Wait for history backend in popup blocker test

chrome/browser/history/history_test_utils.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <memory>
+#include <utility>
+
+#include "base/macros.h"
+#include "base/message_loop/message_loop.h"
+#include "base/task/cancelable_task_tracker.h"
+#include "chrome/browser/history/history_service_factory.h"
+#include "components/history/core/browser/history_db_task.h"
+#include "components/history/core/browser/history_service.h"
+#include "content/public/test/test_utils.h"
+
+namespace {
+
+// Note: WaitableEvent is not used for synchronization between the main thread
+// and history backend thread because the history subsystem posts tasks back
+// to the main thread. Had we tried to Signal an event in such a task
+// and Wait for it on the main thread, the task would not run at all because
+// the main thread would be blocked on the Wait call, resulting in a deadlock.
+
+// A task to be scheduled on the history backend thread.
+// Notifies the main thread after all history backend thread tasks have run.
+class WaitForHistoryTask : public history::HistoryDBTask {
+ public:
+  WaitForHistoryTask() {}
+
+  bool RunOnDBThread(history::HistoryBackend* backend,
+                     history::HistoryDatabase* db) override {
+    return true;
+  }
+
+  void DoneRunOnMainThread() override {
+    base::MessageLoop::current()->QuitWhenIdle();

[Avi (use Gerrit), 2016/11/12 20:35:55]

Use a MessageLoopRunner.

[jochen (gone - plz use gerrit), 2016/11/12 20:48:21]

not sure I understand? MessageLoopRunner doesn't have support for QuitWhenIdle?

[Avi (use Gerrit), 2016/11/12 20:58:41]

If you look at content/public/test/test_utils.cc line 50, it essentially
emulates QuitWhenIdle, so I think it's a drop-in replacement.

[jochen (gone - plz use gerrit), 2016/11/12 21:10:54]

yeah, but it gives up after a few tasks and quits anyways. I can see how that
makes sense for a message loop that has always tasks on it, but the history
backend loop should really be idle...

I'm also a bit hesitant about changing this code, as I just extracted it from
the existing history_browsertest.cc and I don't know for sure whether it depends
on this difference or not.

[Avi (use Gerrit), 2016/11/12 21:23:00]

Fair enough. I'm really not a fan of hammering directly on the MessageLoop,
because MessageLoopRunner has protections against things like the condition
firing before the wait happens, but I'll let this go.

+  }
+
+ private:
+  ~WaitForHistoryTask() override {}
+
+  DISALLOW_COPY_AND_ASSIGN(WaitForHistoryTask);
+};
+
+}  // namespace
+
+void WaitForHistoryBackendToRun(Profile* profile) {
+  base::CancelableTaskTracker task_tracker;
+  std::unique_ptr<history::HistoryDBTask> task(new WaitForHistoryTask());
+  history::HistoryService* history = HistoryServiceFactory::GetForProfile(
+      profile, ServiceAccessType::EXPLICIT_ACCESS);
+  history->ScheduleDBTask(std::move(task), &task_tracker);
+  content::RunMessageLoop();
+}