fix: change servertailnet to register the DERP dialer before setting DERP map (#12137)

I noticed a possible race where tailnet.Conn can try to dial the embedded region before we've set our custom dialer that send the DERP in-memory.  This closes that race and adds a test case for servertailnet with no STUN and an embedded relay

tailnet/configmaps.go

@@ -204,7 +204,8 @@ func (c *configMaps) netMapLocked() *netmap.NetworkMap {
 	nm.Addresses = make([]netip.Prefix, len(c.addresses))
 	copy(nm.Addresses, c.addresses)
 
-	nm.DERPMap = c.derpMap.Clone()
+	// we don't need to set the DERPMap in the network map because we separately
+	// send the DERPMap directly via SetDERPMap
 	nm.Peers = c.peerConfigLocked()
 	nm.SelfNode.Addresses = nm.Addresses
 	nm.SelfNode.AllowedIPs = nm.Addresses

tailnet/conn.go

@@ -116,9 +116,6 @@ func NewConn(options *Options) (conn *Conn, err error) {
 	if len(options.Addresses) == 0 {
 		return nil, xerrors.New("At least one IP range must be provided")
 	}
-	if options.DERPMap == nil {
-		return nil, xerrors.New("DERPMap must be provided")
-	}
 
 	nodePrivateKey := key.NewNode()
 	var nodeID tailcfg.NodeID
@@ -219,7 +216,9 @@ func NewConn(options *Options) (conn *Conn, err error) {
 		magicConn.DiscoPublicKey(),
 	)
 	cfgMaps.setAddresses(options.Addresses)
-	cfgMaps.setDERPMap(options.DERPMap)
+	if options.DERPMap != nil {
+		cfgMaps.setDERPMap(options.DERPMap)
+	}
 	cfgMaps.setBlockEndpoints(options.BlockEndpoints)
 
 	nodeUp := newNodeUpdater(

tailnet/tailnettest/tailnettest.go

@@ -32,22 +32,47 @@ import (
 //go:generate mockgen -destination ./coordinatormock.go -package tailnettest github.com/coder/coder/v2/tailnet Coordinator
 //go:generate mockgen -destination ./coordinateemock.go -package tailnettest github.com/coder/coder/v2/tailnet Coordinatee
 
+type derpAndSTUNCfg struct {
+	DisableSTUN    bool
+	DERPIsEmbedded bool
+}
+
+type DERPAndStunOption func(cfg *derpAndSTUNCfg)
+
+func DisableSTUN(cfg *derpAndSTUNCfg) {
+	cfg.DisableSTUN = true
+}
+
+func DERPIsEmbedded(cfg *derpAndSTUNCfg) {
+	cfg.DERPIsEmbedded = true
+}
+
 // RunDERPAndSTUN creates a DERP mapping for tests.
-func RunDERPAndSTUN(t *testing.T) (*tailcfg.DERPMap, *derp.Server) {
+func RunDERPAndSTUN(t *testing.T, opts ...DERPAndStunOption) (*tailcfg.DERPMap, *derp.Server) {
+	cfg := new(derpAndSTUNCfg)
+	for _, o := range opts {
+		o(cfg)
+	}
 	logf := tailnet.Logger(slogtest.Make(t, nil))
 	d := derp.NewServer(key.NewNode(), logf)
 	server := httptest.NewUnstartedServer(derphttp.Handler(d))
 	server.Config.ErrorLog = tslogger.StdLogger(logf)
 	server.Config.TLSNextProto = make(map[string]func(*http.Server, *tls.Conn, http.Handler))
 	server.StartTLS()
-
-	stunAddr, stunCleanup := stuntest.ServeWithPacketListener(t, nettype.Std{})
 	t.Cleanup(func() {
 		server.CloseClientConnections()
 		server.Close()
 		d.Close()
-		stunCleanup()
 	})
+
+	stunPort := -1
+	if !cfg.DisableSTUN {
+		stunAddr, stunCleanup := stuntest.ServeWithPacketListener(t, nettype.Std{})
+		t.Cleanup(func() {
+			stunCleanup()
+		})
+		stunPort = stunAddr.Port
+	}
 	tcpAddr, ok := server.Listener.Addr().(*net.TCPAddr)
 	if !ok {
 		t.FailNow()
@@ -65,11 +90,12 @@ func RunDERPAndSTUN(t *testing.T) (*tailcfg.DERPMap, *derp.Server) {
 						RegionID:         1,
 						IPv4:             "127.0.0.1",
 						IPv6:             "none",
-						STUNPort:         stunAddr.Port,
+						STUNPort:         stunPort,
 						DERPPort:         tcpAddr.Port,
 						InsecureForTests: true,
 					},
 				},
+				EmbeddedRelay: cfg.DERPIsEmbedded,
 			},
 		},
 	}, d