Fix a possible crash in FindReferences on shutdown

Cancel and wait for all threads started by FindReferences
in its destructor.

Task-number: QTCREATORBUG-25711
Change-Id: I83c3ea832a173a7fff5c52e0c0136dfff2234afe
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmljseditor/qmljsfindreferences.cpp

@@ -815,6 +815,7 @@ FindReferences::FindReferences(QObject *parent)
     m_watcher.setPendingResultsLimit(1);
     connect(&m_watcher, &QFutureWatcherBase::resultsReadyAt, this, &FindReferences::displayResults);
     connect(&m_watcher, &QFutureWatcherBase::finished, this, &FindReferences::searchFinished);
+    m_synchronizer.setCancelOnWait(true);
 }
 
 FindReferences::~FindReferences() = default;
@@ -922,6 +923,7 @@ void FindReferences::findUsages(const QString &fileName, quint32 offset)
     QFuture<Usage> result = Utils::runAsync(&find_helper, ModelManagerInterface::workingCopy(),
                                             modelManager->snapshot(), fileName, offset, QString());
     m_watcher.setFuture(result);
+    m_synchronizer.addFuture(result);
 }
 
 void FindReferences::renameUsages(const QString &fileName, quint32 offset,
@@ -937,6 +939,7 @@ void FindReferences::renameUsages(const QString &fileName, quint32 offset,
     QFuture<Usage> result = Utils::runAsync(&find_helper, ModelManagerInterface::workingCopy(),
                                             modelManager->snapshot(), fileName, offset, newName);
     m_watcher.setFuture(result);
+    m_synchronizer.addFuture(result);
 }
 
 QList<FindReferences::Usage> FindReferences::findUsageOfType(const QString &fileName, const QString &typeName)

src/plugins/qmljseditor/qmljsfindreferences.h

@@ -27,6 +27,8 @@
 
 #include "qmljseditor_global.h"
 
+#include <utils/futuresynchronizer.h>
+
 #include <QObject>
 #include <QFutureWatcher>
 #include <QPointer>
@@ -81,6 +83,7 @@ private:
 
     QPointer<Core::SearchResult> m_currentSearch;
     QFutureWatcher<Usage> m_watcher;
+    Utils::FutureSynchronizer m_synchronizer;
 };
 
 } // namespace QmlJSEditor