[Sync] Refactor ModelSafeWorker::DoWorkAndWaitUntilDone() to avoid code duplication.

components/history/core/browser/history_model_worker.cc

 // Copyright 2012 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 "components/history/core/browser/history_model_worker.h"
 
 #include <utility>
 
-#include "base/synchronization/waitable_event.h"
+#include "base/memory/ptr_util.h"
 #include "components/history/core/browser/history_db_task.h"
 #include "components/history/core/browser/history_service.h"
-#include "components/sync/base/scoped_event_signal.h"
 
 namespace browser_sync {
 
 class WorkerTask : public history::HistoryDBTask {
  public:
-  WorkerTask(const syncer::WorkCallback& work,
-             syncer::ScopedEventSignal scoped_event_signal,
-             syncer::SyncerError* error)
-      : work_(work),
-        scoped_event_signal_(std::move(scoped_event_signal)),
-        error_(error) {}
+  WorkerTask(base::OnceClosure work) : work_(std::move(work)) {}
 
   bool RunOnDBThread(history::HistoryBackend* backend,
                      history::HistoryDatabase* db) override {
-    // Signal the completion event at the end of this scope.
-    auto scoped_event_signal = std::move(scoped_event_signal_);
-
-    // Run the task.
-    *error_ = work_.Run();
-
+    std::move(work_).Run();
     return true;
   }
 
   // Since the DoWorkAndWaitUntilDone() is synchronous, we don't need to run
   // any code asynchronously on the main thread after completion.
   void DoneRunOnMainThread() override {}
 
- protected:
-  ~WorkerTask() override {
-    // The event in |scoped_event_signal_| is signaled at the end of this
-    // scope if this is destroyed before RunOnDBThread runs.
-  }
+ private:
+  // A OnceClosure is deleted right after it runs. This is important to unblock
+  // DoWorkAndWaitUntilDone() right after the task runs.
+  base::OnceClosure work_;
 
-  syncer::WorkCallback work_;
-  syncer::ScopedEventSignal scoped_event_signal_;
-  syncer::SyncerError* error_;
-};
-
-class AddDBThreadObserverTask : public history::HistoryDBTask {
- public:
-  explicit AddDBThreadObserverTask(base::Closure register_callback)
-     : register_callback_(register_callback) {}
-
-  bool RunOnDBThread(history::HistoryBackend* backend,
-                     history::HistoryDatabase* db) override {
-    register_callback_.Run();
-    return true;
-  }
-
-  void DoneRunOnMainThread() override {}
-
- private:
-  ~AddDBThreadObserverTask() override {}
-
-  base::Closure register_callback_;
+  DISALLOW_COPY_AND_ASSIGN(WorkerTask);
 };
 
 namespace {
 
 // Post the work task on |history_service|'s DB thread from the UI
 // thread.
 void PostWorkerTask(
     const base::WeakPtr<history::HistoryService>& history_service,
-    const syncer::WorkCallback& work,
-    syncer::ScopedEventSignal scoped_event_signal,
-    base::CancelableTaskTracker* cancelable_tracker,
-    syncer::SyncerError* error) {
+    base::OnceClosure work,
+    base::CancelableTaskTracker* cancelable_tracker) {
   if (history_service.get()) {
-    std::unique_ptr<history::HistoryDBTask> task(
-        new WorkerTask(work, std::move(scoped_event_signal), error));
-    history_service->ScheduleDBTask(std::move(task), cancelable_tracker);
-  } else {
-    *error = syncer::CANNOT_DO_WORK;
-    // The event in |scoped_event_signal| is signaled at the end of this
-    // scope.
+    history_service->ScheduleDBTask(
+        base::MakeUnique<WorkerTask>(std::move(work)), cancelable_tracker);
   }
 }
 
 }  // namespace
 
 HistoryModelWorker::HistoryModelWorker(
     const base::WeakPtr<history::HistoryService>& history_service,
     const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread)
     : history_service_(history_service), ui_thread_(ui_thread) {
   CHECK(history_service.get());
   DCHECK(ui_thread_->BelongsToCurrentThread());
   cancelable_tracker_.reset(new base::CancelableTaskTracker);
 }
 
-syncer::SyncerError HistoryModelWorker::DoWorkAndWaitUntilDoneImpl(
-    const syncer::WorkCallback& work) {
-  syncer::SyncerError error = syncer::UNSET;
-
-  // Signaled after the task runs or when it is abandoned.
-  base::WaitableEvent work_done_or_abandoned(
-      base::WaitableEvent::ResetPolicy::AUTOMATIC,
-      base::WaitableEvent::InitialState::NOT_SIGNALED);
-
-  if (ui_thread_->PostTask(FROM_HERE,
-                           base::Bind(&PostWorkerTask, history_service_, work,
-                                      base::Passed(syncer::ScopedEventSignal(
-                                          &work_done_or_abandoned)),
-                                      cancelable_tracker_.get(), &error))) {
-    work_done_or_abandoned.Wait();
-  } else {
-    error = syncer::CANNOT_DO_WORK;
-  }
-  return error;
-}
-
 syncer::ModelSafeGroup HistoryModelWorker::GetModelSafeGroup() {
   return syncer::GROUP_HISTORY;
 }
 
 bool HistoryModelWorker::IsOnModelThread() {
   // Ideally HistoryService would expose a way to check whether this is the
   // history DB thread. Since it doesn't, just return true to bypass a CHECK in
   // the sync code.
   return true;
 }
 
 HistoryModelWorker::~HistoryModelWorker() {
   // The base::CancelableTaskTracker class is not thread-safe and must only be
   // used from a single thread but the current object may not be destroyed from
   // the UI thread, so delete it from the UI thread.
   ui_thread_->DeleteSoon(FROM_HERE, cancelable_tracker_.release());
 }
 
+void HistoryModelWorker::ScheduleWork(base::OnceClosure work) {
+  ui_thread_->PostTask(FROM_HERE, base::Bind(&PostWorkerTask, history_service_,
+                                             base::Passed(std::move(work)),
+                                             cancelable_tracker_.get()));
+}
+
 }  // namespace browser_sync