util/eventbus: make internal queue a generic type

In preparation for making the queues carry additional event metadata.

Updates #15160

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

util/eventbus/bus.go

@@ -78,7 +78,7 @@ func (b *Bus) Close() {
 }
 
 func (b *Bus) pump(ctx context.Context) {
-	var vals queue
+	var vals queue[any]
 	acceptCh := func() chan any {
 		if vals.Full() {
 			return nil

util/eventbus/queue.go

@@ -10,32 +10,32 @@ import (
 const maxQueuedItems = 16
 
 // queue is an ordered queue of length up to maxQueuedItems.
-type queue struct {
-	vals  []any
+type queue[T any] struct {
+	vals  []T
 	start int
 }
 
 // canAppend reports whether a value can be appended to q.vals without
 // shifting values around.
-func (q *queue) canAppend() bool {
+func (q *queue[T]) canAppend() bool {
 	return cap(q.vals) < maxQueuedItems || len(q.vals) < cap(q.vals)
 }
 
-func (q *queue) Full() bool {
+func (q *queue[T]) Full() bool {
 	return q.start == 0 && !q.canAppend()
 }
 
-func (q *queue) Empty() bool {
+func (q *queue[T]) Empty() bool {
 	return q.start == len(q.vals)
 }
 
-func (q *queue) Len() int {
+func (q *queue[T]) Len() int {
 	return len(q.vals) - q.start
 }
 
 // Add adds v to the end of the queue. Blocks until append can be
 // done.
-func (q *queue) Add(v any) {
+func (q *queue[T]) Add(v T) {
 	if !q.canAppend() {
 		if q.start == 0 {
 			panic("Add on a full queue")
@@ -54,21 +54,23 @@ func (q *queue) Add(v any) {
 
 // Peek returns the first value in the queue, without removing it from
 // the queue, or nil if the queue is empty.
-func (q *queue) Peek() any {
+func (q *queue[T]) Peek() T {
 	if q.Empty() {
-		return nil
+		var zero T
+		return zero
 	}
 
 	return q.vals[q.start]
 }
 
 // Drop discards the first value in the queue, if any.
-func (q *queue) Drop() {
+func (q *queue[T]) Drop() {
 	if q.Empty() {
 		return
 	}
 
-	q.vals[q.start] = nil
+	var zero T
+	q.vals[q.start] = zero
 	q.start++
 	if q.Empty() {
 		// Reset cursor to start of array, it's free to do.
@@ -78,6 +80,6 @@ func (q *queue) Drop() {
 }
 
 // Snapshot returns a copy of the queue's contents.
-func (q *queue) Snapshot() []any {
+func (q *queue[T]) Snapshot() []T {
 	return slices.Clone(q.vals[q.start:])
 }

util/eventbus/subscribe.go

@@ -27,7 +27,7 @@ type subscriber interface {
 	// processing other potential sources of wakeups, which is how we end
 	// up at this awkward type signature and sharing of internal state
 	// through dispatch.
-	dispatch(ctx context.Context, vals *queue, acceptCh func() chan any) bool
+	dispatch(ctx context.Context, vals *queue[any], acceptCh func() chan any) bool
 	Close()
 }
 
@@ -55,7 +55,7 @@ func newSubscribeState(c *Client) *subscribeState {
 }
 
 func (q *subscribeState) pump(ctx context.Context) {
-	var vals queue
+	var vals queue[any]
 	acceptCh := func() chan any {
 		if vals.Full() {
 			return nil
@@ -155,7 +155,7 @@ func (s *Subscriber[T]) subscribeType() reflect.Type {
 	return reflect.TypeFor[T]()
 }
 
-func (s *Subscriber[T]) dispatch(ctx context.Context, vals *queue, acceptCh func() chan any) bool {
+func (s *Subscriber[T]) dispatch(ctx context.Context, vals *queue[any], acceptCh func() chan any) bool {
 	t := vals.Peek().(T)
 	for {
 		// Keep the cases in this select in sync with subscribeState.pump