do not start new relay if one already exists

p2p/host/relaysvc/relay.go

@@ -65,14 +65,18 @@ func (m *RelayManager) background(ctx context.Context) {
 func (m *RelayManager) reachabilityChanged(r network.Reachability) error {
 	switch r {
 	case network.ReachabilityPublic:
+		m.mutex.Lock()
+		defer m.mutex.Unlock()
+		// ignore events with no reachability change
+		if m.relay != nil {
+			return nil
+		}
 		relay, err := relayv2.New(m.host, m.opts...)
 		if err != nil {
 			return err
 		}
-		m.mutex.Lock()
-		defer m.mutex.Unlock()
 		m.relay = relay
-	case network.ReachabilityPrivate:
+	default:
 		m.mutex.Lock()
 		defer m.mutex.Unlock()
 		if m.relay != nil {

p2p/host/relaysvc/relay_test.go

@@ -0,0 +1,29 @@
+package relaysvc
+
+import (
+	"testing"
+
+	"github.com/libp2p/go-libp2p/core/network"
+	bhost "github.com/libp2p/go-libp2p/p2p/host/blank"
+	swarmt "github.com/libp2p/go-libp2p/p2p/net/swarm/testing"
+	"github.com/stretchr/testify/require"
+)
+
+func TestReachabilityChangeEvent(t *testing.T) {
+	h := bhost.NewBlankHost(swarmt.GenSwarm(t))
+	rmgr := NewRelayManager(h)
+	rmgr.reachabilityChanged(network.ReachabilityPublic)
+	require.NotNil(t, rmgr.relay, "relay should be set on public reachability")
+
+	rmgr.reachabilityChanged(network.ReachabilityPrivate)
+	require.Nil(t, rmgr.relay, "relay should be nil on private reachability")
+
+	rmgr.reachabilityChanged(network.ReachabilityPublic)
+	rmgr.reachabilityChanged(network.ReachabilityUnknown)
+	require.Nil(t, rmgr.relay, "relay should be nil on unknown reachability")
+
+	rmgr.reachabilityChanged(network.ReachabilityPublic)
+	relay := rmgr.relay
+	rmgr.reachabilityChanged(network.ReachabilityPublic)
+	require.Equal(t, relay, rmgr.relay, "relay should not be started on receiving the same event")
+}