arc: Fix thread unsafe behavior of ArcDownloadsWatcherService

chrome/browser/chromeos/arc/arc_downloads_watcher_service.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 "chrome/browser/chromeos/arc/arc_downloads_watcher_service.h"
 
 #include <map>
 #include <memory>
 #include <utility>
 
@@ skipping 58 matching lines @@
 
   return changed_paths;
 }
 
 }  // namespace
 
 // The core part of ArcDownloadsWatcherService to watch for file changes in
 // Downloads directory.
 class ArcDownloadsWatcherService::DownloadsWatcher {
  public:
-  using Callback =
-      base::Callback<void(const std::vector<base::FilePath>& paths)>;
+  using Callback = base::Callback<void(mojo::Array<mojo::String> paths)>;
 
   explicit DownloadsWatcher(const Callback& callback);
   ~DownloadsWatcher();
 
   // Starts watching Downloads directory.
   void Start();
 
  private:
   // Called by base::FilePathWatcher to notify file changes.
   void OnFilePathChanged(const base::FilePath& path, bool error);
@@ skipping 49 matching lines @@
   // as one given to FilePathWatcher::Watch().
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
 
   TimestampMap current_timestamp_map = BuildTimestampMap();
 
   std::vector<base::FilePath> changed_paths =
       CollectChangedPaths(last_timestamp_map_, current_timestamp_map);
 
   last_timestamp_map_ = std::move(current_timestamp_map);
 
-  callback_.Run(changed_paths);
+  mojo::Array<mojo::String> mojo_paths(changed_paths.size());
+  for (size_t i = 0; i < changed_paths.size(); ++i) {
+    mojo_paths[i] = changed_paths[i].value();
+  }
+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE,
+      base::Bind(callback_, base::Passed(std::move(mojo_paths))));
 }
 
 TimestampMap ArcDownloadsWatcherService::DownloadsWatcher::BuildTimestampMap()
     const {
   DCHECK(!downloads_dir_.EndsWithSeparator());
   TimestampMap timestamp_map;
 
   // Enumerate normal files only; directories and symlinks are skipped.
   base::FileEnumerator enumerator(downloads_dir_, true,
                                   base::FileEnumerator::FILES);
@@ skipping 49 matching lines @@
 
 void ArcDownloadsWatcherService::StopWatchingDownloads() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   if (watcher_) {
     BrowserThread::DeleteSoon(BrowserThread::FILE, FROM_HERE,
                               watcher_.release());
   }
 }
 
 void ArcDownloadsWatcherService::OnDownloadsChanged(
-    const std::vector<base::FilePath>& paths) {
-  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+    mojo::Array<mojo::String> paths) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   auto* instance = arc_bridge_service()->file_system()->instance();
   if (!instance)
     return;
-
-  mojo::Array<mojo::String> mojo_paths(paths.size());
-  for (size_t i = 0; i < paths.size(); ++i) {
-    mojo_paths[i] = paths[i].value();
-  }
-  instance->RequestMediaScan(std::move(mojo_paths));
+  instance->RequestMediaScan(std::move(paths));
 }
 
 }  // namespace arc