control/controlclient: skip SetControlClientStatus when queue has newer results later

Updates #1909
Updates #12542
Updates tailscale/corp#26058

Change-Id: I3033d235ca49f9739fdf3deaf603eea4ec3e407e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/controlclient_test.go

@@ -4,8 +4,13 @@
 package controlclient
 
 import (
+	"io"
 	"reflect"
+	"slices"
 	"testing"
+
+	"tailscale.com/types/netmap"
+	"tailscale.com/types/persist"
 )
 
 func fieldsOf(t reflect.Type) (fields []string) {
@@ -62,3 +67,83 @@ func TestStatusEqual(t *testing.T) {
 		}
 	}
 }
+
+// tests [canSkipStatus].
+func TestCanSkipStatus(t *testing.T) {
+	st := new(Status)
+	nm1 := &netmap.NetworkMap{}
+	nm2 := &netmap.NetworkMap{}
+
+	tests := []struct {
+		name   string
+		s1, s2 *Status
+		want   bool
+	}{
+		{
+			name: "nil-s2",
+			s1:   st,
+			s2:   nil,
+			want: false,
+		},
+		{
+			name: "equal",
+			s1:   st,
+			s2:   st,
+			want: false,
+		},
+		{
+			name: "s1-error",
+			s1:   &Status{Err: io.EOF, NetMap: nm1},
+			s2:   &Status{NetMap: nm2},
+			want: false,
+		},
+		{
+			name: "s1-url",
+			s1:   &Status{URL: "foo", NetMap: nm1},
+			s2:   &Status{NetMap: nm2},
+			want: false,
+		},
+		{
+			name: "s1-persist-diff",
+			s1:   &Status{Persist: new(persist.Persist).View(), NetMap: nm1},
+			s2:   &Status{NetMap: nm2},
+			want: false,
+		},
+		{
+			name: "s1-state-diff",
+			s1:   &Status{state: 123, NetMap: nm1},
+			s2:   &Status{NetMap: nm2},
+			want: false,
+		},
+		{
+			name: "s1-no-netmap1",
+			s1:   &Status{NetMap: nil},
+			s2:   &Status{NetMap: nm2},
+			want: false,
+		},
+		{
+			name: "s1-no-netmap2",
+			s1:   &Status{NetMap: nm1},
+			s2:   &Status{NetMap: nil},
+			want: false,
+		},
+		{
+			name: "skip",
+			s1:   &Status{NetMap: nm1},
+			s2:   &Status{NetMap: nm2},
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := canSkipStatus(tt.s1, tt.s2); got != tt.want {
+				t.Errorf("canSkipStatus = %v, want %v", got, tt.want)
+			}
+		})
+	}
+
+	want := []string{"Err", "URL", "NetMap", "Persist", "state"}
+	if f := fieldsOf(reflect.TypeFor[Status]()); !slices.Equal(f, want) {
+		t.Errorf("Status fields = %q; this code was only written to handle fields %q", f, want)
+	}
+}