Don't block FILE thread for media scanning

chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc

Index: chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
diff --git a/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc b/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
index cb43c4c445b344f5b02f3a5048cf2c070f51d20c..689a627b9288f64378edcb5c4fc74fd44e63a7ef 100644
--- a/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
+++ b/chrome/browser/chromeos/arc/arc_downloads_watcher_service.cc
@@ -70,6 +70,36 @@ std::vector<base::FilePath> CollectChangedPaths(
   return changed_paths;
 }
 
+// Scans files under |downloads_dir| recursively and builds a map from file
+// paths (in Android filesystem) to last modified timestamps.
+TimestampMap BuildTimestampMap(base::FilePath downloads_dir) {
+  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);
+  for (base::FilePath cros_path = enumerator.Next(); !cros_path.empty();
+       cros_path = enumerator.Next()) {
+    // Android file path can be obtained by replacing |downloads_dir| prefix
+    // with |kAndroidDownloadDir|.
+    const base::FilePath& android_path =
+        base::FilePath(kAndroidDownloadDir)
+            .Append(
+                cros_path.value().substr(downloads_dir.value().length() + 1));
+    const base::FileEnumerator::FileInfo& info = enumerator.GetInfo();
+    timestamp_map[android_path] = info.GetLastModifiedTime();
+  }
+  return timestamp_map;
+}
+
+std::pair<base::Time, TimestampMap> BuildTimestampMapCallback(
+    base::FilePath downloads_dir) {
+  base::Time update_time = base::Time::Now();

[Shuhei Takahashi, 2016/09/08 04:25:48]

|last_update_time_| and |update_time| sounds a bit confusing, they are actually
different kind of timestamps. How about |last_notify_time_| and |snapshot_time|,
for example?

[dspaid, 2016/09/09 02:10:45]

Done.

+  TimestampMap current_timestamp_map = BuildTimestampMap(downloads_dir);
+  return std::make_pair(update_time, std::move(current_timestamp_map));
+}
+
 }  // namespace
 
 // The core part of ArcDownloadsWatcherService to watch for file changes in
@@ -86,16 +116,21 @@ class ArcDownloadsWatcherService::DownloadsWatcher {
 
  private:
   // Called by base::FilePathWatcher to notify file changes.
+  // Kicks off the update of last_timestamp_map_ if one is not already in
+  // progress.
   void OnFilePathChanged(const base::FilePath& path, bool error);
 
-  // Scans files under |downloads_dir_| recursively and builds a map from file
-  // paths (in Android filesystem) to last modified timestamps.
-  TimestampMap BuildTimestampMap() const;
+  void OnBuildTimestampMap(
+      std::pair<base::Time, TimestampMap> timestamp_and_map);
 
   Callback callback_;
   base::FilePath downloads_dir_;
   std::unique_ptr<base::FilePathWatcher> watcher_;
   TimestampMap last_timestamp_map_;
+  // The timestamp of the last OnFilePathChanged callback received.
+  base::Time last_update_time_;
+  // Whether or not there is an outstanding task to update last_timestamp_map_.
+  bool outstanding_task_;
 
   // Note: This should remain the last member so it'll be destroyed and
   // invalidate the weak pointers before any other members are destroyed.
@@ -106,7 +141,10 @@ class ArcDownloadsWatcherService::DownloadsWatcher {
 
 ArcDownloadsWatcherService::DownloadsWatcher::DownloadsWatcher(
     const Callback& callback)
-    : callback_(callback), weak_ptr_factory_(this) {
+    : callback_(callback),
+      last_update_time_(base::Time()),
+      outstanding_task_(false),
+      weak_ptr_factory_(this) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   downloads_dir_ = DownloadPrefs(ProfileManager::GetActiveUserProfile())
@@ -123,7 +161,10 @@ void ArcDownloadsWatcherService::DownloadsWatcher::Start() {
 
   // Initialize with the current timestamp map and avoid initial notification.
   // It is not needed since MediaProvider scans whole storage area on boot.
-  last_timestamp_map_ = BuildTimestampMap();
+  last_update_time_ = base::Time::Now();
+  last_timestamp_map_ = BuildTimestampMap(downloads_dir_);
+
+  outstanding_task_ = false;
 
   watcher_ = base::MakeUnique<base::FilePathWatcher>();
   // On Linux, base::FilePathWatcher::Watch() always returns true.
@@ -138,9 +179,23 @@ void ArcDownloadsWatcherService::DownloadsWatcher::OnFilePathChanged(
   // On Linux, |error| is always false. Also, |path| is always the same path
   // as one given to FilePathWatcher::Watch().
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  if (!outstanding_task_) {
+    base::PostTaskAndReplyWithResult(
+        BrowserThread::GetBlockingPool(), FROM_HERE,
+        base::Bind(&BuildTimestampMapCallback, downloads_dir_),
+        base::Bind(&DownloadsWatcher::OnBuildTimestampMap,
+                   weak_ptr_factory_.GetWeakPtr()));
+    outstanding_task_ = true;
+  } else {
+    last_update_time_ = base::Time::Now();
+  }
+}
 
-  TimestampMap current_timestamp_map = BuildTimestampMap();
-
+void ArcDownloadsWatcherService::DownloadsWatcher::OnBuildTimestampMap(
+    std::pair<base::Time, TimestampMap> timestamp_and_map) {
+  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  DCHECK(outstanding_task_);
+  TimestampMap current_timestamp_map = timestamp_and_map.second;
   std::vector<base::FilePath> changed_paths =
       CollectChangedPaths(last_timestamp_map_, current_timestamp_map);
 
@@ -153,28 +208,15 @@ void ArcDownloadsWatcherService::DownloadsWatcher::OnFilePathChanged(
   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);
-  for (base::FilePath cros_path = enumerator.Next(); !cros_path.empty();
-       cros_path = enumerator.Next()) {
-    // Android file path can be obtained by replacing |downloads_dir_| prefix
-    // with |kAndroidDownloadDir|.
-    const base::FilePath& android_path =
-        base::FilePath(kAndroidDownloadDir)
-            .Append(
-                cros_path.value().substr(downloads_dir_.value().length() + 1));
-    const base::FileEnumerator::FileInfo& info = enumerator.GetInfo();
-    timestamp_map[android_path] = info.GetLastModifiedTime();
+  if (last_update_time_ > timestamp_and_map.first) {
+    base::PostTaskAndReplyWithResult(
+        BrowserThread::GetBlockingPool(), FROM_HERE,
+        base::Bind(&BuildTimestampMapCallback, downloads_dir_),
+        base::Bind(&DownloadsWatcher::OnBuildTimestampMap,
+                   weak_ptr_factory_.GetWeakPtr()));
+  } else {
+    outstanding_task_ = false;
   }
-  return timestamp_map;
 }
 
 ArcDownloadsWatcherService::ArcDownloadsWatcherService(