ipn: add methods on Prefs to get/set exit node being advertised

This code was copied in a few places (Windows, Android), so unify it
and add tests.

Change-Id: Id0510c0f5974761365a2045279d1fb498feca11e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/prefs.go

@@ -426,6 +426,47 @@ func (p *Prefs) AdminPageURL() string {
 	return url + "/admin/machines"
 }
 
+// AdvertisesExitNode reports whether p is advertising both the v4 and
+// v6 /0 exit node routes.
+func (p *Prefs) AdvertisesExitNode() bool {
+	if p == nil {
+		return false
+	}
+	var v4, v6 bool
+	for _, r := range p.AdvertiseRoutes {
+		if r.Bits() != 0 {
+			continue
+		}
+		if r.IP().Is4() {
+			v4 = true
+		} else if r.IP().Is6() {
+			v6 = true
+		}
+	}
+	return v4 && v6
+}
+
+// SetRunExitNode mutates p (if non-nil) to add or remove the two
+// /0 exit node routes.
+func (p *Prefs) SetRunExitNode(runExit bool) {
+	if p == nil {
+		return
+	}
+	all := p.AdvertiseRoutes
+	p.AdvertiseRoutes = p.AdvertiseRoutes[:0]
+	for _, r := range all {
+		if r.Bits() != 0 {
+			p.AdvertiseRoutes = append(p.AdvertiseRoutes, r)
+		}
+	}
+	if !runExit {
+		return
+	}
+	p.AdvertiseRoutes = append(p.AdvertiseRoutes,
+		netaddr.IPPrefixFrom(netaddr.IPv4(0, 0, 0, 0), 0),
+		netaddr.IPPrefixFrom(netaddr.IPv6Unspecified(), 0))
+}
+
 // PrefsFromBytes deserializes Prefs from a JSON blob. If
 // enforceDefaults is true, Prefs.RouteAll and Prefs.AllowSingleHosts
 // are forced on.

ipn/prefs_test.go

@@ -646,3 +646,35 @@ func TestMaskedPrefsPretty(t *testing.T) {
 		}
 	}
 }
+
+func TestPrefsExitNode(t *testing.T) {
+	var p *Prefs
+	if p.AdvertisesExitNode() {
+		t.Errorf("nil shouldn't advertise exit node")
+	}
+	p = NewPrefs()
+	if p.AdvertisesExitNode() {
+		t.Errorf("default shouldn't advertise exit node")
+	}
+	p.AdvertiseRoutes = []netaddr.IPPrefix{
+		netaddr.MustParseIPPrefix("10.0.0.0/16"),
+	}
+	p.SetRunExitNode(true)
+	if got, want := len(p.AdvertiseRoutes), 3; got != want {
+		t.Errorf("routes = %d; want %d", got, want)
+	}
+	p.SetRunExitNode(true)
+	if got, want := len(p.AdvertiseRoutes), 3; got != want {
+		t.Errorf("routes = %d; want %d", got, want)
+	}
+	if !p.AdvertisesExitNode() {
+		t.Errorf("not advertising after enable")
+	}
+	p.SetRunExitNode(false)
+	if p.AdvertisesExitNode() {
+		t.Errorf("advertising after disable")
+	}
+	if got, want := len(p.AdvertiseRoutes), 1; got != want {
+		t.Errorf("routes = %d; want %d", got, want)
+	}
+}