cachedAsyncFn.js

function cachedAsyncFn(fn) {
  var cached, queue;
  return function (callback) {
    if (cached) {
      process.nextTick(function () {
        callback.apply(null, cached);
      });
    } else if (queue) {
      queue.push(callback);
    } else {
      queue = [callback];
      fn(function () {
        cached = Array.prototype.slice.call(arguments);
        queue.forEach(function (cb) {
          cb.apply(null, cached);
        });
        queue = null;
      });
    }
  };
}

//// Example

var getConnection = cachedAsyncFn( asyncFnThatGivesMeAConnection );

getConnection(function (err, conn) {
  // i have connection
});
getConnection(function (err, conn) {
  // me too! no race condition...
});