mediaview: Support watchers in ArcDocumentsProviderRoot.

chrome/browser/chromeos/arc/fileapi/arc_documents_provider_root.h

Index: chrome/browser/chromeos/arc/fileapi/arc_documents_provider_root.h
diff --git a/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_root.h b/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_root.h
index fd8f8807e3b3ac777b6b68b4a91e35f4d1777311..031518912ba9e649f85d387ed5979b5dc81e5f0a 100644
--- a/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_root.h
+++ b/chrome/browser/chromeos/arc/fileapi/arc_documents_provider_root.h
@@ -5,6 +5,8 @@
 #ifndef CHROME_BROWSER_CHROMEOS_ARC_FILEAPI_ARC_DOCUMENTS_PROVIDER_ROOT_H_
 #define CHROME_BROWSER_CHROMEOS_ARC_FILEAPI_ARC_DOCUMENTS_PROVIDER_ROOT_H_
 
+#include <stdint.h>
+
 #include <map>
 #include <string>
 #include <vector>
@@ -16,6 +18,7 @@
 #include "base/optional.h"
 #include "components/arc/common/file_system.mojom.h"
 #include "storage/browser/fileapi/async_file_util.h"
+#include "storage/browser/fileapi/watcher_manager.h"
 
 class GURL;
 
@@ -30,6 +33,9 @@ class ArcDocumentsProviderRoot {
  public:
   using GetFileInfoCallback = storage::AsyncFileUtil::GetFileInfoCallback;
   using ReadDirectoryCallback = storage::AsyncFileUtil::ReadDirectoryCallback;
+  using ChangeType = storage::WatcherManager::ChangeType;
+  using WatcherCallback = base::Callback<void(ChangeType type)>;
+  using StatusCallback = base::Callback<void(base::File::Error error)>;
   using ResolveToContentUrlCallback =
       base::Callback<void(const GURL& content_url)>;
 
@@ -46,6 +52,49 @@ class ArcDocumentsProviderRoot {
   void ReadDirectory(const base::FilePath& path,
                      const ReadDirectoryCallback& callback);
 
+  // Installs a document watcher.
+  //
+  // It is not allowed to install multiple watchers at the same file path;
+  // if attempted, duplicated requests will fail.
+  //
+  // Currently, watchers can be installed only on directories, and only
+  // directory content changes are notified.
+  //
+  // NOTES ABOUT CORRECTNESS AND CONSISTENCY:
+  //
+  // Document watchers are not always correct and they may miss some updates or
+  // even notify incorrect update events for several reasons, such as:
+  //
+  //   - Racy file system operations.

[hidehiko, 2017/02/23 12:10:54]

Could you add a simple racy operations as an example?

[Shuhei Takahashi, 2017/03/02 07:06:01]

Sure, added more examples.

Also I added mention of a notable case about directory moves (I forgot to add
this once).

+  //   - Broken DocumentsProviders.
+  //   - Duplicated file name handling in this class.
+  //
+  // However, consistency of installed watchers is guaranteed. That is, after
+  // a watcher is installed on a file path X, an attempt to uninstall a watcher
+  // at X will always success (unless it is automatically uninstalled by
+  // a DELETED event).
+  //
+  // Unfortunately it is too difficult (or maybe theoretically impossible) to
+  // implement a perfect Android document watcher which never misses document
+  // updates. So the current implementation gives up correctness, but instead
+  // focuses on following two goals:
+  //
+  //   1. Keep the implementation simple, rather than trying hard to catch
+  //      race conditions or minor cases. Even if we return wrong results, the
+  //      worst consequence is just that users do not see the latest contents
+  //      until they refresh UI.
+  //
+  //   2. Keep consistency of installed watchers so that the caller can avoid
+  //      dangling watchers.
+  void AddWatcher(const base::FilePath& path,
+                  const WatcherCallback& watcher_callback,
+                  const StatusCallback& callback);
+
+  // Uninstalls a document watcher.
+  // See the documentation of AddWatcher() above.
+  void RemoveWatcher(const base::FilePath& path,
+                     const StatusCallback& callback);
+
   // Resolves a file path into a content:// URL pointing to the file
   // on DocumentsProvider. Returns URL that can be passed to
   // ArcContentFileSystemFileSystemReader to read the content.
@@ -82,6 +131,17 @@ class ArcDocumentsProviderRoot {
       base::File::Error error,
       NameToThinDocumentMap mapping);
 
+  void AddWatcherWithDocumentId(const base::FilePath& path,
+                                const WatcherCallback& watcher_callback,
+                                const StatusCallback& callback,
+                                const std::string& document_id);
+  void AddWatcherDone(const base::FilePath& path,
+                      const StatusCallback& callback,
+                      int64_t watcher_id);
+  void AddWatcherAborted(const StatusCallback& callback, bool success);
+
+  void RemoveWatcherDone(const StatusCallback& callback, bool success);
+
   void ResolveToContentUrlWithDocumentId(
       const ResolveToContentUrlCallback& callback,
       const std::string& document_id);
@@ -110,6 +170,15 @@ class ArcDocumentsProviderRoot {
 
   const std::string authority_;
   const std::string root_document_id_;
+
+  // Map from a file path to a watcher ID.
+  //
+  // Note that we do not use a document ID as a key here to guarantee that
+  // a watch installed by AddWatcher() can be always identified in
+  // RemoveWatcher() with the same file path specified.
+  // See the documentation of AddWatcher() for more details.
+  std::map<base::FilePath, int64_t> path_to_watcher_id_;
+
   base::WeakPtrFactory<ArcDocumentsProviderRoot> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ArcDocumentsProviderRoot);