Lock serversLock in Topology.String.

GODRIVER-1302

Author: bartle-stripe

x/mongo/driver/topology/topology.go

@@ -591,6 +591,8 @@ func (t *Topology) addServer(addr address.Address) error {
 func (t *Topology) String() string {
 	desc := t.Description()
 	str := fmt.Sprintf("Type: %s\nServers:\n", desc.Kind)
+	t.serversLock.Lock()
+	defer t.serversLock.Unlock()
 	for _, s := range t.servers {
 		str += s.String() + "\n"
 	}

x/mongo/driver/topology/topology_test.go

@@ -455,3 +455,27 @@ func TestMinPoolSize(t *testing.T) {
 		t.Errorf("topology.Connect shouldn't error. got: %v", err)
 	}
 }
+
+func TestTopology_String_Race(t *testing.T) {
+	ch := make(chan bool)
+	topo := &Topology{
+		servers: make(map[address.Address]*Server),
+	}
+
+	go func() {
+		topo.serversLock.Lock()
+		srv := &Server{}
+		srv.desc.Store(description.Server{})
+		topo.servers[address.Address("127.0.0.1:27017")] = srv
+		topo.serversLock.Unlock()
+		ch <- true
+	}()
+
+	go func() {
+		topo.String()
+		ch <- true
+	}()
+
+	<-ch
+	<-ch
+}