fileSystemProvider: First cut at implementing fileSystemProvider API

chrome/common/extensions/api/file_system_provider.idl

+// Copyright 2013 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.
+
+[platforms=("chromeos")]
+namespace fileSystemProvider {
+  // TODO(satorux): Switch to DOMError once crbug.com/313131 is fixed.
+  // http://www.w3.org/TR/file-system-api/#errors-and-exceptions
+  dictionary Error {
+    // On success, the value is "OK". On error, one of names described at the
+    // following URL is set.
+    // http://www.w3.org/TR/file-system-api/#errors-and-exceptions
+    DOMString name;
+
+    // Error message, that is present in case of a failure.
+    DOMString message;
+  };
+
+  // Callback to receive the result of mount() function. <code>error</code>
+  // indicates if the request was successful or not. See <code>Error</code>
+  // for details.
+  //
+  // On success, <code>fileSystemID</code> will be a unique ID for the file
+  // system just mounted. <code>fileSystemID</code> is undefined on error.
+  // The ID is used to distinguish multiple file systems mounted from a
+  // single File System Provider.
+  callback MountCallback = void(Error error,
+                                DOMString fileSystemId);

[not at google - send to devlin, 2013/11/01 00:18:10]

I understand why you want to do this - setting chrome.runtime.lastError to a
DOMError would be a confusing API, and I agree that you'll want to use a
DOMError to match the file system API.

If you want consistency with DOM, how about using 2 callbacks? A success
callback and a failure callback. That model will be easier to move to a
Promise-based API when we do that, compared to including the error inline.

[satorux1, 2013/11/01 01:02:04]

That would be great but can we do that? Actually, kinuko@ also suggested that
way, and the API would look like:

  mount(DOMString displayName,
        MountCallback callback,
        ErrorCallback errorCallback);

However, seems to me that extension function can only take one callback. My
understanding is that "result_ = Results::Create(...); SendResponse(true)" will
run the callback passed as the last parameter of an API function).

Maybe we can emulate the two-callback API with custom bindings?

+
+  interface Functions {
+    // Mounts a file system with the given
+    // <code>displayName</code>. <code>displayName</code> will be shown in
+    // the left panel of Files.app. <code>displayName</code> can contain any
+    // characters including '/'. <code>displayName</code> should be
+    // descritive but doesn't have to be unique. Duplicate display names are
+    // uniquified by adding suffix like "(1)" in the Files.app UI. An empty
+    // display name is not allowed.
+    static void mount(DOMString displayName,
+                      MountCallback callback);
+  };
+};