Flush WidgetInputHandler messages before running layout tests.

Layout test initialization is non-deterministic. WidgetInputHandler::OnFocus()
races against the contents of the layout test, which may also attempt to
set/unset focus -- the ordering between the two is non-deterministic. This CL
adds a FlushForTesting() message to WidgetInputHandler to make the ordering
explicit.

Bug: 889036, 889952
Change-Id: I26adca82915e75a9941c93b60a555f0c16084014
Reviewed-on: https://chromium-review.googlesource.com/c/1255782Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596321}

content/browser/renderer_host/render_widget_host_impl.cc

@@ -269,6 +269,10 @@ std::vector<DropData::Metadata> DropDataToMetaData(const DropData& drop_data) {
 
 class UnboundWidgetInputHandler : public mojom::WidgetInputHandler {
  public:
+  void FlushForTesting(FlushForTestingCallback callback) override {
+    CHECK(false) << "Flush for testing called on an unbound interface.";
+  }
+
   void SetFocus(bool focused) override {
     DLOG(WARNING) << "Input request on unbound interface";
   }
@@ -981,6 +985,12 @@ void RenderWidgetHostImpl::Blur() {
   focused_widget->SetPageFocus(false);
 }
 
+void RenderWidgetHostImpl::FlushForTesting(FlushForTestingCallback callback) {
+  // Flush IPC messages on WidgetInputHandler to ensure that SetFocus() messages
+  // have been processed.
+  GetWidgetInputHandler()->FlushForTesting(std::move(callback));
+}
+
 void RenderWidgetHostImpl::SetPageFocus(bool focused) {
   is_focused_ = focused;
 

content/browser/renderer_host/render_widget_host_impl.h

@@ -187,6 +187,7 @@ class CONTENT_EXPORT RenderWidgetHostImpl
   void NotifyTextDirection() override;
   void Focus() override;
   void Blur() override;
+  void FlushForTesting(FlushForTestingCallback callback) override;
   void SetActive(bool active) override;
   void ForwardMouseEvent(const blink::WebMouseEvent& mouse_event) override;
   void ForwardWheelEvent(const blink::WebMouseWheelEvent& wheel_event) override;

content/common/input/input_handler.mojom

@@ -198,6 +198,10 @@ interface WidgetInputHandlerHost {
 // an input interface for an associated Widget object. See FrameInputHandler
 // for an interface at the frame level.
 interface WidgetInputHandler {
+  // A simple mechanism by which the caller can ensure that all previous
+  // messages have been flushed.
+  FlushForTesting() => ();
+
   // Tells widget focus has been changed.
   SetFocus(bool focused);
 

content/public/browser/render_widget_host.h

@@ -165,6 +165,10 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Sender {
   virtual void Focus() = 0;
   virtual void Blur() = 0;
 
+  // Tests may need to flush IPCs to ensure deterministic behavior.
+  using FlushForTestingCallback = base::OnceClosure;
+  virtual void FlushForTesting(FlushForTestingCallback callback) = 0;
+
   // Sets whether the renderer should show controls in an active state.  On all
   // platforms except mac, that's the same as focused. On mac, the frontmost
   // window will show active controls even if the focus is not in the web

content/renderer/input/widget_input_handler_impl.cc

@@ -67,6 +67,18 @@ void WidgetInputHandlerImpl::SetBinding(
       base::BindOnce(&WidgetInputHandlerImpl::Release, base::Unretained(this)));
 }
 
+void WidgetInputHandlerImpl::FlushForTesting(FlushForTestingCallback callback) {
+  auto run_on_this_thread = base::BindOnce(
+      [](scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+         FlushForTestingCallback callback) {
+        task_runner->PostTask(FROM_HERE, std::move(callback));
+      },
+      base::ThreadTaskRunnerHandle::Get(), std::move(callback));
+
+  // Hop to main thread to ensure everything there is flushed.
+  RunOnMainThread(std::move(run_on_this_thread));
+}
+
 void WidgetInputHandlerImpl::SetFocus(bool focused) {
   RunOnMainThread(
       base::BindOnce(&RenderWidget::OnSetFocus, render_widget_, focused));

content/renderer/input/widget_input_handler_impl.h

@@ -32,6 +32,7 @@ class WidgetInputHandlerImpl : public mojom::WidgetInputHandler {
       mojom::WidgetInputHandlerAssociatedRequest interface_request);
   void SetBinding(mojom::WidgetInputHandlerRequest interface_request);
 
+  void FlushForTesting(FlushForTestingCallback callback) override;
   void SetFocus(bool focused) override;
   void MouseCaptureLost() override;
   void SetEditCommandsForNextKeyEvent(

content/shell/browser/layout_test/blink_test_controller.cc

@@ -408,6 +408,14 @@ bool BlinkTestController::PrepareForLayoutTest(
     if (is_devtools_js_test) {
       LoadDevToolsJSTest();
     } else {
+      // Flush IPC messages on the widget.
+      base::RunLoop run_loop;
+      main_window_->web_contents()
+          ->GetRenderViewHost()
+          ->GetWidget()
+          ->FlushForTesting(run_loop.QuitClosure());
+      run_loop.Run();
+
       // Loading the URL will immediately start the layout test. Manually call
       // LoadURLWithParams on the WebContents to avoid extraneous calls from
       // content::Shell such as SetFocus(), which could race with the layout

content/test/mock_widget_input_handler.cc

@@ -26,6 +26,10 @@ MockWidgetInputHandler::MockWidgetInputHandler(
 
 MockWidgetInputHandler::~MockWidgetInputHandler() {}
 
+void MockWidgetInputHandler::FlushForTesting(FlushForTestingCallback callback) {
+  std::move(callback).Run();
+}
+
 void MockWidgetInputHandler::SetFocus(bool focused) {
   dispatched_messages_.emplace_back(
       std::make_unique<DispatchedFocusMessage>(focused));

content/test/mock_widget_input_handler.h

@@ -192,6 +192,7 @@ class MockWidgetInputHandler : public mojom::WidgetInputHandler {
   };
 
   // mojom::WidgetInputHandler override.
+  void FlushForTesting(FlushForTestingCallback callback) override;
   void SetFocus(bool focused) override;
   void MouseCaptureLost() override;
   void SetEditCommandsForNextKeyEvent(