util/set: add new set package for SetHandle type

We use this pattern in a number of places (in this repo and elsewhere)
and I was about to add a fourth to this repo which was crossing the line.
Add this type instead so they're all the same.

Also, we have another Set type (SliceSet, which tracks its keys in
order) in another repo we can move to this package later.

Change-Id: Ibbdcdba5443fae9b6956f63990bdb9e9443cefa9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/monitor/monitor.go

@@ -17,6 +17,7 @@ import (
 
 	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/set"
 )
 
 // pollWallTimeInterval is how often we check the time to check
@@ -54,9 +55,6 @@ type osMon interface {
 // callback.
 type ChangeFunc func(changed bool, state *interfaces.State)
 
-// An allocated callbackHandle's address is the Mon.cbs map key.
-type callbackHandle byte
-
 // Mon represents a monitoring instance.
 type Mon struct {
 	logf   logger.Logf
@@ -65,8 +63,8 @@ type Mon struct {
 	stop   chan struct{} // closed on Stop
 
 	mu         sync.Mutex // guards all following fields
-	cbs        map[*callbackHandle]ChangeFunc
-	ruleDelCB  map[*callbackHandle]RuleDeleteCallback
+	cbs        set.HandleSet[ChangeFunc]
+	ruleDelCB  set.HandleSet[RuleDeleteCallback]
 	ifState    *interfaces.State
 	gwValid    bool       // whether gw and gwSelfIP are valid
 	gw         netip.Addr // our gateway's IP
@@ -86,7 +84,6 @@ func New(logf logger.Logf) (*Mon, error) {
 	logf = logger.WithPrefix(logf, "monitor: ")
 	m := &Mon{
 		logf:     logf,
-		cbs:      map[*callbackHandle]ChangeFunc{},
 		change:   make(chan struct{}, 1),
 		stop:     make(chan struct{}),
 		lastWall: wallTime(),
@@ -144,10 +141,9 @@ func (m *Mon) GatewayAndSelfIP() (gw, myIP netip.Addr, ok bool) {
 // notified (in their own goroutine) when the network state changes.
 // To remove this callback, call unregister (or close the monitor).
 func (m *Mon) RegisterChangeCallback(callback ChangeFunc) (unregister func()) {
-	handle := new(callbackHandle)
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	m.cbs[handle] = callback
+	handle := m.cbs.Add(callback)
 	return func() {
 		m.mu.Lock()
 		defer m.mu.Unlock()
@@ -165,13 +161,9 @@ type RuleDeleteCallback func(table uint8, priority uint32)
 // notified (in their own goroutine) when a Linux ip rule is deleted.
 // To remove this callback, call unregister (or close the monitor).
 func (m *Mon) RegisterRuleDeleteCallback(callback RuleDeleteCallback) (unregister func()) {
-	handle := new(callbackHandle)
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	if m.ruleDelCB == nil {
-		m.ruleDelCB = map[*callbackHandle]RuleDeleteCallback{}
-	}
-	m.ruleDelCB[handle] = callback
+	handle := m.ruleDelCB.Add(callback)
 	return func() {
 		m.mu.Lock()
 		defer m.mu.Unlock()