util/eventbus: [DRAFT] add sketch of Subscribe with funcsbradfitz/evsub

Updates #DRAFT

Change-Id: Id1f208bdd55a9ae4eccc07afc44eade6e67db5bb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

2 files changed, 52 insertions, 1 deletions

diff --git a/util/eventbus/client.go b/util/eventbus/client.go
index 7c0268886..b5333292a 100644
--- a/util/eventbus/client.go
+++ b/util/eventbus/client.go
@@ -147,6 +147,26 @@ func Subscribe[T any](c *Client) *Subscriber[T] {
 	return s
 }
 
+// SubscribeFunc is like [Subscribe] but calls the provided func
+// for each event of type T.
+//
+// The func is not called from a new goroutine. It is called
+// from the eventbus's goroutine.
+func SubscribeFunc[T any](c *Client, f func(T)) *SubscriberFunc[T] {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if c.isClosed() {
+		panic("cannot SubscribeFunc on a closed client")
+	}
+
+	r := c.subscribeStateLocked()
+	s := newSubscriberFunc(r, f)
+	r.addSubscriber(s)
+
+	return &SubscriberFunc[T]{s: s}
+}
+
 // Publish returns a publisher for event type T using the given client.
 // It panics if c is closed.
 func Publish[T any](c *Client) *Publisher[T] {
diff --git a/util/eventbus/subscribe.go b/util/eventbus/subscribe.go
index ef155e621..cfa810daa 100644
--- a/util/eventbus/subscribe.go
+++ b/util/eventbus/subscribe.go
@@ -179,10 +179,21 @@ func (s *subscribeState) closed() <-chan struct{} {
 // A Subscriber delivers one type of event from a [Client].
 type Subscriber[T any] struct {
 	stop       stopFlag
-	read       chan T
+	read       chan T  // mutually exclusive with readFunc
+	readFunc   func(T) // mutually exclusive with read
 	unregister func()
 }
 
+// SubscriberFunc is like [Subscriber] but has no channel
+// for delivery. They're returned by [SubscribeFunc].
+type SubscriberFunc[T any] struct {
+	s *Subscriber[T] // but don't use its Events or Done methods
+}
+
+func (s *SubscriberFunc[T]) Close() {
+	s.s.Close()
+}
+
 func newSubscriber[T any](r *subscribeState) *Subscriber[T] {
 	return &Subscriber[T]{
 		read:       make(chan T),
@@ -190,6 +201,13 @@ func newSubscriber[T any](r *subscribeState) *Subscriber[T] {
 	}
 }
 
+func newSubscriberFunc[T any](r *subscribeState, f func(T)) *Subscriber[T] {
+	return &Subscriber[T]{
+		readFunc:   f,
+		unregister: func() { r.deleteSubscriber(reflect.TypeFor[T]()) },
+	}
+}
+
 func newMonitor[T any](attach func(fn func(T)) (cancel func())) *Subscriber[T] {
 	ret := &Subscriber[T]{
 		read: make(chan T, 100), // arbitrary, large
@@ -211,6 +229,19 @@ func (s *Subscriber[T]) monitor(debugEvent T) {
 
 func (s *Subscriber[T]) dispatch(ctx context.Context, vals *queue[DeliveredEvent], acceptCh func() chan DeliveredEvent, snapshot chan chan []DeliveredEvent) bool {
 	t := vals.Peek().Event.(T)
+
+	if s.readFunc != nil {
+		select {
+		case <-ctx.Done():
+			return false
+		case ch := <-snapshot:
+			ch <- vals.Snapshot()
+		default:
+		}
+		s.readFunc(t)
+		vals.Drop()
+		return true
+	}
 	for {
 		// Keep the cases in this select in sync with subscribeState.pump
 		// above. The only different should be that this select