util/eventbus: add internal hook type for debugging

Publicly exposed debugging functions will use these hooks to
observe dataflow in the bus.

Updates #15160

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

util/eventbus/debug.go

@@ -0,0 +1,62 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package eventbus
+
+import (
+	"slices"
+	"sync"
+	"sync/atomic"
+)
+
+// A hook collects hook functions that can be run as a group.
+type hook[T any] struct {
+	sync.Mutex
+	fns []hookFn[T]
+}
+
+var hookID atomic.Uint64
+
+// add registers fn to be called when the hook is run. Returns an
+// unregistration function that removes fn from the hook when called.
+//
+//lint:ignore U1000 Not used yet, but will be in an upcoming change
+func (h *hook[T]) add(fn func(T)) (remove func()) {
+	id := hookID.Add(1)
+	h.Lock()
+	defer h.Unlock()
+	h.fns = append(h.fns, hookFn[T]{id, fn})
+	return func() { h.remove(id) }
+}
+
+// remove removes the function with the given ID from the hook.
+//
+//lint:ignore U1000 Not used yet, but will be in an upcoming change
+func (h *hook[T]) remove(id uint64) {
+	h.Lock()
+	defer h.Unlock()
+	h.fns = slices.DeleteFunc(h.fns, func(f hookFn[T]) bool { return f.ID == id })
+}
+
+// active reports whether any functions are registered with the
+// hook. This can be used to skip expensive work when the hook is
+// inactive.
+func (h *hook[T]) active() bool {
+	h.Lock()
+	defer h.Unlock()
+	return len(h.fns) > 0
+}
+
+// run calls all registered functions with the value v.
+func (h *hook[T]) run(v T) {
+	h.Lock()
+	defer h.Unlock()
+	for _, fn := range h.fns {
+		fn.Fn(v)
+	}
+}
+
+type hookFn[T any] struct {
+	ID uint64
+	Fn func(T)
+}