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.go

@@ -12,6 +12,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"strings"
 
@@ -147,6 +148,73 @@ type Prefs struct {
 	Persist *persist.Persist `json:"Config"`
 }
 
+// MaskedPrefs is a Prefs with an associated bitmask of which fields are set.
+type MaskedPrefs struct {
+	Prefs
+
+	ControlURLSet       bool `json:",omitempty"`
+	RouteAllSet         bool `json:",omitempty"`
+	AllowSingleHostsSet bool `json:",omitempty"`
+	ExitNodeIDSet       bool `json:",omitempty"`
+	ExitNodeIPSet       bool `json:",omitempty"`
+	CorpDNSSet          bool `json:",omitempty"`
+	WantRunningSet      bool `json:",omitempty"`
+	ShieldsUpSet        bool `json:",omitempty"`
+	AdvertiseTagsSet    bool `json:",omitempty"`
+	HostnameSet         bool `json:",omitempty"`
+	OSVersionSet        bool `json:",omitempty"`
+	DeviceModelSet      bool `json:",omitempty"`
+	NotepadURLsSet      bool `json:",omitempty"`
+	ForceDaemonSet      bool `json:",omitempty"`
+	AdvertiseRoutesSet  bool `json:",omitempty"`
+	NoSNATSet           bool `json:",omitempty"`
+	NetfilterModeSet    bool `json:",omitempty"`
+}
+
+// ApplyEdits mutates p, assigning fields from m.Prefs for each MaskedPrefs
+// Set field that's true.
+func (p *Prefs) ApplyEdits(m *MaskedPrefs) {
+	if p == nil {
+		panic("can't edit nil Prefs")
+	}
+	pv := reflect.ValueOf(p).Elem()
+	mv := reflect.ValueOf(m).Elem()
+	mpv := reflect.ValueOf(&m.Prefs).Elem()
+	fields := mv.NumField()
+	for i := 1; i < fields; i++ {
+		if mv.Field(i).Bool() {
+			newFieldValue := mpv.Field(i - 1)
+			pv.Field(i - 1).Set(newFieldValue)
+		}
+	}
+}
+
+func (m *MaskedPrefs) Pretty() string {
+	if m == nil {
+		return "MaskedPrefs{<nil>}"
+	}
+	var sb strings.Builder
+	sb.WriteString("MaskedPrefs{")
+	mv := reflect.ValueOf(m).Elem()
+	mt := mv.Type()
+	mpv := reflect.ValueOf(&m.Prefs).Elem()
+	first := true
+	for i := 1; i < mt.NumField(); i++ {
+		name := mt.Field(i).Name
+		if mv.Field(i).Bool() {
+			if !first {
+				sb.WriteString(" ")
+			}
+			first = false
+			fmt.Fprintf(&sb, "%s=%#v",
+				strings.TrimSuffix(name, "Set"),
+				mpv.Field(i-1).Interface())
+		}
+	}
+	sb.WriteString("}")
+	return sb.String()
+}
+
 // IsEmpty reports whether p is nil or pointing to a Prefs zero value.
 func (p *Prefs) IsEmpty() bool { return p == nil || p.Equals(&Prefs{}) }