state: compare peer-visible Hostinfo through views

Field by field: no allocation, no reflection.

Updates #3531

hscontrol/state/maprequest_test.go

@@ -3,6 +3,7 @@ package state
 import (
 	"fmt"
 	"net/netip"
+	"reflect"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -860,12 +861,60 @@ func TestPeerHostinfoEqual(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			changed := base()
 			tt.mutate(changed)
-			require.Equal(t, tt.want, peerHostinfoEqual(base(), changed))
+			require.Equal(t, tt.want, peerHostinfoEqual(base().View(), changed.View()))
 		})
 	}
 
-	require.True(t, peerHostinfoEqual(nil, nil))
-	require.False(t, peerHostinfoEqual(nil, base()))
+	require.True(t, peerHostinfoEqual(tailcfg.HostinfoView{}, tailcfg.HostinfoView{}))
+	require.False(t, peerHostinfoEqual(tailcfg.HostinfoView{}, base().View()))
+}
+
+// TestPeerHostinfoEqualSeesEveryField fails when tailscale adds a field to
+// [tailcfg.Service] or [tailcfg.Location] that servicesEqual or locationEqual
+// does not compare.
+func TestPeerHostinfoEqualSeesEveryField(t *testing.T) {
+	set := func(t *testing.T, f reflect.Value) {
+		t.Helper()
+
+		switch {
+		case f.Kind() == reflect.String:
+			f.SetString("x")
+		case f.CanInt():
+			f.SetInt(1)
+		case f.CanUint():
+			f.SetUint(1)
+		case f.CanFloat():
+			f.SetFloat(1)
+		default:
+			t.Fatalf("field kind %s not handled; compare it", f.Kind())
+		}
+	}
+
+	for _, field := range reflect.VisibleFields(reflect.TypeFor[tailcfg.Service]()) {
+		if !field.IsExported() {
+			continue
+		}
+
+		t.Run("Service."+field.Name, func(t *testing.T) {
+			var svc tailcfg.Service
+			set(t, reflect.ValueOf(&svc).Elem().FieldByIndex(field.Index))
+
+			a := &tailcfg.Hostinfo{Services: []tailcfg.Service{{}}}
+			b := &tailcfg.Hostinfo{Services: []tailcfg.Service{svc}}
+			assert.False(t, peerHostinfoEqual(a.View(), b.View()))
+		})
+	}
+
+	for _, field := range reflect.VisibleFields(reflect.TypeFor[tailcfg.Location]()) {
+		t.Run("Location."+field.Name, func(t *testing.T) {
+			var loc tailcfg.Location
+			set(t, reflect.ValueOf(&loc).Elem().FieldByIndex(field.Index))
+
+			a := &tailcfg.Hostinfo{Location: &tailcfg.Location{}}
+			b := &tailcfg.Hostinfo{Location: &loc}
+			assert.False(t, peerHostinfoEqual(a.View(), b.View()))
+		})
+	}
 }
 
 func TestNetInfoEqualIgnoringDERP(t *testing.T) {