Fix hang when dragging files from file picker onto QWebEngineView

The method WebContentsAdapter::updateDragPosition actively waits for the
UpdateDragCursor message, sent by the renderer. This active wait does
not work whenever we're currently in a base::MessageLoop::RunTask call,
because of its internal recursion guard nestable_tasks_allowed. Add a
check for nestable_tasks_allowed and bail out if we know that the active
wait will fail. This fixes the hang.

Ensure that the modal file picker dialog is shown outside of
base::MessageLoop::RunTask. This enables drag 'n drop updates from the
file picker to QWebEngineView.

Task-number: QTBUG-56488
Change-Id: Ia13ada9c19e7780e12ca633ab1caeac352aca2a9
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

2 files changed, 16 insertions, 1 deletions

diff --git a/src/core/web_contents_adapter.cpp b/src/core/web_contents_adapter.cpp
index 7285648fd..fc54c98ed 100644
--- a/src/core/web_contents_adapter.cpp
+++ b/src/core/web_contents_adapter.cpp
@@ -1159,6 +1159,16 @@ Qt::DropAction WebContentsAdapter::updateDragPosition(QDragMoveEvent *e, const Q
     rvh->DragTargetDragOver(toGfx(e->pos()), toGfx(screenPos), toWeb(e->possibleActions()),
                             blink::WebInputEvent::LeftButtonDown);
 
+    base::MessageLoop *currentMessageLoop = base::MessageLoop::current();
+    DCHECK(currentMessageLoop);
+    if (!currentMessageLoop->NestableTasksAllowed()) {
+        // We're already inside a MessageLoop::RunTask call, and scheduled tasks will not be
+        // executed. That means, updateDragAction will never be called, and the RunLoop below will
+        // remain blocked forever.
+        qWarning("WebContentsAdapter::updateDragPosition called from MessageLoop::RunTask.");
+        return Qt::IgnoreAction;
+    }
+
     // Wait until we get notified via RenderViewHostDelegateView::UpdateDragCursor. This calls
     // WebContentsAdapter::updateDragAction that will eventually quit the nested loop.
     base::RunLoop loop;
diff --git a/src/core/web_contents_delegate_qt.cpp b/src/core/web_contents_delegate_qt.cpp
index 06ae9c6da..757ae853c 100644
--- a/src/core/web_contents_delegate_qt.cpp
+++ b/src/core/web_contents_delegate_qt.cpp
@@ -74,6 +74,7 @@
 #include "ui/events/latency_info.h"
 
 #include <QDesktopServices>
+#include <QTimer>
 
 namespace QtWebEngineCore {
 
@@ -325,7 +326,11 @@ void WebContentsDelegateQt::RunFileChooser(content::WebContents *web_contents, c
         acceptedMimeTypes.append(toQt(*it));
 
     FilePickerController *controller = new FilePickerController(static_cast<FilePickerController::FileChooserMode>(params.mode), web_contents, toQt(params.default_file_name.value()), acceptedMimeTypes);
-    m_viewClient->runFileChooser(controller);
+
+    // Defer the call to not block base::MessageLoop::RunTask with modal dialogs.
+    QTimer::singleShot(0, [this, controller] () {
+        m_viewClient->runFileChooser(controller);
+    });
 }
 
 bool WebContentsDelegateQt::AddMessageToConsole(content::WebContents *source, int32_t level, const base::string16 &message, int32_t line_no, const base::string16 &source_id)