drive: Make callback parameters mandatory

Looking at the code, most callback parameters always take callbacks
hence can be mandatory. DriveSystemService::ClearCacheAndRemountFileSystem()
was an exception, so add a dummy callback for now with a TODO comment.

BUG=126634
TEST=none

Review URL: https://codereview.chromium.org/11446056

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@171715 0039d316-1c4b-4281-b951-d872f2087c98

13 files changed, 109 insertions, 38 deletions

diff --git a/chrome/browser/chromeos/drive/drive_scheduler.cc b/chrome/browser/chromeos/drive/drive_scheduler.cc
index 8c356c3..3e55b50 100644
--- a/chrome/browser/chromeos/drive/drive_scheduler.cc
+++ b/chrome/browser/chromeos/drive/drive_scheduler.cc
@@ -31,15 +31,18 @@ DriveScheduler::JobInfo::JobInfo(JobType in_job_type, FilePath in_file_path)
       total_bytes(0),
       file_path(in_file_path),
       state(STATE_NONE) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 }
 
 DriveScheduler::QueueEntry::QueueEntry(JobType in_job_type,
                                        FilePath in_file_path)
     : job_info(in_job_type, in_file_path),
       is_recursive(false) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 }
 
 DriveScheduler::QueueEntry::~QueueEntry() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 }
 
 DriveScheduler::DriveScheduler(
@@ -79,6 +82,7 @@ void DriveScheduler::Initialize() {
 void DriveScheduler::GetAccountMetadata(
     const google_apis::GetDataCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(
       new QueueEntry(TYPE_GET_ACCOUNT_METADATA, FilePath()));
@@ -92,6 +96,7 @@ void DriveScheduler::GetAccountMetadata(
 void DriveScheduler::GetApplicationInfo(
     const google_apis::GetDataCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(
       new QueueEntry(TYPE_GET_APPLICATION_INFO, FilePath()));
@@ -106,6 +111,7 @@ void DriveScheduler::Copy(const FilePath& src_file_path,
                           const FilePath& dest_file_path,
                           const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(new QueueEntry(TYPE_COPY, src_file_path));
   new_job->dest_file_path = dest_file_path;
@@ -124,6 +130,7 @@ void DriveScheduler::GetDocuments(
     const std::string& directory_resource_id,
     const google_apis::GetDataCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(
       new QueueEntry(TYPE_GET_DOCUMENTS, FilePath()));
@@ -144,6 +151,7 @@ void DriveScheduler::TransferFileFromRemoteToLocal(
     const FilePath& local_dest_file_path,
     const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(new QueueEntry(TYPE_TRANSFER_REMOTE_TO_LOCAL,
                                                 remote_src_file_path));
@@ -160,6 +168,7 @@ void DriveScheduler::TransferFileFromLocalToRemote(
     const FilePath& remote_dest_file_path,
     const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(new QueueEntry(TYPE_TRANSFER_LOCAL_TO_REMOTE,
                                                 local_src_file_path));
@@ -176,6 +185,7 @@ void DriveScheduler::TransferRegularFile(
     const FilePath& remote_dest_file_path,
     const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(new QueueEntry(TYPE_TRANSFER_REGULAR_FILE,
                                                 local_src_file_path));
@@ -191,6 +201,7 @@ void DriveScheduler::Move(const FilePath& src_file_path,
                           const FilePath& dest_file_path,
                           const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(new QueueEntry(TYPE_MOVE, src_file_path));
   new_job->dest_file_path = dest_file_path;
@@ -205,6 +216,7 @@ void DriveScheduler::Remove(const FilePath& file_path,
                             bool is_recursive,
                             const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   scoped_ptr<QueueEntry> new_job(new QueueEntry(TYPE_REMOVE, file_path));
   new_job->is_recursive = is_recursive;
@@ -354,6 +366,8 @@ void DriveScheduler::DoJobLoop() {
 }
 
 bool DriveScheduler::ShouldStopJobLoop() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
   // Should stop if the gdata feature was disabled while running the fetch
   // loop.
   if (profile_->GetPrefs()->GetBoolean(prefs::kDisableDrive))
@@ -411,6 +425,8 @@ void DriveScheduler::ResetThrottleAndContinueJobLoop() {
 scoped_ptr<DriveScheduler::QueueEntry> DriveScheduler::OnJobDone(
     int job_id,
     DriveFileError error) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
   JobMap::iterator job_iter = job_info_map_.find(job_id);
   DCHECK(job_iter != job_info_map_.end());
 
@@ -435,21 +451,24 @@ scoped_ptr<DriveScheduler::QueueEntry> DriveScheduler::OnJobDone(
 }
 
 void DriveScheduler::OnFileOperationJobDone(int job_id, DriveFileError error) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
   scoped_ptr<DriveScheduler::QueueEntry> job_info = OnJobDone(job_id, error);
 
   if (!job_info)
     return;
 
   // Handle the callback.
-  if (!job_info->file_operation_callback.is_null()) {
-    MessageLoop::current()->PostTask(FROM_HERE,
-        base::Bind(job_info->file_operation_callback, error));
-  }
+  base::MessageLoopProxy::current()->PostTask(
+      FROM_HERE,
+      base::Bind(job_info->file_operation_callback, error));
 }
 
 void DriveScheduler::OnGetDataJobDone(int job_id,
                                       google_apis::GDataErrorCode error,
                                       scoped_ptr<base::Value> feed_data) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
   DriveFileError drive_error(util::GDataToDriveFileError(error));
 
   scoped_ptr<QueueEntry> job_info = OnJobDone(job_id, drive_error);
@@ -457,13 +476,12 @@ void DriveScheduler::OnGetDataJobDone(int job_id,
   if (!job_info)
     return;
 
-    // Handle the callback.
-  if (!job_info->get_data_callback.is_null()) {
-    MessageLoop::current()->PostTask(FROM_HERE,
-        base::Bind(job_info->get_data_callback,
-                   error,
-                   base::Passed(&feed_data)));
-  }
+  // Handle the callback.
+  base::MessageLoopProxy::current()->PostTask(
+      FROM_HERE,
+      base::Bind(job_info->get_data_callback,
+                 error,
+                 base::Passed(&feed_data)));
 }
 
 void DriveScheduler::OnConnectionTypeChanged(
diff --git a/chrome/browser/chromeos/drive/drive_scheduler.h b/chrome/browser/chromeos/drive/drive_scheduler.h
index 7f1c6f8..5f8c2ba 100644
--- a/chrome/browser/chromeos/drive/drive_scheduler.h
+++ b/chrome/browser/chromeos/drive/drive_scheduler.h
@@ -89,17 +89,21 @@ class DriveScheduler
   void Initialize();
 
   // Adds a GetAccountMetadata operation to the queue.
+  // |callback| must not be null.
   void GetAccountMetadata(const google_apis::GetDataCallback& callback);
 
   // Adds a GetApplicationInfo operation to the queue.
+  // |callback| must not be null.
   void GetApplicationInfo(const google_apis::GetDataCallback& callback);
 
   // Adds a copy operation to the queue.
+  // |callback| must not be null.
   void Copy(const FilePath& src_file_path,
             const FilePath& dest_file_path,
             const FileOperationCallback& callback);
 
   // Adds a GetDocuments operation to the queue.
+  // |callback| must not be null.
   void GetDocuments(const GURL& feed_url,
                     int64 start_changestamp,
                     const std::string& search_query,
@@ -108,26 +112,31 @@ class DriveScheduler
                     const google_apis::GetDataCallback& callback);
 
   // Adds a transfer operation to the queue.
+  // |callback| must not be null.
   void TransferFileFromRemoteToLocal(const FilePath& remote_src_file_path,
                                      const FilePath& local_dest_file_path,
                                      const FileOperationCallback& callback);
 
   // Adds a transfer operation to the queue.
+  // |callback| must not be null.
   void TransferFileFromLocalToRemote(const FilePath& local_src_file_path,
                                      const FilePath& remote_dest_file_path,
                                      const FileOperationCallback& callback);
 
   // Adds a transfer operation to the queue.
+  // |callback| must not be null.
   void TransferRegularFile(const FilePath& local_src_file_path,
                            const FilePath& remote_dest_file_path,
                            const FileOperationCallback& callback);
 
   // Adds a move operation to the queue.
+  // |callback| must not be null.
   void Move(const FilePath& src_file_path,
             const FilePath& dest_file_path,
             const FileOperationCallback& callback);
 
   // Adds a remove operation to the queue.
+  // |callback| must not be null.
   void Remove(const FilePath& file_path,
               bool is_recursive,
               const FileOperationCallback& callback);
diff --git a/chrome/browser/chromeos/drive/drive_system_service.h b/chrome/browser/chromeos/drive/drive_system_service.h
index dbf54fa..da38b49 100644
--- a/chrome/browser/chromeos/drive/drive_system_service.h
+++ b/chrome/browser/chromeos/drive/drive_system_service.h
@@ -55,8 +55,10 @@ class DriveSystemService : public ProfileKeyedService,
   DriveWebAppsRegistry* webapps_registry() { return webapps_registry_.get(); }
   EventLogger* event_logger() { return event_logger_.get(); }
 
-  // Clears all the local cache files and in-memory data, and remounts the file
-  // system.
+  // Clears all the local cache files and in-memory data, and remounts the
+  // file system. |callback| is called with true when this operation is done
+  // successfully. Otherwise, |callback| is called with false.
+  // |callback| must not be null.
   void ClearCacheAndRemountFileSystem(
       const base::Callback<void(bool)>& callback);
 
diff --git a/chrome/browser/chromeos/drive/file_system/copy_operation.cc b/chrome/browser/chromeos/drive/file_system/copy_operation.cc
index fbc033b..d495f45 100644
--- a/chrome/browser/chromeos/drive/file_system/copy_operation.cc
+++ b/chrome/browser/chromeos/drive/file_system/copy_operation.cc
@@ -58,16 +58,6 @@ DriveFileError GetLocalFileInfoOnBlockingPool(
       DRIVE_FILE_OK : DRIVE_FILE_ERROR_NOT_FOUND;
 }
 
-// Helper function called upon completion of AddUploadFile invoked by
-// OnTransferCompleted.
-// TODO(mtomasz): The same method is in drive_file_system.cc. Share it.
-void OnAddUploadFileCompleted(const FileOperationCallback& callback,
-                              DriveFileError error) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  if (!callback.is_null())
-    callback.Run(error);
-}
-
 // Checks if a local file at |local_file_path| is a JSON file referencing a
 // hosted document on blocking pool, and if so, gets the resource ID of the
 // document.
@@ -201,6 +191,7 @@ void CopyOperation::TransferRegularFile(
     const FilePath& remote_dest_file_path,
     const FileOperationCallback& callback) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   int64* file_size = new int64;
   std::string* content_type = new std::string;
@@ -439,11 +430,10 @@ void CopyOperation::StartFileUpload(
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   DCHECK(file_size);
   DCHECK(content_type);
+  DCHECK(!params.callback.is_null());
 
   if (error != DRIVE_FILE_OK) {
-    if (!params.callback.is_null())
-      params.callback.Run(error);
-
+    params.callback.Run(error);
     return;
   }
 
@@ -465,13 +455,13 @@ void CopyOperation::StartFileUploadAfterGetEntryInfo(
     DriveFileError error,
     scoped_ptr<DriveEntryProto> entry_proto) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!params.callback.is_null());
 
   if (entry_proto.get() && !entry_proto->file_info().is_directory())
     error = DRIVE_FILE_ERROR_NOT_A_DIRECTORY;
 
   if (error != DRIVE_FILE_OK) {
-    if (!params.callback.is_null())
-      params.callback.Run(error);
+    params.callback.Run(error);
     return;
   }
   DCHECK(entry_proto.get());
@@ -496,6 +486,7 @@ void CopyOperation::OnTransferCompleted(
     const FilePath& file_path,
     scoped_ptr<DocumentEntry> document_entry) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
 
   if (error == google_apis::DRIVE_UPLOAD_OK && document_entry.get()) {
     drive_file_system_->AddUploadedFile(
@@ -503,8 +494,8 @@ void CopyOperation::OnTransferCompleted(
         document_entry.Pass(),
         file_path,
         DriveCache::FILE_OPERATION_COPY,
-        base::Bind(&OnAddUploadFileCompleted, callback));
-  } else if (!callback.is_null()) {
+        callback);
+  } else {
     callback.Run(DriveUploadErrorToDriveFileError(error));
   }
 }
diff --git a/chrome/browser/chromeos/drive/file_system/copy_operation.h b/chrome/browser/chromeos/drive/file_system/copy_operation.h
index 02c77f1..8aa1380 100644
--- a/chrome/browser/chromeos/drive/file_system/copy_operation.h
+++ b/chrome/browser/chromeos/drive/file_system/copy_operation.h
@@ -47,7 +47,7 @@ class CopyOperation {
 
   // Performs the copy operation on the file at drive path |src_file_path|
   // with a target of |dest_file_path|. Invokes |callback| when finished with
-  // the result of the operation.
+  // the result of the operation. |callback| must not be null.
   virtual void Copy(const FilePath& src_file_path,
                     const FilePath& dest_file_path,
                     const FileOperationCallback& callback);
@@ -81,6 +81,7 @@ class CopyOperation {
   // path within Drive file system.
   //
   // Must be called from *UI* thread. |callback| is run on the calling thread.
+  // |callback| must not be null.
   virtual void TransferRegularFile(const FilePath& local_file_path,
                                    const FilePath& remote_dest_file_path,
                                    const FileOperationCallback& callback);
diff --git a/chrome/browser/chromeos/drive/file_system/drive_operations.cc b/chrome/browser/chromeos/drive/file_system/drive_operations.cc
index 030dbe1..6e181ac 100644
--- a/chrome/browser/chromeos/drive/file_system/drive_operations.cc
+++ b/chrome/browser/chromeos/drive/file_system/drive_operations.cc
@@ -4,18 +4,24 @@
 
 #include "chrome/browser/chromeos/drive/file_system/drive_operations.h"
 
+#include "base/callback.h"
 #include "chrome/browser/chromeos/drive/file_system/copy_operation.h"
 #include "chrome/browser/chromeos/drive/file_system/move_operation.h"
 #include "chrome/browser/chromeos/drive/file_system/remove_operation.h"
 #include "chrome/browser/chromeos/drive/file_system/update_operation.h"
+#include "content/public/browser/browser_thread.h"
+
+using content::BrowserThread;
 
 namespace drive {
 namespace file_system {
 
 DriveOperations::DriveOperations() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 }
 
 DriveOperations::~DriveOperations() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 }
 
 void DriveOperations::Init(
@@ -26,6 +32,8 @@ void DriveOperations::Init(
     google_apis::DriveUploaderInterface* uploader,
     scoped_refptr<base::SequencedTaskRunner> blocking_task_runner,
     OperationObserver* observer) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
   copy_operation_.reset(new file_system::CopyOperation(drive_service,
                                                        drive_file_system,
                                                        metadata,
@@ -50,6 +58,8 @@ void DriveOperations::InitForTesting(CopyOperation* copy_operation,
                                      MoveOperation* move_operation,
                                      RemoveOperation* remove_operation,
                                      UpdateOperation* update_operation) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
   copy_operation_.reset(copy_operation);
   move_operation_.reset(move_operation);
   remove_operation_.reset(remove_operation);
@@ -59,6 +69,9 @@ void DriveOperations::InitForTesting(CopyOperation* copy_operation,
 void DriveOperations::Copy(const FilePath& src_file_path,
                            const FilePath& dest_file_path,
                            const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   copy_operation_->Copy(src_file_path, dest_file_path, callback);
 }
 
@@ -66,6 +79,9 @@ void DriveOperations::TransferFileFromRemoteToLocal(
     const FilePath& remote_src_file_path,
     const FilePath& local_dest_file_path,
     const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   copy_operation_->TransferFileFromRemoteToLocal(remote_src_file_path,
                                                  local_dest_file_path,
                                                  callback);
@@ -75,6 +91,9 @@ void DriveOperations::TransferFileFromLocalToRemote(
     const FilePath& local_src_file_path,
     const FilePath& remote_dest_file_path,
     const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   copy_operation_->TransferFileFromLocalToRemote(local_src_file_path,
                                                  remote_dest_file_path,
                                                  callback);
@@ -84,6 +103,9 @@ void DriveOperations::TransferRegularFile(
     const FilePath& local_src_file_path,
     const FilePath& remote_dest_file_path,
     const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   copy_operation_->TransferRegularFile(local_src_file_path,
                                        remote_dest_file_path,
                                        callback);
@@ -92,18 +114,27 @@ void DriveOperations::TransferRegularFile(
 void DriveOperations::Move(const FilePath& src_file_path,
                            const FilePath& dest_file_path,
                            const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   move_operation_->Move(src_file_path, dest_file_path, callback);
 }
 
 void DriveOperations::Remove(const FilePath& file_path,
                              bool is_recursive,
                              const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   remove_operation_->Remove(file_path, is_recursive, callback);
 }
 
 void DriveOperations::UpdateFileByResourceId(
     const std::string& resource_id,
     const FileOperationCallback& callback) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   update_operation_->UpdateFileByResourceId(resource_id, callback);
 }
 
diff --git a/chrome/browser/chromeos/drive/file_system/drive_operations.h b/chrome/browser/chromeos/drive/file_system/drive_operations.h
index 602492a..d25b2d8 100644
--- a/chrome/browser/chromeos/drive/file_system/drive_operations.h
+++ b/chrome/browser/chromeos/drive/file_system/drive_operations.h
@@ -51,36 +51,43 @@ class DriveOperations {
                       UpdateOperation* update_operation);
 
   // Wrapper function for copy_operation_.
+  // |callback| must not be null.
   void Copy(const FilePath& src_file_path,
             const FilePath& dest_file_path,
             const FileOperationCallback& callback);
 
   // Wrapper function for copy_operation_.
+  // |callback| must not be null.
   void TransferFileFromRemoteToLocal(const FilePath& remote_src_file_path,
                                      const FilePath& local_dest_file_path,
                                      const FileOperationCallback& callback);
 
   // Wrapper function for copy_operation_.
+  // |callback| must not be null.
   void TransferFileFromLocalToRemote(const FilePath& local_src_file_path,
                                      const FilePath& remote_dest_file_path,
                                      const FileOperationCallback& callback);
 
   // Wrapper function for copy_operation_.
+  // |callback| must not be null.
   void TransferRegularFile(const FilePath& local_src_file_path,
                            const FilePath& remote_dest_file_path,
                            const FileOperationCallback& callback);
 
   // Wrapper function for move_operation_.
+  // |callback| must not be null.
   void Move(const FilePath& src_file_path,
             const FilePath& dest_file_path,
             const FileOperationCallback& callback);
 
   // Wrapper function for remove_operation_.
+  // |callback| must not be null.
   void Remove(const FilePath& file_path,
               bool is_recursive,
               const FileOperationCallback& callback);
 
   // Wrapper function for update_operation_.
+  // |callback| must not be null.
   void UpdateFileByResourceId(const std::string& resource_id,
                               const FileOperationCallback& callback);
 
diff --git a/chrome/browser/chromeos/drive/file_system/move_operation.cc b/chrome/browser/chromeos/drive/file_system/move_operation.cc
index 956530b..c62ebd7 100644
--- a/chrome/browser/chromeos/drive/file_system/move_operation.cc
+++ b/chrome/browser/chromeos/drive/file_system/move_operation.cc
@@ -109,8 +109,9 @@ void MoveOperation::OnFilePathUpdated(const FileOperationCallback& callback,
                                         DriveFileError error,
                                         const FilePath& /* file_path */) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  if (!callback.is_null())
-    callback.Run(error);
+  DCHECK(!callback.is_null());
+
+  callback.Run(error);
 }
 
 void MoveOperation::Rename(const FilePath& file_path,
diff --git a/chrome/browser/chromeos/drive/file_system/move_operation.h b/chrome/browser/chromeos/drive/file_system/move_operation.h
index 9f8f010..d62eef8 100644
--- a/chrome/browser/chromeos/drive/file_system/move_operation.h
+++ b/chrome/browser/chromeos/drive/file_system/move_operation.h
@@ -40,7 +40,7 @@ class MoveOperation {
 
   // Performs the move operation on the file at drive path |src_file_path|
   // with a target of |dest_file_path|.  Invokes |callback| when finished with
-  // the result of the operation.
+  // the result of the operation. |callback| must not be null.
   virtual void Move(const FilePath& src_file_path,
                     const FilePath& dest_file_path,
                     const FileOperationCallback& callback);
diff --git a/chrome/browser/chromeos/drive/file_system/remove_operation.cc b/chrome/browser/chromeos/drive/file_system/remove_operation.cc
index 86e9eb4..aae5c1f 100644
--- a/chrome/browser/chromeos/drive/file_system/remove_operation.cc
+++ b/chrome/browser/chromeos/drive/file_system/remove_operation.cc
@@ -107,11 +107,13 @@ void RemoveOperation::NotifyDirectoryChanged(
     const FileOperationCallback& callback,
     DriveFileError error,
     const FilePath& directory_path) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!callback.is_null());
+
   if (error == DRIVE_FILE_OK)
     observer_->OnDirectoryChangedByOperation(directory_path);
 
-  if (!callback.is_null())
-    callback.Run(error);
+  callback.Run(error);
 }
 
 }  // namespace file_system
diff --git a/chrome/browser/chromeos/drive/file_system/remove_operation.h b/chrome/browser/chromeos/drive/file_system/remove_operation.h
index 2d61c32..3054ceb 100644
--- a/chrome/browser/chromeos/drive/file_system/remove_operation.h
+++ b/chrome/browser/chromeos/drive/file_system/remove_operation.h
@@ -41,6 +41,7 @@ class RemoveOperation {
 
   // Perform the remove operation on the file at drive path |file_path|.
   // Invokes |callback| when finished with the result of the operation.
+  // |callback| must not be null.
   virtual void Remove(const FilePath& file_path,
                       bool is_recursive,
                       const FileOperationCallback& callback);
@@ -62,7 +63,7 @@ class RemoveOperation {
       google_apis::GDataErrorCode status);
 
   // Sends notification for directory changes. Notifies of directory changes,
-  // and runs |callback| with |error|. |callback| may be null.
+  // and runs |callback| with |error|. |callback| must not be null.
   void NotifyDirectoryChanged(
       const FileOperationCallback& callback,
       DriveFileError error,
diff --git a/chrome/browser/chromeos/drive/file_system/update_operation.h b/chrome/browser/chromeos/drive/file_system/update_operation.h
index 7eb7e51..0349a5e 100644
--- a/chrome/browser/chromeos/drive/file_system/update_operation.h
+++ b/chrome/browser/chromeos/drive/file_system/update_operation.h
@@ -94,6 +94,7 @@ class UpdateOperation {
       scoped_ptr<google_apis::DocumentEntry> document_entry);
 
   // Part of UpdateFileByResourceId().
+  // |callback| must not be null.
   void OnUpdatedFileRefreshed(const FileOperationCallback& callback,
                               DriveFileError error,
                               const FilePath& drive_file_path,
diff --git a/chrome/browser/chromeos/extensions/file_browser_private_api.cc b/chrome/browser/chromeos/extensions/file_browser_private_api.cc
index 81c0587..6f794a80 100644
--- a/chrome/browser/chromeos/extensions/file_browser_private_api.cc
+++ b/chrome/browser/chromeos/extensions/file_browser_private_api.cc
@@ -398,6 +398,12 @@ void LogDefaultTask(const std::set<std::string>& mime_types,
   }
 }
 
+// Does nothing with a bool parameter. Used as a placeholder for calling
+// ClearCacheAndRemountFileSystem(). TODO(yoshiki): Handle an error from
+// ClearCacheAndRemountFileSystem() properly: http://crbug.com/140511.
+void DoNothingWithBool(bool /* success */) {
+}
+
 }  // namespace
 
 class RequestLocalFileSystemFunction::LocalFileSystemCallbackDispatcher {
@@ -2787,7 +2793,8 @@ bool ClearDriveCacheFunction::RunImpl() {
 
   // TODO(yoshiki): Receive a callback from JS-side and pass it to
   // ClearCacheAndRemountFileSystem(). http://crbug.com/140511
-  system_service->ClearCacheAndRemountFileSystem(base::Callback<void(bool)>());
+  system_service->ClearCacheAndRemountFileSystem(
+      base::Bind(&DoNothingWithBool));
 
   SendResponse(true);
   return true;