ipn/ipnlocal: add a C2N endpoint for fetching a netmap

For debugging purposes, add a new C2N endpoint returning the current
netmap. Optionally, coordination server can send a new "candidate" map
response, which the client will generate a separate netmap for.
Coordination server can later compare two netmaps, detecting unexpected
changes to the client state.

Updates tailscale/corp#32095

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

control/controlclient/map_test.go

@@ -20,6 +20,7 @@ import (
 	"go4.org/mem"
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
+	"tailscale.com/ipn"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
 	"tailscale.com/tstime"
@@ -27,6 +28,7 @@ import (
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
+	"tailscale.com/types/persist"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/eventbus/eventbustest"
 	"tailscale.com/util/mak"
@@ -1419,3 +1421,27 @@ func TestNetmapDisplayMessageIntegration(t *testing.T) {
 		t.Errorf("unexpected message contents (-want +got):\n%s", diff)
 	}
 }
+
+func TestNetmapForMapResponseForDebug(t *testing.T) {
+	mr := &tailcfg.MapResponse{
+		Node: &tailcfg.Node{
+			ID:   1,
+			Name: "foo.bar.ts.net.",
+		},
+		Peers: []*tailcfg.Node{
+			{ID: 2, Name: "peer1.bar.ts.net.", HomeDERP: 1},
+			{ID: 3, Name: "peer2.bar.ts.net.", HomeDERP: 1},
+		},
+	}
+	ms := newTestMapSession(t, nil)
+	nm1 := ms.netmapForResponse(mr)
+
+	prefs := &ipn.Prefs{Persist: &persist.Persist{PrivateNodeKey: ms.privateNodeKey}}
+	nm2, err := NetmapFromMapResponseForDebug(t.Context(), prefs.View().Persist(), mr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(nm1, nm2) {
+		t.Errorf("mismatch\nnm1: %s\nnm2: %s\n", logger.AsJSON(nm1), logger.AsJSON(nm2))
+	}
+}