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/client.go

@@ -0,0 +1,100 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package eventbus
+
+import (
+	"reflect"
+	"sync"
+
+	"tailscale.com/util/set"
+)
+
+// A Client can publish and subscribe to events on its attached
+// bus. See [Publish] to publish events, and [Subscribe] to receive
+// events.
+//
+// Subscribers that share the same client receive events one at a
+// time, in the order they were published.
+type Client struct {
+	name string
+	bus  *Bus
+
+	mu  sync.Mutex
+	pub set.Set[publisher]
+	sub *subscribeState // Lazily created on first subscribe
+}
+
+// Close closes the client. Implicitly closes all publishers and
+// subscribers obtained from this client.
+func (c *Client) Close() {
+	var (
+		pub set.Set[publisher]
+		sub *subscribeState
+	)
+
+	c.mu.Lock()
+	pub, c.pub = c.pub, nil
+	sub, c.sub = c.sub, nil
+	c.mu.Unlock()
+
+	if sub != nil {
+		sub.close()
+	}
+	for p := range pub {
+		p.Close()
+	}
+}
+
+func (c *Client) subscribeState() *subscribeState {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.sub == nil {
+		c.sub = newSubscribeState(c)
+	}
+	return c.sub
+}
+
+func (c *Client) addPublisher(pub publisher) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.pub.Add(pub)
+}
+
+func (c *Client) deletePublisher(pub publisher) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.pub.Delete(pub)
+}
+
+func (c *Client) addSubscriber(t reflect.Type, s *subscribeState) {
+	c.bus.subscribe(t, s)
+}
+
+func (c *Client) deleteSubscriber(t reflect.Type, s *subscribeState) {
+	c.bus.unsubscribe(t, s)
+}
+
+func (c *Client) publish() chan<- any {
+	return c.bus.write
+}
+
+func (c *Client) shouldPublish(t reflect.Type) bool {
+	return c.bus.shouldPublish(t)
+}
+
+// Subscribe requests delivery of events of type T through the given
+// Queue. Panics if the queue already has a subscriber for T.
+func Subscribe[T any](c *Client) *Subscriber[T] {
+	return newSubscriber[T](c.subscribeState())
+}
+
+// Publisher returns a publisher for event type T using the given
+// client.
+func Publish[T any](c *Client) *Publisher[T] {
+	ret := newPublisher[T](c)
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.pub.Add(ret)
+	return ret
+}