cmd/tailscale/cli,ipn/ipnlocal,wgengine/magicsock: implement tailscale debug peer-relay-servers (#16577)

Updates tailscale/corp#30036

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -356,6 +356,12 @@ func debugCmd() *ffcli.Command {
 				ShortHelp:  "Print Go's runtime/debug.BuildInfo",
 				Exec:       runGoBuildInfo,
 			},
+			{
+				Name:       "peer-relay-servers",
+				ShortUsage: "tailscale debug peer-relay-servers",
+				ShortHelp:  "Print the current set of candidate peer relay servers",
+				Exec:       runPeerRelayServers,
+			},
 		}...),
 	}
 }
@@ -1327,3 +1333,17 @@ func runDebugResolve(ctx context.Context, args []string) error {
 	}
 	return nil
 }
+
+func runPeerRelayServers(ctx context.Context, args []string) error {
+	if len(args) > 0 {
+		return errors.New("unexpected arguments")
+	}
+	v, err := localClient.DebugResultJSON(ctx, "peer-relay-servers")
+	if err != nil {
+		return err
+	}
+	e := json.NewEncoder(os.Stdout)
+	e.SetIndent("", "  ")
+	e.Encode(v)
+	return nil
+}

ipn/ipnlocal/local.go

@@ -6956,6 +6956,10 @@ func (b *LocalBackend) DebugReSTUN() error {
 	return nil
 }
 
+func (b *LocalBackend) DebugPeerRelayServers() set.Set[netip.AddrPort] {
+	return b.MagicConn().PeerRelays()
+}
+
 // ControlKnobs returns the node's control knobs.
 func (b *LocalBackend) ControlKnobs() *controlknobs.Knobs {
 	return b.sys.ControlKnobs()

ipn/localapi/localapi.go

@@ -696,6 +696,12 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 			break
 		}
 		h.b.DebugForcePreferDERP(n)
+	case "peer-relay-servers":
+		servers := h.b.DebugPeerRelayServers()
+		err = json.NewEncoder(w).Encode(servers)
+		if err == nil {
+			return
+		}
 	case "":
 		err = fmt.Errorf("missing parameter 'action'")
 	default:

wgengine/magicsock/magicsock.go

@@ -3907,3 +3907,8 @@ func (le *lazyEndpoint) FromPeer(peerPublicKey [32]byte) {
 	le.c.peerMap.setNodeKeyForEpAddr(le.src, pubKey)
 	le.c.logf("magicsock: lazyEndpoint.FromPeer(%v) setting epAddr(%v) in peerMap for node(%v)", pubKey.ShortString(), le.src, ep.nodeAddr)
 }
+
+// PeerRelays returns the current set of candidate peer relays.
+func (c *Conn) PeerRelays() set.Set[netip.AddrPort] {
+	return c.relayManager.getServers()
+}

wgengine/magicsock/relaymanager.go

@@ -57,6 +57,7 @@ type relayManager struct {
 	newServerEndpointCh   chan newRelayServerEndpointEvent
 	rxHandshakeDiscoMsgCh chan relayHandshakeDiscoMsgEvent
 	serversCh             chan set.Set[netip.AddrPort]
+	getServersCh          chan chan set.Set[netip.AddrPort]
 
 	discoInfoMu            sync.Mutex // guards the following field
 	discoInfoByServerDisco map[key.DiscoPublic]*relayHandshakeDiscoInfo
@@ -185,10 +186,29 @@ func (r *relayManager) runLoop() {
 			if !r.hasActiveWorkRunLoop() {
 				return
 			}
+		case getServersCh := <-r.getServersCh:
+			r.handleGetServersRunLoop(getServersCh)
+			if !r.hasActiveWorkRunLoop() {
+				return
+			}
 		}
 	}
 }
 
+func (r *relayManager) handleGetServersRunLoop(getServersCh chan set.Set[netip.AddrPort]) {
+	servers := make(set.Set[netip.AddrPort], len(r.serversByAddrPort))
+	for server := range r.serversByAddrPort {
+		servers.Add(server)
+	}
+	getServersCh <- servers
+}
+
+func (r *relayManager) getServers() set.Set[netip.AddrPort] {
+	ch := make(chan set.Set[netip.AddrPort])
+	relayManagerInputEvent(r, nil, &r.getServersCh, ch)
+	return <-ch
+}
+
 func (r *relayManager) handleServersUpdateRunLoop(update set.Set[netip.AddrPort]) {
 	for k, v := range r.serversByAddrPort {
 		if !update.Contains(k) {
@@ -244,6 +264,7 @@ func (r *relayManager) init() {
 		r.newServerEndpointCh = make(chan newRelayServerEndpointEvent)
 		r.rxHandshakeDiscoMsgCh = make(chan relayHandshakeDiscoMsgEvent)
 		r.serversCh = make(chan set.Set[netip.AddrPort])
+		r.getServersCh = make(chan chan set.Set[netip.AddrPort])
 		r.runLoopStoppedCh = make(chan struct{}, 1)
 		r.runLoopStoppedCh <- struct{}{}
 	})

wgengine/magicsock/relaymanager_test.go

@@ -32,4 +32,19 @@ func TestRelayManagerInitAndIdle(t *testing.T) {
 	rm = relayManager{}
 	rm.handleRelayServersSet(make(set.Set[netip.AddrPort]))
 	<-rm.runLoopStoppedCh
+
+	rm = relayManager{}
+	rm.getServers()
+	<-rm.runLoopStoppedCh
+}
+
+func TestRelayManagerGetServers(t *testing.T) {
+	rm := relayManager{}
+	servers := make(set.Set[netip.AddrPort], 1)
+	servers.Add(netip.MustParseAddrPort("192.0.2.1:7"))
+	rm.handleRelayServersSet(servers)
+	got := rm.getServers()
+	if !servers.Equal(got) {
+		t.Errorf("got %v != want %v", got, servers)
+	}
 }