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/queue.go

@@ -0,0 +1,83 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package eventbus
+
+import (
+	"slices"
+)
+
+const maxQueuedItems = 16
+
+// queue is an ordered queue of length up to maxQueuedItems.
+type queue struct {
+	vals  []any
+	start int
+}
+
+// canAppend reports whether a value can be appended to q.vals without
+// shifting values around.
+func (q *queue) canAppend() bool {
+	return cap(q.vals) < maxQueuedItems || len(q.vals) < cap(q.vals)
+}
+
+func (q *queue) Full() bool {
+	return q.start == 0 && !q.canAppend()
+}
+
+func (q *queue) Empty() bool {
+	return q.start == len(q.vals)
+}
+
+func (q *queue) 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) {
+	if !q.canAppend() {
+		if q.start == 0 {
+			panic("Add on a full queue")
+		}
+
+		// Slide remaining values back to the start of the array.
+		n := copy(q.vals, q.vals[q.start:])
+		toClear := len(q.vals) - n
+		clear(q.vals[len(q.vals)-toClear:])
+		q.vals = q.vals[:n]
+		q.start = 0
+	}
+
+	q.vals = append(q.vals, v)
+}
+
+// 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 {
+	if q.Empty() {
+		return nil
+	}
+
+	return q.vals[q.start]
+}
+
+// Drop discards the first value in the queue, if any.
+func (q *queue) Drop() {
+	if q.Empty() {
+		return
+	}
+
+	q.vals[q.start] = nil
+	q.start++
+	if q.Empty() {
+		// Reset cursor to start of array, it's free to do.
+		q.start = 0
+		q.vals = q.vals[:0]
+	}
+}
+
+// Snapshot returns a copy of the queue's contents.
+func (q *queue) Snapshot() []any {
+	return slices.Clone(q.vals[q.start:])
+}