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/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:])
 }