device: avoid cycle-leaky runtime.SetFinalizer when unnecessary

In tailscale/wireguard-go#65, @lkosewsk reproduced a memory leak seen
in prod with lots of wireguard-go instances being created and
destroyed, where they were still being retained forever due to cycles
in the runtime.SetFinalizer reference graph.

Really we shouldn't be using runtime.SetFinalizer anywhere. But we
still use it on mobile platforms in WaitPool. But those platforms
don't have thousands of tsnet.Server instances coming & going, so this
is a half fix: avoid the finalizer registration on Linux, etc where
the queue doesn't need to be drained and there's no WaitPool
accounting. Just let GC handle it, without adding finalizer cycle
complexity.

Updates tailscale/corp#42776

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

device/channels.go

@@ -83,10 +83,16 @@ func newAutodrainingInboundQueue(device *Device) *autodrainingInboundQueue {
 	q := &autodrainingInboundQueue{
 		c: make(chan *QueueInboundElementsContainer, QueueInboundSize),
 	}
+	if device.needsInboundQueueFinalizer() {
 		runtime.SetFinalizer(q, device.flushInboundQueue)
+	}
 	return q
 }
 
+func (device *Device) needsInboundQueueFinalizer() bool {
+	return device.pool.messageBuffers.hasAccounting()
+}
+
 func (device *Device) flushInboundQueue(q *autodrainingInboundQueue) {
 	for {
 		select {
@@ -116,10 +122,16 @@ func newAutodrainingOutboundQueue(device *Device) *autodrainingOutboundQueue {
 	q := &autodrainingOutboundQueue{
 		c: make(chan *QueueOutboundElementsContainer, QueueOutboundSize),
 	}
+	if device.needsOutboundQueueFinalizer() {
 		runtime.SetFinalizer(q, device.flushOutboundQueue)
+	}
 	return q
 }
 
+func (device *Device) needsOutboundQueueFinalizer() bool {
+	return device.pool.messageBuffers.hasAccounting()
+}
+
 func (device *Device) flushOutboundQueue(q *autodrainingOutboundQueue) {
 	for {
 		select {

device/pools.go

@@ -25,6 +25,10 @@ func NewWaitPool(max uint32, new func() any) *WaitPool {
 	return p
 }
 
+func (p *WaitPool) hasAccounting() bool {
+	return p != nil && p.max != 0
+}
+
 func (p *WaitPool) Get() any {
 	if p.max != 0 {
 		p.lock.Lock()