Take into account options.offset in onReadFileRequested.

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

Index: chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/background.js
diff --git a/chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/background.js b/chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/background.js
index bf78eb4ef8804de0b4d64d1f2e4dc6fd2f306fe5..9514ba7c7c377097f859ae1661f087eb4b416944 100644
--- a/chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/background.js
+++ b/chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/background.js
@@ -89,11 +89,19 @@ function onReadFileRequested(options, onSuccess, onError) {
   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. */);