Remove race condition from bdns_test.go (letsencrypt#1906)

This PR, makes testing the bdns package more reliable. A race condition in TestMain was resulting in the test running before the test dns server had started. This is fixed by actively polling for the DNS server to be ready before starting the test suite.

Furthermore, a 1 millisecond server read/write timeout was proving to time out on occasion. This is fixed increasing to a 1 second read/write timeout to increase test reliability. 

FYI: ran package bdns tests 1000 times with 22 failures previously, after this PR ran 1000 times with 0 failures.

fixes letsencrypt#1317

bdns/dns_test.go

@@ -171,12 +171,10 @@ func mockDNSQuery(w dns.ResponseWriter, r *dns.Msg) {
 	return
 }
 
-func serveLoopResolver(stopChan chan bool) chan bool {
+func serveLoopResolver(stopChan chan bool) {
 	dns.HandleFunc(".", mockDNSQuery)
-	server := &dns.Server{Addr: dnsLoopbackAddr, Net: "tcp", ReadTimeout: time.Millisecond, WriteTimeout: time.Millisecond}
-	waitChan := make(chan bool, 1)
+	server := &dns.Server{Addr: dnsLoopbackAddr, Net: "tcp", ReadTimeout: time.Second, WriteTimeout: time.Second}
 	go func() {
-		waitChan <- true
 		err := server.ListenAndServe()
 		if err != nil {
 			fmt.Println(err)
@@ -190,13 +188,32 @@ func serveLoopResolver(stopChan chan bool) chan bool {
 			fmt.Println(err)
 		}
 	}()
-	return waitChan
+}
+
+func pollServer() {
+	backoff := time.Duration(200 * time.Millisecond)
+	ctx, _ := context.WithTimeout(context.Background(), time.Duration(5*time.Second))
+	ticker := time.NewTicker(backoff)
+
+	for {
+		select {
+		case <-ctx.Done():
+			fmt.Fprintln(os.Stderr, "Timeout reached while testing for the dns server to come up")
+			os.Exit(1)
+		case <-ticker.C:
+			conn, _ := dns.DialTimeout("tcp", dnsLoopbackAddr, backoff)
+			if conn != nil {
+				_ = conn.Close()
+				return
+			}
+		}
+	}
 }
 
 func TestMain(m *testing.M) {
 	stop := make(chan bool, 1)
-	wait := serveLoopResolver(stop)
-	<-wait
+	serveLoopResolver(stop)
+	pollServer()
 	ret := m.Run()
 	stop <- true
 	os.Exit(ret)