Take into account options.offset in onReadFileRequested.

chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/background.js

 // Copyright 2014 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.
 
 'use strict';
 
 // Fake data similar to a file system structure.
 var MODIFICATION_DATE = new Date();
 var SHORT_CONTENTS = 'Just another example.';
 var LONGER_CONTENTS = 'It works!\nEverything gets displayed correctly.';
@@ skipping 71 matching lines @@
 
 function onReadFileRequested(options, onSuccess, onError) {
   if (!openedFiles[options.openRequestId]) {
     onError('INVALID_OPERATION');
     return;
   }
 
   var contents =
       METADATA[openedFiles[options.openRequestId]].contents;
 
+  var length = contents.length - options.offset;
+  if (length < 0) {
+    length = 0;
+  }
+  if (length > options.length) {
+    length = options.length;
+  }

[mtomasz, 2017/05/29 07:10:17]

I think we could simplify:

var remaining = Math.max(0, contents.length - options.offset);
var length = Math.min(remaining, options.length);

WDYT?

[tetsui, 2017/05/29 07:22:37]

Done.

+
   // Write the contents as ASCII text.
-  var buffer = new ArrayBuffer(options.length);
+  var buffer = new ArrayBuffer(length);
   var bufferView = new Uint8Array(buffer);
-  for (var i = 0; i < options.length; i++) {
-    bufferView[i] = contents.charCodeAt(i);
+  for (var i = 0; i < length; i++) {
+    bufferView[i] = contents.charCodeAt(i + options.offset);
   }
 
   onSuccess(buffer, false /* Last call. */);
 }
 
 function onMountRequested(onSuccess, onError) {
   chrome.fileSystemProvider.mount(
       {fileSystemId: 'sample-file-system', displayName: 'Sample File System'},
       function() {
         if (chrome.runtime.lastError) {
@@ skipping 23 matching lines @@
 chrome.fileSystemProvider.onGetMetadataRequested.addListener(
     onGetMetadataRequested);
 chrome.fileSystemProvider.onReadDirectoryRequested.addListener(
     onReadDirectoryRequested);
 chrome.fileSystemProvider.onOpenFileRequested.addListener(onOpenFileRequested);
 chrome.fileSystemProvider.onCloseFileRequested.addListener(
     onCloseFileRequested);
 chrome.fileSystemProvider.onReadFileRequested.addListener(onReadFileRequested);
 chrome.fileSystemProvider.onMountRequested.addListener(onMountRequested);
 chrome.fileSystemProvider.onUnmountRequested.addListener(onUnmountRequested);