util/eventbus: initial implementation of an in-process event bus

Updates #15160

Signed-off-by: David Anderson <dave@tailscale.com>
Co-authored-by: M. J. Fromberger <fromberger@tailscale.com>

util/eventbus/publish.go

@@ -0,0 +1,79 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package eventbus
+
+import (
+	"context"
+	"reflect"
+)
+
+// publisher is a uniformly typed wrapper around Publisher[T], so that
+// debugging facilities can look at active publishers.
+type publisher interface {
+	publisherName() string
+}
+
+// A Publisher publishes events on the bus.
+type Publisher[T any] struct {
+	bus     *Bus
+	name    string
+	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] {
+	ctx, cancel := context.WithCancel(context.Background())
+	ret := &Publisher[T]{
+		bus:     b,
+		name:    name,
+		stopCtx: ctx,
+		stop:    cancel,
+	}
+	b.addPublisher(ret)
+	return ret
+}
+
+func (p *Publisher[T]) publisherName() string { return p.name }
+
+// Publish publishes event v on the bus.
+func (p *Publisher[T]) Publish(v T) {
+	// Check for just a stopped publisher or bus before trying to
+	// write, so that once closed Publish consistently does nothing.
+	select {
+	case <-p.stopCtx.Done():
+		return
+	case <-p.bus.stop.WaitChan():
+		return
+	default:
+	}
+
+	select {
+	case p.bus.write <- v:
+	case <-p.stopCtx.Done():
+	case <-p.bus.stop.WaitChan():
+	}
+}
+
+// ShouldPublish reports whether anyone is subscribed to events of
+// type T.
+//
+// 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)
+}