syncs: add generic Pool (#12759)

Pool is a type-safe wrapper over sync.Pool.

Updates tailscale/corp#11038
Updates #cleanup

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/pool.go

@@ -0,0 +1,31 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syncs
+
+import "sync"
+
+// Pool is the generic version of [sync.Pool].
+type Pool[T any] struct {
+	pool sync.Pool
+
+	// New optionally specifies a function to generate
+	// a value when Get would otherwise return the zero value of T.
+	// It may not be changed concurrently with calls to Get.
+	New func() T
+}
+
+// Get selects an arbitrary item from the Pool, removes it from the Pool,
+// and returns it to the caller. See [sync.Pool.Get].
+func (p *Pool[T]) Get() T {
+	x, ok := p.pool.Get().(T)
+	if !ok && p.New != nil {
+		x = p.New()
+	}
+	return x
+}
+
+// Put adds x to the pool.
+func (p *Pool[T]) Put(x T) {
+	p.pool.Put(x)
+}