mediaview: Change the contract of watcher callbacks.

components/arc/common/file_system.mojom

 // 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.
 //
 // Next MinVersion: 4
 
 module arc.mojom;
 
 // Represents a document in Android DocumentsProvider.
 // See Android docs of DocumentsContract.Document for details.
@@ skipping 19 matching lines @@
 };
 
 // Describes the type of a change made to a document.
 [Extensible]
 enum ChangeType {
   // Indicates that the child document list of the watched directory was
   // changed. Note that a watcher can be installed only on directory for now.
   CHANGED = 0,
 
   // Indicates that the watched document itself was deleted.
-  // When OnDocumentChanged() is called with this change type, the corresponding
-  // watcher has been already uninstalled and the host should not call
-  // RemoveWatcher().
+  // Even if OnDocumentChanged() is called with this change type, the
+  // corresponding watcher is not uninstalled automatically. The host must
+  // call RemoveWatcher() to clean up an orphaned watcher.
   DELETED = 1,
 };
 
 
 // Next method ID: 1
 interface FileSystemHost {
   // Called when a watched document was changed.
-  // |type| describes the type of change made to the document. If |type| is
-  // DELETED, the watcher has been already removed and the host should not call
-  // RemoveWatcher().
+  // |type| describes the type of change made to the document.
   [MinVersion=3] OnDocumentChanged@0(int64 watcher_id, ChangeType type);
 };
 
 // Next method ID: 8
 interface FileSystemInstance {
   // Notes about Android Documents Provider:
   //
   // In Android Storage Access Framework, a document is uniquely identified by
   // a pair of "authority" and "document ID".
   //
@@ skipping 12 matching lines @@
   //
   // Note: Currently, watchers can be installed only on directories, and only
   // directory content changes are notified.
   //
   // On success, a positive unique integer is returned as a watcher ID.
   // FileSystemHost.OnDocumentChanged() will be called with the watcher ID
   // on directory content changes.
   // On failure, -1 is returned.
   //
   // It is allowed to install multiple watchers to the same directory. In that
   // case, different watcher IDs are returned.

[hidehiko, 2017/03/02 14:20:28]

Optional: How about adding the lifetime of watcher? E.g.;

Watcher IDs are not persistent. I.e., on connection lost, including ARC crash,
all watcher IDs are invalidated.

[Shuhei Takahashi, 2017/03/02 16:13:15]

Sounds great! Let's do so.

   [MinVersion=3] AddWatcher@6(string authority, string document_id) =>
       (int64 watcher_id);
 
   // Queries child documents of the directory specified by |authority| and
   // |parent_document_id| in Documents Provider.
   // If such a directory does not exist, null is returned.
   [MinVersion=2] GetChildDocuments@4(string authority,
                                      string parent_document_id) =>
       (array<Document>? documents);
 
@@ skipping 14 matching lines @@
   // Asks the ContentResolver to get a FD to read the file specified by the
   // URL.
   [MinVersion=1] OpenFileToRead@2(string url) => (handle? fd);
 
   // Uninstalls a document watcher.
   //
   // After this method call returns, OnDocumentChanged() will never be called
   // with the watcher ID. Whether OnDocumentChanged() is called or not after
   // this method is called and before this method returns is undefined.
   //
-  // It fails if the specified watcher does not exist. Note that a watcher
-  // can be automatically uninstalled when a watched document is deleted
-  // (notified by OnDocumentChanged() with |type| = DELETED).
+  // It fails if the specified watcher does not exist.
   [MinVersion=3] RemoveWatcher@7(int64 watcher_id) => (bool success);
 
   // Requests MediaProvider to scan specified files.
   // When the specified file does not exist, the corresponding entry in
   // MediaProvider is removed.
   RequestMediaScan@0(array<string> paths);
 };