vcsbase: fix bug causing redundant output

In VCSJobRunner::task() some execution paths left a signal/slot
connection, causing redundant output emission.
This would occur for example after a VCS job fails : if the next
job succeeds then its output is emitted twice, because the previous
connection for signal output() was not destroyed.
The bug is now fixed by ensuring disconnect is done for all execution
paths (thanks to helper DisconnectSignalHelper).

Change-Id: I67a7ba2829208f7e71158cb17a99575d79c9f9f7
Merge-request: 364
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt.nokia.com/2997

src/plugins/vcsbase/vcsjobrunner.cpp

@@ -49,6 +49,30 @@
 #include <QtCore/QWaitCondition>
 #include <QtCore/QSharedPointer>
 
+namespace {
+
+//Helper class to automatically disconnect a signal from all its receivers.
+//The disconnect occurs on destruction of the helper object.
+class DisconnectSignalHelper
+{
+public:
+    DisconnectSignalHelper(QObject *sender, const char *signal) :
+        m_sender(sender), m_signal(signal)
+    {
+    }
+
+    ~DisconnectSignalHelper()
+    {
+        QObject::disconnect(m_sender, m_signal, 0, 0);
+    }
+
+private:
+    QObject *m_sender;
+    const char *m_signal;
+};
+
+} // Anonymous namespace
+
 /*!
     \class  VCSBase::VCSJob
 
@@ -290,6 +314,11 @@ void VCSJobRunner::task(const QSharedPointer<VCSJob> &job)
         break;
     }
 
+    //the signal connection is to last only for the duration of a job/task.  next time a new
+    //output signal connection must be made
+    DisconnectSignalHelper autoDisconnectOutputSig(this, SIGNAL(output(QByteArray)));
+    Q_UNUSED(autoDisconnectOutputSig);
+
     const QStringList args = d->m_standardArguments + taskData->arguments();
     emit commandStarted(VCSBase::VCSBaseOutputWindow::msgExecutionLogEntry(taskData->workingDirectory(), d->m_binary, args));
     //infom the user of what we are going to try and perform
@@ -341,9 +370,6 @@ void VCSJobRunner::task(const QSharedPointer<VCSJob> &job)
     }
 
     vcsProcess->close();
-    //the signal connection is to last only for the duration of a job/task.  next time a new
-    //output signal connection must be made
-    disconnect(this, SIGNAL(output(QByteArray)), 0, 0);
 }
 
 } // namespace VCSBase