Fix a crash with newest library

This is basically a replica of
4c0020385a
for Spectral.

src/imageprovider.cpp

@@ -4,39 +4,38 @@
 #include <jobs/mediathumbnailjob.h>
 
 #include <QtCore/QDebug>
+#include <QtCore/QThread>
 #include <QtCore/QReadWriteLock>
 
 using QMatrixClient::BaseJob;
-using QMatrixClient::Connection;
 
-ThumbnailResponse::ThumbnailResponse(Connection* c, QString mediaId,
-                                     const QSize& requestedSize)
-    : c(c),
-      mediaId(std::move(mediaId)),
-      requestedSize(requestedSize),
-      errorStr("Image request hasn't started") {
-  moveToThread(c->thread());
+ThumbnailResponse::ThumbnailResponse(QMatrixClient::Connection* c,
+    QString mediaId, const QSize& requestedSize)
+  : c(c),
+    mediaId(std::move(mediaId)),
+    requestedSize(requestedSize),
+    errorStr("Image request hasn't started") {
   if (requestedSize.isEmpty()) {
     errorStr.clear();
     emit finished();
     return;
   }
+  if (mediaId.count('/') != 1) {
+    errorStr =
+        tr("Media id '%1' doesn't follow server/mediaId pattern")
+            .arg(mediaId);
+    emit finished();
+    return;
+  }
   // Execute a request on the main thread asynchronously
+  moveToThread(c->thread());
   QMetaObject::invokeMethod(this, &ThumbnailResponse::startRequest,
                             Qt::QueuedConnection);
 }
 
 void ThumbnailResponse::startRequest() {
   // Runs in the main thread, not QML thread
-  if (mediaId.count('/') != 1) {
-    errorStr =
-        QStringLiteral("Media id '%1' doesn't follow server/mediaId pattern")
-            .arg(mediaId);
-    emit finished();
-    return;
-  }
-
-  QWriteLocker _(&lock);
+  Q_ASSERT(QThread::currentThread() == c->thread());
   job = c->getThumbnail(mediaId, requestedSize);
   // Connect to any possible outcome including abandonment
   // to make sure the QML thread is not left stuck forever.
@@ -44,13 +43,16 @@ void ThumbnailResponse::startRequest() {
 }
 
 void ThumbnailResponse::prepareResult() {
+  Q_ASSERT(QThread::currentThread() == job->thread());
+  Q_ASSERT(job->error() != BaseJob::Pending);  
   {
     QWriteLocker _(&lock);
-    Q_ASSERT(job->error() != BaseJob::Pending);
-
     if (job->error() == BaseJob::Success) {
       image = job->thumbnail();
       errorStr.clear();
+    } else if (job->error() == BaseJob::Abandoned) {
+      errorStr = tr("Image request has been cancelled");
+      qDebug() << "ThumbnailResponse: cancelled for" << mediaId;  
     } else {
       errorStr = job->errorString();
       qWarning() << "ThumbnailResponse: no valid image for" << mediaId << "-"
@@ -61,6 +63,13 @@ void ThumbnailResponse::prepareResult() {
   emit finished();
 }
 
+void ThumbnailResponse::doCancel() {
+  // Runs in the main thread, not QML thread
+  Q_ASSERT(QThread::currentThread() == job->thread());
+  if (job)
+    job->abandon();  
+}
+
 QQuickTextureFactory* ThumbnailResponse::textureFactory() const {
   QReadLocker _(&lock);
   return QQuickTextureFactory::textureFactoryForImage(image);
@@ -72,12 +81,8 @@ QString ThumbnailResponse::errorString() const {
 }
 
 void ThumbnailResponse::cancel() {
-  QWriteLocker _(&lock);
-  if (job) {
-    job->abandon();
-    job = nullptr;
-  }
-  errorStr = "Image request has been cancelled";
+  QMetaObject::invokeMethod(this, &ThumbnailResponse::doCancel,
+                            Qt::QueuedConnection);
 }
 
 QQuickImageResponse* ImageProvider::requestImageResponse(