[SyncFS] Make routines using metadata database async

chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc

 // Copyright 2014 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 <algorithm>
 #include <stack>
 
 #include "base/file_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
@@ skipping 485 matching lines @@
   drive::FakeDriveService* fake_drive_service() {
     return static_cast<drive::FakeDriveService*>(
         remote_sync_service_->GetDriveService());
   }
 
   FakeDriveServiceHelper* fake_drive_service_helper() {
     return fake_drive_service_helper_.get();
   }
 
   MetadataDatabase* metadata_database() {
-    return remote_sync_service_->GetMetadataDatabase();
+    return remote_sync_service_->GetMetadataDatabaseForTesting();

[nhiroki, 2014/05/29 04:12:58]

As chatted locally, this wrapper function easily hides the consumption I
mentioned in the previous comment. How about removing getter for metadata
database and accessing it asynchronously?

[peria, 2014/06/02 04:30:28]

Yes, direct calls of MetadataDatabase's methods are dangerous.
However, this getter is very useful for this test class.
I replaced all MetadataDatabase's methods with
PostTaskAndReplyWithResult and added a comment for it.
And I will add DCHECK() to check if MetadataDatabase runs on
worker task runner.

WDYT?

   }
 
  private:
   content::TestBrowserThreadBundle thread_bundle_;
   ScopedEnableSyncFSV2 enable_syncfs_v2_;
 
   base::ScopedTempDir base_dir_;
   scoped_ptr<leveldb::Env> in_memory_env_;
   TestingProfile profile_;
 
@@ skipping 1093 matching lines @@
 
   EXPECT_EQ(1u, CountApp());
   EXPECT_EQ(1u, CountLocalFile(app_id));
 
   EXPECT_EQ(2u, CountMetadata());
   EXPECT_EQ(2u, CountTracker());
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system