util/eventbus: track additional event context in publish queue

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>

util/eventbus/bus.go

@@ -16,8 +16,8 @@ import (
 // subscribers.
 type Bus struct {
 	router   *worker
-	write    chan any
-	snapshot chan chan []any
+	write    chan publishedEvent
+	snapshot chan chan []publishedEvent
 
 	topicsMu sync.Mutex // guards everything below.
 	topics   map[reflect.Type][]*subscribeState
@@ -31,8 +31,8 @@ type Bus struct {
 // and [Bus.Queue] and [Subscribe] to make event subscribers.
 func New() *Bus {
 	ret := &Bus{
-		write:    make(chan any),
-		snapshot: make(chan chan []any),
+		write:    make(chan publishedEvent),
+		snapshot: make(chan chan []publishedEvent),
 		topics:   map[reflect.Type][]*subscribeState{},
 		clients:  set.Set[*Client]{},
 	}
@@ -78,8 +78,8 @@ func (b *Bus) Close() {
 }
 
 func (b *Bus) pump(ctx context.Context) {
-	var vals queue[any]
-	acceptCh := func() chan any {
+	var vals queue[publishedEvent]
+	acceptCh := func() chan publishedEvent {
 		if vals.Full() {
 			return nil
 		}
@@ -92,12 +92,12 @@ func (b *Bus) pump(ctx context.Context) {
 		// queue space for it.
 		for !vals.Empty() {
 			val := vals.Peek()
-			dests := b.dest(reflect.ValueOf(val).Type())
+			dests := b.dest(reflect.ValueOf(val.Event).Type())
 			for _, d := range dests {
 			deliverOne:
 				for {
 					select {
-					case d.write <- val:
+					case d.write <- val.Event:
 						break deliverOne
 					case <-d.closed():
 						// Queue closed, don't block but continue

util/eventbus/client.go

@@ -75,7 +75,7 @@ func (c *Client) deleteSubscriber(t reflect.Type, s *subscribeState) {
 	c.bus.unsubscribe(t, s)
 }
 
-func (c *Client) publish() chan<- any {
+func (c *Client) publish() chan<- publishedEvent {
 	return c.bus.write
 }
 

util/eventbus/publish.go

@@ -5,8 +5,15 @@ package eventbus
 
 import (
 	"reflect"
+	"time"
 )
 
+type publishedEvent struct {
+	Event     any
+	From      *Client
+	Published time.Time
+}
+
 // publisher is a uniformly typed wrapper around Publisher[T], so that
 // debugging facilities can look at active publishers.
 type publisher interface {
@@ -52,8 +59,14 @@ func (p *Publisher[T]) Publish(v T) {
 	default:
 	}
 
+	evt := publishedEvent{
+		Event:     v,
+		From:      p.client,
+		Published: time.Now(),
+	}
+
 	select {
-	case p.client.publish() <- v:
+	case p.client.publish() <- evt:
 	case <-p.stop.Done():
 	}
 }