From 09268b375cbbe076ebccf64decb2b8ee304c6957 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Wed, 3 Jun 2026 16:09:03 +0000
Subject: [PATCH] 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 | 16 ++++++++++++++--
 device/pools.go    |  4 ++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/device/channels.go b/device/channels.go
index 1eaec56..45b2a76 100644
--- a/device/channels.go
+++ b/device/channels.go
@@ -83,10 +83,16 @@ func newAutodrainingInboundQueue(device *Device) *autodrainingInboundQueue {
 	q := &autodrainingInboundQueue{
 		c: make(chan *QueueInboundElementsContainer, QueueInboundSize),
 	}
-	runtime.SetFinalizer(q, device.flushInboundQueue)
+	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),
 	}
-	runtime.SetFinalizer(q, device.flushOutboundQueue)
+	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 {
diff --git a/device/pools.go b/device/pools.go
index b7536a3..173486e 100644
--- a/device/pools.go
+++ b/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()