ipn: replace SetWantRunning(bool) with EditPrefs(MaskedPrefs)

This adds a new ipn.MaskedPrefs embedding a ipn.Prefs, along with a
bunch of "has bits", kept in sync with tests & reflect.

Then it adds a Prefs.ApplyEdits(MaskedPrefs) method.

Then the ipn.Backend interface loses its weirdo SetWantRunning(bool)
method (that I added in 483141094c for "tailscale down")
and replaces it with EditPrefs (alongside the existing SetPrefs for now).

Then updates 'tailscale down' to use EditPrefs instead of SetWantRunning.

In the future, we can use this to do more interesting things with the
CLI, reconfiguring only certain properties without the reset-the-world
"tailscale up".

Updates #1436

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

ipn/prefs_test.go

@@ -5,11 +5,13 @@
 package ipn
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -432,3 +434,146 @@ func TestLoadPrefsFileWithZeroInIt(t *testing.T) {
 	}
 	t.Fatalf("unexpected prefs=%#v, err=%v", p, err)
 }
+
+func TestMaskedPrefsFields(t *testing.T) {
+	have := map[string]bool{}
+	for _, f := range fieldsOf(reflect.TypeOf(Prefs{})) {
+		if f == "Persist" {
+			// This one can't be edited.
+			continue
+		}
+		have[f] = true
+	}
+	for _, f := range fieldsOf(reflect.TypeOf(MaskedPrefs{})) {
+		if f == "Prefs" {
+			continue
+		}
+		if !strings.HasSuffix(f, "Set") {
+			t.Errorf("unexpected non-/Set$/ field %q", f)
+			continue
+		}
+		bare := strings.TrimSuffix(f, "Set")
+		_, ok := have[bare]
+		if !ok {
+			t.Errorf("no corresponding Prefs.%s field for MaskedPrefs.%s", bare, f)
+			continue
+		}
+		delete(have, bare)
+	}
+	for f := range have {
+		t.Errorf("missing MaskedPrefs.%sSet for Prefs.%s", f, f)
+	}
+
+	// And also make sure they line up in the right order, which
+	// ApplyEdits assumes.
+	pt := reflect.TypeOf(Prefs{})
+	mt := reflect.TypeOf(MaskedPrefs{})
+	for i := 0; i < mt.NumField(); i++ {
+		name := mt.Field(i).Name
+		if i == 0 {
+			if name != "Prefs" {
+				t.Errorf("first field of MaskedPrefs should be Prefs")
+			}
+			continue
+		}
+		prefName := pt.Field(i - 1).Name
+		if prefName+"Set" != name {
+			t.Errorf("MaskedField[%d] = %s; want %sSet", i-1, name, prefName)
+		}
+	}
+}
+
+func TestPrefsApplyEdits(t *testing.T) {
+	tests := []struct {
+		name  string
+		prefs *Prefs
+		edit  *MaskedPrefs
+		want  *Prefs
+	}{
+		{
+			name: "no_change",
+			prefs: &Prefs{
+				Hostname: "foo",
+			},
+			edit: &MaskedPrefs{},
+			want: &Prefs{
+				Hostname: "foo",
+			},
+		},
+		{
+			name: "set1_decoy1",
+			prefs: &Prefs{
+				Hostname: "foo",
+			},
+			edit: &MaskedPrefs{
+				Prefs: Prefs{
+					Hostname:    "bar",
+					DeviceModel: "ignore-this", // not set
+				},
+				HostnameSet: true,
+			},
+			want: &Prefs{
+				Hostname: "bar",
+			},
+		},
+		{
+			name:  "set_several",
+			prefs: &Prefs{},
+			edit: &MaskedPrefs{
+				Prefs: Prefs{
+					Hostname:    "bar",
+					DeviceModel: "galaxybrain",
+				},
+				HostnameSet:    true,
+				DeviceModelSet: true,
+			},
+			want: &Prefs{
+				Hostname:    "bar",
+				DeviceModel: "galaxybrain",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.prefs.Clone()
+			got.ApplyEdits(tt.edit)
+			if !got.Equals(tt.want) {
+				gotj, _ := json.Marshal(got)
+				wantj, _ := json.Marshal(tt.want)
+				t.Errorf("fail.\n got: %s\nwant: %s\n", gotj, wantj)
+			}
+		})
+	}
+}
+
+func TestMaskedPrefsPretty(t *testing.T) {
+	tests := []struct {
+		m    *MaskedPrefs
+		want string
+	}{
+		{
+			m:    &MaskedPrefs{},
+			want: "MaskedPrefs{}",
+		},
+		{
+			m: &MaskedPrefs{
+				Prefs: Prefs{
+					Hostname:         "bar",
+					DeviceModel:      "galaxybrain",
+					AllowSingleHosts: true,
+					RouteAll:         false,
+				},
+				RouteAllSet:    true,
+				HostnameSet:    true,
+				DeviceModelSet: true,
+			},
+			want: `MaskedPrefs{RouteAll=false Hostname="bar" DeviceModel="galaxybrain"}`,
+		},
+	}
+	for i, tt := range tests {
+		got := tt.m.Pretty()
+		if got != tt.want {
+			t.Errorf("%d.\n got: %#q\nwant: %#q\n", i, got, tt.want)
+		}
+	}
+}