fs.readFile 跟踪解读

fs.readFile = function(path, options, callback_) {
  var callback = maybeCallback(arguments[arguments.length - 1]);

  if (!options || typeof options === 'function') {
    options = { encoding: null, flag: 'r' };
  } else if (typeof options === 'string') {
    options = { encoding: options, flag: 'r' };
  } else if (typeof options !== 'object') {
    throwOptionsError(options);
  }

  var encoding = options.encoding;
  assertEncoding(encoding);

  var flag = options.flag || 'r';

  if (!nullCheck(path, callback))
    return;

  var context = new ReadFileContext(callback, encoding);
  context.isUserFd = isFd(path); // file descriptor ownership
  var req = new FSReqWrap();
  req.context = context;
  req.oncomplete = readFileAfterOpen;

  if (context.isUserFd) {
    process.nextTick(function() {
      req.oncomplete(null, path);
    });
    return;
  }

  binding.open(pathModule._makeLong(path),
               stringToFlags(flag),
               0o666,
               req);
};

重点callback 未跟踪到