util/eventbus: initial debugging facilities for the event bus

Enables monitoring events as they flow, listing bus clients, and
snapshotting internal queues to troubleshoot stalls.

Updates #15160

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

util/eventbus/client.go

@@ -19,13 +19,15 @@ import (
 type Client struct {
 	name         string
 	bus          *Bus
-	publishDebug hook[publishedEvent]
+	publishDebug hook[PublishedEvent]
 
 	mu  sync.Mutex
 	pub set.Set[publisher]
 	sub *subscribeState // Lazily created on first subscribe
 }
 
+func (c *Client) Name() string { return c.name }
+
 // Close closes the client. Implicitly closes all publishers and
 // subscribers obtained from this client.
 func (c *Client) Close() {
@@ -47,6 +49,16 @@ func (c *Client) Close() {
 	}
 }
 
+func (c *Client) snapshotSubscribeQueue() []DeliveredEvent {
+	return c.peekSubscribeState().snapshotQueue()
+}
+
+func (c *Client) peekSubscribeState() *subscribeState {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.sub
+}
+
 func (c *Client) subscribeState() *subscribeState {
 	c.mu.Lock()
 	defer c.mu.Unlock()
@@ -76,7 +88,7 @@ func (c *Client) deleteSubscriber(t reflect.Type, s *subscribeState) {
 	c.bus.unsubscribe(t, s)
 }
 
-func (c *Client) publish() chan<- publishedEvent {
+func (c *Client) publish() chan<- PublishedEvent {
 	return c.bus.write
 }