Synchronization Issue with as*Async methods (ThreadStateException)

[joshgontijo]

From #143

Hello,
There is a synchronization issue in the method com.mashape.unirest.http.HttpClientHelper.requestAsync.
To explain the problem I will post the relevant method code here:

public static <T> Future<HttpResponse<T>> requestAsync(HttpRequest request, final Class<T> responseClass, Callback<T> callback) {
        HttpUriRequest requestObj = prepareRequest(request, true);

        CloseableHttpAsyncClient asyncHttpClient = ClientFactory.getAsyncHttpClient();

        if (!asyncHttpClient.isRunning()) { //ENTER CS
            asyncHttpClient.start();
            AsyncIdleConnectionMonitorThread asyncIdleConnectionMonitorThread = (AsyncIdleConnectionMonitorThread) Options.getOption(Option.ASYNC_MONITOR);
            asyncIdleConnectionMonitorThread.start(); // <-- this line throws ThreadStateException
        }
//... rest of the code
}

The problem is that if several threads attempt to perform requests with as*Async operations at the same time then it can happen that two threads will enter if in the line marked with ENTER_CS on the same time start the asyncHttpClient and then the asyncIdleConnectionMonitorThread. Since the asyncIdleConnectionMonitorThread's start method will be called twice - the second time will throw ThreadStateException (starting a thread that is already started).

Here is a simple program that reproduce the bug

public static void main(String[] args)  {
        int threads = 8;
        CountDownLatch letch = new CountDownLatch(1);
        for (int i = 0; i < threads; i++) {
            new Thread(() -> {
                try {
                    letch.await();
                    Unirest.get("http://www.google.com").asStringAsync().get();
                } catch (InterruptedException | ExecutionException  ex) {
                    Logger.getLogger(Bug.class.getName()).log(Level.SEVERE, null, ex);
                }

            }).start();
        }

        letch.countDown();
    }