util/eventbus: add a helper program for bus development

The demo program generates a stream of made up bus events between
a number of bus actors, as a way to generate some interesting activity
to show on the bus debug page.

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

util/eventbus/debug-demo/main.go

@@ -0,0 +1,103 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// debug-demo is a program that serves a bus's debug interface over
+// HTTP, then generates some fake traffic from a handful of
+// clients. It is an aid to development, to have something to present
+// on the debug interfaces while writing them.
+package main
+
+import (
+	"log"
+	"math/rand/v2"
+	"net/http"
+	"net/netip"
+	"time"
+
+	"tailscale.com/tsweb"
+	"tailscale.com/types/key"
+	"tailscale.com/util/eventbus"
+)
+
+func main() {
+	b := eventbus.New()
+	c := b.Client("RouteMonitor")
+	go testPub[RouteAdded](c, 5*time.Second)
+	go testPub[RouteRemoved](c, 5*time.Second)
+	c = b.Client("ControlClient")
+	go testPub[PeerAdded](c, 3*time.Second)
+	go testPub[PeerRemoved](c, 6*time.Second)
+	c = b.Client("Portmapper")
+	go testPub[PortmapAcquired](c, 10*time.Second)
+	go testPub[PortmapLost](c, 15*time.Second)
+	go testSub[RouteAdded](c)
+	c = b.Client("WireguardConfig")
+	go testSub[PeerAdded](c)
+	go testSub[PeerRemoved](c)
+	c = b.Client("Magicsock")
+	go testPub[PeerPathChanged](c, 5*time.Second)
+	go testSub[RouteAdded](c)
+	go testSub[RouteRemoved](c)
+	go testSub[PortmapAcquired](c)
+	go testSub[PortmapLost](c)
+
+	m := http.NewServeMux()
+	d := tsweb.Debugger(m)
+	b.Debugger().RegisterHTTP(d)
+
+	m.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/debug/bus", http.StatusFound)
+	})
+	log.Printf("Serving debug interface at http://localhost:8185/debug/bus")
+	http.ListenAndServe(":8185", m)
+}
+
+func testPub[T any](c *eventbus.Client, every time.Duration) {
+	p := eventbus.Publish[T](c)
+	for {
+		jitter := time.Duration(rand.N(2000)) * time.Millisecond
+		time.Sleep(jitter)
+		var zero T
+		log.Printf("%s publish: %T", c.Name(), zero)
+		p.Publish(zero)
+		time.Sleep(every)
+	}
+}
+
+func testSub[T any](c *eventbus.Client) {
+	s := eventbus.Subscribe[T](c)
+	for v := range s.Events() {
+		log.Printf("%s received: %T", c.Name(), v)
+	}
+}
+
+type RouteAdded struct {
+	Prefix   netip.Prefix
+	Via      netip.Addr
+	Priority int
+}
+type RouteRemoved struct {
+	Prefix netip.Addr
+}
+
+type PeerAdded struct {
+	ID  int
+	Key key.NodePublic
+}
+type PeerRemoved struct {
+	ID  int
+	Key key.NodePublic
+}
+
+type PortmapAcquired struct {
+	Endpoint netip.Addr
+}
+type PortmapLost struct {
+	Endpoint netip.Addr
+}
+
+type PeerPathChanged struct {
+	ID         int
+	EndpointID int
+	Quality    int
+}