DiffEditor: Fix a crash when "No difference"

It may happen that async task associated with diffing one pair
of files may not report any value. This happens when both
file contents are the same. In this case taking a result from
async task on a successful done will lead to crash.

Add AsyncTask::isResultAvailable() method. Use it in client code
just in case, where needed.

Fix DiffFilesController, so that no result is allowed for all
running tasks (i.e. make the main group optional).
Collect list of optional results instead of direct results.
The empty optional on the list means the result wasn't delivered
by async task and it's skipped.

Fixes: QTCREATORBUG-28750
Change-Id: I4ca678a187fad619bae470da3e806e8c8da61127
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

3 files changed, 14 insertions, 7 deletions

diff --git a/src/plugins/diffeditor/diffeditorplugin.cpp b/src/plugins/diffeditor/diffeditorplugin.cpp
index e18a0e2797..2a81320665 100644
--- a/src/plugins/diffeditor/diffeditorplugin.cpp
+++ b/src/plugins/diffeditor/diffeditorplugin.cpp
@@ -109,24 +109,25 @@ DiffFilesController::DiffFilesController(IDocument *document)
     setDisplayName(Tr::tr("Diff"));
     using namespace Tasking;
 
-    const TreeStorage<QList<FileData>> storage;
+    const TreeStorage<QList<std::optional<FileData>>> storage;
 
     const auto setupTree = [this, storage](TaskTree &taskTree) {
-        QList<FileData> *outputList = storage.activeStorage();
+        QList<std::optional<FileData>> *outputList = storage.activeStorage();
 
         const auto setupDiff = [this](AsyncTask<FileData> &async, const ReloadInput &reloadInput) {
             async.setAsyncCallData(DiffFile(ignoreWhitespace(), contextLineCount()), reloadInput);
             async.setFutureSynchronizer(Internal::DiffEditorPlugin::futureSynchronizer());
         };
         const auto onDiffDone = [outputList](const AsyncTask<FileData> &async, int i) {
-            (*outputList)[i] = async.result();
+            if (async.isResultAvailable())
+                (*outputList)[i] = async.result();
         };
 
         const QList<ReloadInput> inputList = reloadInputList();
         outputList->resize(inputList.size());
 
         using namespace std::placeholders;
-        QList<TaskItem> tasks {parallel, continueOnDone};
+        QList<TaskItem> tasks {parallel, optional};
         for (int i = 0; i < inputList.size(); ++i) {
             tasks.append(Async<FileData>(std::bind(setupDiff, _1, inputList.at(i)),
                                          std::bind(onDiffDone, _1, i)));
@@ -134,7 +135,13 @@ DiffFilesController::DiffFilesController(IDocument *document)
         taskTree.setupRoot(tasks);
     };
     const auto onTreeDone = [this, storage] {
-        setDiffFiles(*storage.activeStorage());
+        const QList<std::optional<FileData>> &results = *storage.activeStorage();
+        QList<FileData> finalList;
+        for (const std::optional<FileData> &result : results) {
+            if (result.has_value())
+                finalList.append(*result);
+        }
+        setDiffFiles(finalList);
     };
     const auto onTreeError = [this, storage] {
         setDiffFiles({});
diff --git a/src/plugins/diffeditor/sidebysidediffeditorwidget.cpp b/src/plugins/diffeditor/sidebysidediffeditorwidget.cpp
index 5bc887b90a..da69e60784 100644
--- a/src/plugins/diffeditor/sidebysidediffeditorwidget.cpp
+++ b/src/plugins/diffeditor/sidebysidediffeditorwidget.cpp
@@ -874,7 +874,7 @@ void SideBySideDiffEditorWidget::showDiff()
     m_controller.setBusyShowing(true);
 
     connect(m_asyncTask.get(), &AsyncTaskBase::done, this, [this] {
-        if (m_asyncTask->isCanceled()) {
+        if (m_asyncTask->isCanceled() || !m_asyncTask->isResultAvailable()) {
             for (SideDiffEditorWidget *editor : m_editor)
                 editor->clearAll(Tr::tr("Retrieving data failed."));
         } else {
diff --git a/src/plugins/diffeditor/unifieddiffeditorwidget.cpp b/src/plugins/diffeditor/unifieddiffeditorwidget.cpp
index 8c8bee05c2..b937b919f7 100644
--- a/src/plugins/diffeditor/unifieddiffeditorwidget.cpp
+++ b/src/plugins/diffeditor/unifieddiffeditorwidget.cpp
@@ -458,7 +458,7 @@ void UnifiedDiffEditorWidget::showDiff()
     m_asyncTask->setFutureSynchronizer(DiffEditorPlugin::futureSynchronizer());
     m_controller.setBusyShowing(true);
     connect(m_asyncTask.get(), &AsyncTaskBase::done, this, [this] {
-        if (m_asyncTask->isCanceled()) {
+        if (m_asyncTask->isCanceled() || !m_asyncTask->isResultAvailable()) {
             setPlainText(Tr::tr("Retrieving data failed."));
         } else {
             const ShowResult result = m_asyncTask->result();