TUN-3777: Fix /ready endpoint for classic tunnels

Classic tunnels flow was triggering an event for RegisteringTunnel for
every connection that was about to be established, and then a Connected
event for every connection established.

However, the RegistreringTunnel event had no connection ID, always
causing it to unset/disconnect the 0th connection making the /ready
endpoint report incorrect numbers for classic tunnels.

cmd/cloudflared/ui/launch_ui.go

@@ -135,7 +135,7 @@ func (data *uiModel) Launch(
 			data.edgeURL = event.URL
 		case connection.RegisteringTunnel:
 			if data.edgeURL == "" {
-				tunnelHostText.SetText("Registering tunnel...")
+				tunnelHostText.SetText(fmt.Sprintf("Registering tunnel connection %d...", event.Index))
 			}
 		}
 		app.Draw()

connection/observer.go

@@ -99,8 +99,8 @@ func trialZoneMsg(url string) []string {
 	}
 }
 
-func (o *Observer) sendRegisteringEvent() {
-	o.sendEvent(Event{EventType: RegisteringTunnel})
+func (o *Observer) sendRegisteringEvent(connIndex uint8) {
+	o.sendEvent(Event{Index: connIndex, EventType: RegisteringTunnel})
 }
 
 func (o *Observer) sendConnectedEvent(connIndex uint8, location string) {

connection/observer_test.go

@@ -58,8 +58,8 @@ func TestObserverEventsDontBlock(t *testing.T) {
 	})
 
 	mu.Lock() // block the callback
-	for i := 0; i < 2 * observerChannelBufferSize; i++ {
-		observer.sendRegisteringEvent()
+	for i := 0; i < 2*observerChannelBufferSize; i++ {
+		observer.sendRegisteringEvent(0)
 	}
 	if pending := timeout.Stop(); pending {
 		// release the callback if timer hasn't expired yet

connection/rpc.go

@@ -138,7 +138,7 @@ const (
 )
 
 func (h *h2muxConnection) registerTunnel(ctx context.Context, credentialSetter CredentialManager, classicTunnel *ClassicTunnelConfig, registrationOptions *tunnelpogs.RegistrationOptions) error {
-	h.observer.sendRegisteringEvent()
+	h.observer.sendRegisteringEvent(registrationOptions.ConnectionID)
 
 	stream, err := h.newRPCStream(ctx, register)
 	if err != nil {

metrics/readiness_test.go

@@ -98,6 +98,15 @@ func TestReadinessEventHandling(t *testing.T) {
 	assert.EqualValues(t, http.StatusOK, code)
 	assert.EqualValues(t, 1, ready)
 
+	// Regression test for TUN-3777
+	rs.OnTunnelEvent(connection.Event{
+		Index:     1,
+		EventType: connection.RegisteringTunnel,
+	})
+	code, ready = rs.makeResponse()
+	assert.NotEqualValues(t, http.StatusOK, code)
+	assert.Zero(t, ready)
+
 	// other disconnected => not ok
 	rs.OnTunnelEvent(connection.Event{
 		Index:     1,