Fix race condition during initialization (#163)

Currently, async workers are started before HTTP interface is available
to get their requests. It fixes by ensuring that async workers are
started after HTTP interface is up.

Essentially we are getting rid of an error message during bootstrap:

        ERRO[0000] Could not fetch task error=Get http://127.0.0.1:8080/tasks: dial tcp 127.0.0.1:8080: getsockopt: connection refused

api/runner/async_runner.go

@@ -96,8 +96,11 @@ func runTask(task *models.Task) (drivers.RunResult, error) {
 }
 
 // RunAsyncRunner pulls tasks off a queue and processes them
-func RunAsyncRunner(ctx context.Context, wgAsync *sync.WaitGroup, tasksrv, port string, n int) {
-	u := tasksrvURL(tasksrv, port)
+func RunAsyncRunner(ctx context.Context, tasksrv, port string, n int) {
+	u, h := tasksrvURL(tasksrv, port)
+	if isHostOpen(h) {
+		return
+	}
 
 	var wg sync.WaitGroup
 	for i := 0; i < n; i++ {
@@ -107,7 +110,15 @@ func RunAsyncRunner(ctx context.Context, wgAsync *sync.WaitGroup, tasksrv, port
 
 	wg.Wait()
 	<-ctx.Done()
-	wgAsync.Done()
+}
+
+func isHostOpen(host string) bool {
+	conn, err := net.Dial("tcp", host)
+	available := err == nil
+	if available {
+		conn.Close()
+	}
+	return available
 }
 
 func startAsyncRunners(ctx context.Context, wg *sync.WaitGroup, i int, url string, runTask func(task *models.Task) (drivers.RunResult, error)) {
@@ -150,7 +161,7 @@ func startAsyncRunners(ctx context.Context, wg *sync.WaitGroup, i int, url strin
 	}
 }
 
-func tasksrvURL(tasksrv, port string) string {
+func tasksrvURL(tasksrv, port string) (parsedURL, host string) {
 	parsed, err := url.Parse(tasksrv)
 	if err != nil {
 		log.Fatalf("cannot parse TASKSRV endpoint: %v", err)
@@ -168,5 +179,5 @@ func tasksrvURL(tasksrv, port string) string {
 		parsed.Host = net.JoinHostPort(parsed.Host, port)
 	}
 
-	return parsed.String()
+	return parsed.String(), parsed.Host
 }