Clang: Avoid blocking UI if references are requested and clangbackend crashes

The two cases in which the UI might be blocked are:
 * Invocation of the editor context menu
 * Rename Symbol Under Cursor

Cancel the future if the backend is restarted and on the other side,
check whether the future is cancelled.

Change-Id: If2315da1f66f15eab1531fcd8da1dff851a9a4e6
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangbackendipcintegration.cpp

@@ -99,7 +99,7 @@ IpcReceiver::IpcReceiver()
 
 IpcReceiver::~IpcReceiver()
 {
-    deleteAndClearWaitingAssistProcessors();
+    reset();
 }
 
 void IpcReceiver::setAliveHandler(const IpcReceiver::AliveHandler &handler)
@@ -116,12 +116,6 @@ void IpcReceiver::addExpectedCodeCompletedMessage(
     m_assistProcessorsTable.insert(ticket, processor);
 }
 
-void IpcReceiver::deleteAndClearWaitingAssistProcessors()
-{
-    qDeleteAll(m_assistProcessorsTable.begin(), m_assistProcessorsTable.end());
-    m_assistProcessorsTable.clear();
-}
-
 void IpcReceiver::deleteProcessorsOfEditorWidget(TextEditor::TextEditorWidget *textEditorWidget)
 {
     QMutableHashIterator<quint64, ClangCompletionAssistProcessor *> it(m_assistProcessorsTable);
@@ -155,6 +149,18 @@ bool IpcReceiver::isExpectingCodeCompletedMessage() const
     return !m_assistProcessorsTable.isEmpty();
 }
 
+void IpcReceiver::reset()
+{
+    // Clean up waiting assist processors
+    qDeleteAll(m_assistProcessorsTable.begin(), m_assistProcessorsTable.end());
+    m_assistProcessorsTable.clear();
+
+    // Clean up futures for references
+    for (ReferencesEntry &entry : m_referencesTable)
+        entry.futureInterface.cancel();
+    m_referencesTable.clear();
+}
+
 void IpcReceiver::alive()
 {
     if (printAliveMessage())
@@ -719,7 +725,7 @@ void IpcCommunicator::onConnectedToBackend()
     if (m_connectedCount > 1)
         logRestartedDueToUnexpectedFinish();
 
-    m_ipcReceiver.deleteAndClearWaitingAssistProcessors();
+    m_ipcReceiver.reset();
     m_ipcSender.reset(new IpcSender(m_connection));
 
     initializeBackendWithCurrentData();

src/plugins/clangcodemodel/clangbackendipcintegration.h

@@ -73,14 +73,14 @@ public:
     void setAliveHandler(const AliveHandler &handler);
 
     void addExpectedCodeCompletedMessage(quint64 ticket, ClangCompletionAssistProcessor *processor);
-    void deleteAndClearWaitingAssistProcessors();
     void deleteProcessorsOfEditorWidget(TextEditor::TextEditorWidget *textEditorWidget);
 
     QFuture<CppTools::CursorInfo> addExpectedReferencesMessage(quint64 ticket,
                                                                QTextDocument *textDocument);
-
     bool isExpectingCodeCompletedMessage() const;
 
+    void reset();
+
 private:
     void alive() override;
     void echo(const ClangBackEnd::EchoMessage &message) override;