Bug 634666 - File streams perform disk I/O when being initialized. r=philikon a=blocking-fennec

services/sync/modules/util.js

@@ -1140,7 +1140,8 @@ let Utils = {
 
     let fos = Cc["@mozilla.org/network/safe-file-output-stream;1"]
                 .createInstance(Ci.nsIFileOutputStream);
-    fos.init(file, MODE_WRONLY | MODE_CREATE | MODE_TRUNCATE, PERMS_FILE, 0);
+    fos.init(file, MODE_WRONLY | MODE_CREATE | MODE_TRUNCATE, PERMS_FILE,
+             fos.DEFER_OPEN || 0);
     let is = this._utf8Converter.convertToInputStream(out);
     NetUtil.asyncCopy(is, fos, function (result) {
       if (typeof callback == "function") {