net/dnsfallback: add singleflight to recursive resolver

This prevents running more than one recursive resolution for the same
hostname in parallel, which can use excessive amounts of CPU when called
in a tight loop. Additionally, add tests that hit the network (when
run with a flag) to test the lookup behaviour.

Updates tailscale/corp#15261

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I39351e1d2a8782dd4c52cb04b3bd982eb651c81e

net/dnsfallback/dnsfallback_test.go

@@ -4,13 +4,17 @@
 package dnsfallback
 
 import (
+	"context"
 	"encoding/json"
+	"flag"
 	"os"
 	"path/filepath"
 	"reflect"
 	"testing"
 
+	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/logger"
 )
 
 func TestGetDERPMap(t *testing.T) {
@@ -170,3 +174,30 @@ func TestCacheUnchanged(t *testing.T) {
 		t.Fatalf("didn't find non-empty regular file; mode=%v size=%d", st.Mode(), st.Size())
 	}
 }
+
+var extNetwork = flag.Bool("use-external-network", false, "use the external network in tests")
+
+func TestLookup(t *testing.T) {
+	if !*extNetwork {
+		t.Skip("skipping test without --use-external-network")
+	}
+
+	logf, closeLogf := logger.LogfCloser(t.Logf)
+	defer closeLogf()
+
+	netMon, err := netmon.New(logf)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	resolver := &fallbackResolver{
+		logf:           logf,
+		netMon:         netMon,
+		waitForCompare: true,
+	}
+	addrs, err := resolver.Lookup(context.Background(), "controlplane.tailscale.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("addrs: %+v", addrs)
+}