Add some guard rails and docs (#1863)

options.go

@@ -245,7 +245,9 @@ func EnableRelayService(opts ...relayv2.Option) Option {
 //
 // Dependencies:
 //   - Relay (enabled by default)
-//   - Routing (to find relays), or StaticRelays/DefaultStaticRelays.
+//   - Either:
+//     1. A list of static relays
+//     2. A PeerSource function that provides a chan of relays. See `autorelay.WithPeerSource`
 //
 // This subsystem performs automatic address rewriting to advertise relay addresses when it
 // detects that the node is publicly unreachable (e.g. behind a NAT).

p2p/host/autorelay/autorelay_test.go

@@ -444,3 +444,15 @@ func TestMaxAge(t *testing.T) {
 	}
 	require.Contains(t, ids, relays[0])
 }
+
+func TestIncorrectInit(t *testing.T) {
+	// Check if we panic if we do not correctly initialize the autorelay system.
+	// Common since it's easy to initialize without passing in the correct options: https://github.com/libp2p/go-libp2p/issues/1852
+
+	defer func() {
+		if r := recover(); r == nil {
+			t.Errorf("Expected to panic")
+		}
+	}()
+	_ = newPrivateNode(t)
+}

p2p/host/autorelay/relay_finder.go

@@ -83,6 +83,10 @@ type relayFinder struct {
 }
 
 func newRelayFinder(host *basic.BasicHost, peerSource func(context.Context, int) <-chan peer.AddrInfo, conf *config) *relayFinder {
+	if peerSource == nil && len(conf.staticRelays) == 0 {
+		panic("Can not create a new relayFinder. Need a Peer Source fn or a list of static relays. Refer to the documentation around `libp2p.EnableAutoRelay`")
+	}
+
 	return &relayFinder{
 		bootTime:                   conf.clock.Now(),
 		host:                       host,