()
+{
+    return false;
+}
+
 QT_END_NAMESPACE

src/sql/kernel/qsqldriver.h

@@ -71,7 +71,7 @@ public:
     enum DriverFeature { Transactions, QuerySize, BLOB, Unicode, PreparedQueries,
                          NamedPlaceholders, PositionalPlaceholders, LastInsertId,
                          BatchOperations, SimpleLocking, LowPrecisionNumbers,
-                         EventNotifications, FinishQuery, MultipleResultSets };
+                         EventNotifications, FinishQuery, MultipleResultSets, CancelQuery };
 
     enum StatementType { WhereStatement, SelectStatement, UpdateStatement,
                          InsertStatement, DeleteStatement };
@@ -120,6 +120,9 @@ public:
     void setNumericalPrecisionPolicy(QSql::NumericalPrecisionPolicy precisionPolicy);
     QSql::NumericalPrecisionPolicy numericalPrecisionPolicy() const;
 
+public Q_SLOTS:
+    virtual bool cancelQuery();
+
 Q_SIGNALS:
     void notification(const QString &name);
     void notification(const QString &name, QSqlDriver::NotificationSource source, const QVariant &payload);