netcheck: work behind UDP-blocked networks again, add tests

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

netcheck/netcheck_test.go

@@ -5,10 +5,16 @@
 package netcheck
 
 import (
+	"context"
 	"net"
+	"reflect"
+	"strings"
 	"testing"
+	"time"
 
+	"tailscale.com/derp/derpmap"
 	"tailscale.com/stun"
+	"tailscale.com/stun/stuntest"
 )
 
 func TestHairpinSTUN(t *testing.T) {
@@ -29,3 +35,66 @@ func TestHairpinSTUN(t *testing.T) {
 		t.Fatal("expected value")
 	}
 }
+
+func TestBasic(t *testing.T) {
+	stunAddr, cleanup := stuntest.Serve(t)
+	defer cleanup()
+
+	c := &Client{
+		DERP: derpmap.NewTestWorld(stunAddr),
+		Logf: t.Logf,
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+	defer cancel()
+
+	r, err := c.GetReport(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !r.UDP {
+		t.Error("want UDP")
+	}
+	if len(r.DERPLatency) != 1 {
+		t.Errorf("expected 1 key in DERPLatency; got %+v", r.DERPLatency)
+	}
+	if _, ok := r.DERPLatency[stunAddr]; !ok {
+		t.Errorf("expected key %q in DERPLatency; got %+v", stunAddr, r.DERPLatency)
+	}
+	if r.GlobalV4 == "" {
+		t.Error("expected GlobalV4 set")
+	}
+	if r.PreferredDERP != 1 {
+		t.Errorf("PreferredDERP = %v; want 1", r.PreferredDERP)
+	}
+}
+
+func TestWorksWhenUDPBlocked(t *testing.T) {
+	blackhole, err := net.ListenPacket("udp4", ":0")
+	if err != nil {
+		t.Fatalf("failed to open blackhole STUN listener: %v", err)
+	}
+	defer blackhole.Close()
+
+	stunAddr := blackhole.LocalAddr().String()
+	stunAddr = strings.Replace(stunAddr, "0.0.0.0:", "127.0.0.1:", 1)
+
+	c := &Client{
+		DERP: derpmap.NewTestWorld(stunAddr),
+		Logf: t.Logf,
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 250*time.Millisecond)
+	defer cancel()
+
+	r, err := c.GetReport(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := &Report{
+		DERPLatency: map[string]time.Duration{},
+	}
+
+	if !reflect.DeepEqual(r, want) {
+		t.Errorf("mismatch\n got: %+v\nwant: %+v\n", r, want)
+	}
+}