Asynchronous functions should create their own fibers. More info at https://www.mixmax.com/blog/node-fibers-using-synchronize-js.

async-functions-should-create-own-fibers.js

function processFile(fileName, done) {
  sync.fiber(function() {
    var data = sync.await(fs.readFile(fileName, sync.defer()));
    return /* do something with the data */;
  }, done);
}