util/eventbus: rework to have a Client abstraction

The Client carries both publishers and subscribers for a single
actor. This makes the APIs for publish and subscribe look more
similar, and this structure is a better fit for upcoming debug
facilities.

Updates #15160

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

util/eventbus/publish.go

@@ -11,35 +11,41 @@ import (
 // publisher is a uniformly typed wrapper around Publisher[T], so that
 // debugging facilities can look at active publishers.
 type publisher interface {
-	publisherName() string
+	publishType() reflect.Type
+	Close()
 }
 
-// A Publisher publishes events on the bus.
+// A Publisher publishes typed events on a bus.
 type Publisher[T any] struct {
-	bus     *Bus
-	name    string
+	client  *Client
 	stopCtx context.Context
 	stop    context.CancelFunc
 }
 
-// PublisherOf returns a publisher for event type T on the given bus.
-//
-// The publisher's name should be a short, human-readable string that
-// identifies this event publisher. The name is only visible through
-// debugging APIs.
-func PublisherOf[T any](b *Bus, name string) *Publisher[T] {
+func newPublisher[T any](c *Client) *Publisher[T] {
 	ctx, cancel := context.WithCancel(context.Background())
 	ret := &Publisher[T]{
-		bus:     b,
-		name:    name,
+		client:  c,
 		stopCtx: ctx,
 		stop:    cancel,
 	}
-	b.addPublisher(ret)
+	c.addPublisher(ret)
 	return ret
 }
 
-func (p *Publisher[T]) publisherName() string { return p.name }
+// Close closes the publisher.
+//
+// Calls to Publish after Close silently do nothing.
+func (p *Publisher[T]) Close() {
+	// Just unblocks any active calls to Publish, no other
+	// synchronization needed.
+	p.stop()
+	p.client.deletePublisher(p)
+}
+
+func (p *Publisher[T]) publishType() reflect.Type {
+	return reflect.TypeFor[T]()
+}
 
 // Publish publishes event v on the bus.
 func (p *Publisher[T]) Publish(v T) {
@@ -48,32 +54,21 @@ func (p *Publisher[T]) Publish(v T) {
 	select {
 	case <-p.stopCtx.Done():
 		return
-	case <-p.bus.stop.WaitChan():
-		return
 	default:
 	}
 
 	select {
-	case p.bus.write <- v:
+	case p.client.publish() <- v:
 	case <-p.stopCtx.Done():
-	case <-p.bus.stop.WaitChan():
 	}
 }
 
-// ShouldPublish reports whether anyone is subscribed to events of
-// type T.
+// ShouldPublish reports whether anyone is subscribed to the events
+// that this publisher emits.
 //
 // ShouldPublish can be used to skip expensive event construction if
 // nobody seems to care. Publishers must not assume that someone will
 // definitely receive an event if ShouldPublish returns true.
 func (p *Publisher[T]) ShouldPublish() bool {
-	dests := p.bus.dest(reflect.TypeFor[T]())
-	return len(dests) > 0
-}
-
-// Close closes the publisher, indicating that no further events will
-// be published with it.
-func (p *Publisher[T]) Close() {
-	p.stop()
-	p.bus.deletePublisher(p)
+	return p.client.shouldPublish(reflect.TypeFor[T]())
 }