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>

health/health.go

@@ -19,15 +19,16 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/multierr"
+	"tailscale.com/util/set"
 )
 
 var (
 	// mu guards everything in this var block.
 	mu sync.Mutex
 
-	sysErr    = map[Subsystem]error{}                     // error key => err (or nil for no error)
-	watchers  = map[*watchHandle]func(Subsystem, error){} // opt func to run if error state changes
-	warnables = map[*Warnable]struct{}{}                  // set of warnables
+	sysErr    = map[Subsystem]error{}                   // error key => err (or nil for no error)
+	watchers  = set.HandleSet[func(Subsystem, error)]{} // opt func to run if error state changes
+	warnables = map[*Warnable]struct{}{}                // set of warnables
 	timer     *time.Timer
 
 	debugHandler = map[string]http.Handler{}
@@ -148,8 +149,6 @@ func AppendWarnableDebugFlags(base []string) []string {
 	return ret
 }
 
-type watchHandle byte
-
 // RegisterWatcher adds a function that will be called if an
 // error changes state either to unhealthy or from unhealthy. It is
 // not called on transition from unknown to healthy. It must be non-nil
@@ -157,8 +156,7 @@ type watchHandle byte
 func RegisterWatcher(cb func(key Subsystem, err error)) (unregister func()) {
 	mu.Lock()
 	defer mu.Unlock()
-	handle := new(watchHandle)
-	watchers[handle] = cb
+	handle := watchers.Add(cb)
 	if timer == nil {
 		timer = time.AfterFunc(time.Minute, timerSelfCheck)
 	}
@@ -174,23 +172,23 @@ func RegisterWatcher(cb func(key Subsystem, err error)) (unregister func()) {
 }
 
 // SetRouterHealth sets the state of the wgengine/router.Router.
-func SetRouterHealth(err error) { set(SysRouter, err) }
+func SetRouterHealth(err error) { setErr(SysRouter, err) }
 
 // RouterHealth returns the wgengine/router.Router error state.
 func RouterHealth() error { return get(SysRouter) }
 
 // SetDNSHealth sets the state of the net/dns.Manager
-func SetDNSHealth(err error) { set(SysDNS, err) }
+func SetDNSHealth(err error) { setErr(SysDNS, err) }
 
 // DNSHealth returns the net/dns.Manager error state.
 func DNSHealth() error { return get(SysDNS) }
 
 // SetDNSOSHealth sets the state of the net/dns.OSConfigurator
-func SetDNSOSHealth(err error) { set(SysDNSOS, err) }
+func SetDNSOSHealth(err error) { setErr(SysDNSOS, err) }
 
 // SetDNSManagerHealth sets the state of the Linux net/dns manager's
 // discovery of the /etc/resolv.conf situation.
-func SetDNSManagerHealth(err error) { set(SysDNSManager, err) }
+func SetDNSManagerHealth(err error) { setErr(SysDNSManager, err) }
 
 // DNSOSHealth returns the net/dns.OSConfigurator error state.
 func DNSOSHealth() error { return get(SysDNSOS) }
@@ -213,7 +211,7 @@ func get(key Subsystem) error {
 	return sysErr[key]
 }
 
-func set(key Subsystem, err error) {
+func setErr(key Subsystem, err error) {
 	mu.Lock()
 	defer mu.Unlock()
 	setLocked(key, err)