internal: rewrite TestDialWithMultipleBackendsNotSendingServerPreface (#2438)

- Remove the slice of servers approach, since there's specific
logic at server 2 that's different from server 1. This has the
advantage of making the test more readable without sacrificing
anything (given the previous point).

- Defer server close at initialization time instead of at the
end.

- Remove a time.Sleep(time.Second): use timeout + select around
serverDone instead.

- Use a goroutine to keep the connection reading, instead of
using a for loop in the server goroutine. This causes the
defer close(server2Done) to happen immediately after preface
is sent, which combined with the aforementioned time.Sleep
removal causes the test to go from 1.00s to ~0.05s.

clientconn_test.go

@@ -62,72 +62,59 @@ func assertState(wantState connectivity.State, cc *ClientConn) (connectivity.Sta
 }
 
 func (s) TestDialWithMultipleBackendsNotSendingServerPreface(t *testing.T) {
-	numServers := 2
-	servers := make([]net.Listener, numServers)
-	var err error
-	for i := 0; i < numServers; i++ {
-		servers[i], err = net.Listen("tcp", "localhost:0")
+	lis1, err := net.Listen("tcp", "localhost:0")
 	if err != nil {
 		t.Fatalf("Error while listening. Err: %v", err)
 	}
-	}
-	dones := make([]chan struct{}, numServers)
-	for i := 0; i < numServers; i++ {
-		dones[i] = make(chan struct{})
-	}
-	for i := 0; i < numServers; i++ {
-		go func(i int) {
-			defer func() {
-				close(dones[i])
-			}()
-			conn, err := servers[i].Accept()
+	defer lis1.Close()
+	lis1Addr := resolver.Address{Addr: lis1.Addr().String()}
+	lis1Done := make(chan struct{})
+	// 1st listener accepts the connection and immediately closes it.
+	go func() {
+		defer close(lis1Done)
+		conn, err := lis1.Accept()
 		if err != nil {
 			t.Errorf("Error while accepting. Err: %v", err)
 			return
 		}
-			defer conn.Close()
-			switch i {
-			case 0: // 1st server accepts the connection and immediately closes it.
-			case 1: // 2nd server accepts the connection and sends settings frames.
-				framer := http2.NewFramer(conn, conn)
-				if err := framer.WriteSettings(http2.Setting{}); err != nil {
-					t.Errorf("Error while writing settings frame. %v", err)
+		conn.Close()
+	}()
+
+	lis2, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		t.Fatalf("Error while listening. Err: %v", err)
+	}
+	defer lis2.Close()
+	lis2Done := make(chan struct{})
+	lis2Addr := resolver.Address{Addr: lis2.Addr().String()}
+	// 2nd listener should get a connection attempt since the first one failed.
+	go func() {
+		defer close(lis2Done)
+		_, err := lis2.Accept() // Closing the client will clean up this conn.
+		if err != nil {
+			t.Errorf("Error while accepting. Err: %v", err)
 			return
 		}
-				conn.SetDeadline(time.Now().Add(time.Second))
-				buf := make([]byte, 1024)
-				for { // Make sure the connection stays healthy.
-					_, err = conn.Read(buf)
-					if err == nil {
-						continue
-					}
-					if nerr, ok := err.(net.Error); !ok || !nerr.Timeout() {
-						t.Errorf("Server expected the conn.Read(_) to timeout instead got error: %v", err)
-					}
-					return
-				}
-			}
-		}(i)
-	}
+	}()
+
 	r, cleanup := manual.GenerateAndRegisterManualResolver()
 	defer cleanup()
-	resolvedAddrs := make([]resolver.Address, numServers)
-	for i := 0; i < numServers; i++ {
-		resolvedAddrs[i] = resolver.Address{Addr: servers[i].Addr().String()}
-	}
-	r.InitialAddrs(resolvedAddrs)
+	r.InitialAddrs([]resolver.Address{lis1Addr, lis2Addr})
 	client, err := Dial(r.Scheme()+":///test.server", WithInsecure())
 	if err != nil {
-		t.Errorf("Dial failed. Err: %v", err)
-	} else {
+		t.Fatalf("Dial failed. Err: %v", err)
+	}
 	defer client.Close()
+	timeout := time.After(5 * time.Second)
+	select {
+	case <-timeout:
+		t.Fatal("timed out waiting for server 1 to finish")
+	case <-lis1Done:
 	}
-	time.Sleep(time.Second) // Close the servers after a second for cleanup.
-	for _, s := range servers {
-		s.Close()
-	}
-	for _, done := range dones {
-		<-done
+	select {
+	case <-timeout:
+		t.Fatal("timed out waiting for server 2 to finish")
+	case <-lis2Done:
 	}
 }