control/controlclient, tailcfg: turn active route discovery on by default

Updates #483

control/controlclient/direct.go

@@ -483,6 +483,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 		Endpoints:       ep,
 		Stream:          allowStream,
 		Hostinfo:        hostinfo,
+		DebugForceDisco: Debug.ForceDisco,
 	}
 	if c.newDecompressor != nil {
 		request.Compress = "zstd"
@@ -785,14 +786,20 @@ type debug struct {
 	NetMap     bool
 	OnlyDisco  bool
 	Disco      bool
+	ForceDisco bool // ask control server to not filter out our disco key
 }
 
 func initDebug() debug {
-	return debug{
+	d := debug{
 		NetMap:     envBool("TS_DEBUG_NETMAP"),
 		OnlyDisco:  os.Getenv("TS_DEBUG_USE_DISCO") == "only",
-		Disco:     os.Getenv("TS_DEBUG_USE_DISCO") == "only" || envBool("TS_DEBUG_USE_DISCO"),
+		ForceDisco: os.Getenv("TS_DEBUG_USE_DISCO") == "only" || envBool("TS_DEBUG_USE_DISCO"),
 	}
+	if d.ForceDisco || os.Getenv("TS_DEBUG_USE_DISCO") == "" {
+		// This is now defaults to on.
+		d.Disco = true
+	}
+	return d
 }
 
 func envBool(k string) bool {

tailcfg/tailcfg.go

@@ -454,6 +454,12 @@ type MapRequest struct {
 	IncludeIPv6 bool     // include IPv6 endpoints in returned Node Endpoints
 	Stream      bool     // if true, multiple MapResponse objects are returned
 	Hostinfo    *Hostinfo
+
+	// DebugForceDisco is a temporary flag during the deployment
+	// of magicsock active discovery. It says that that the client
+	// has environment variables explicitly turning discovery on,
+	// so control should not disable it.
+	DebugForceDisco bool `json:"debugForceDisco,omitempty"`
 }
 
 // PortRange represents a range of UDP or TCP port numbers.