From 430d378f7d70a6386e61190050c2c781f3877124 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Sun, 17 Oct 2021 17:11:27 -0700
Subject: [PATCH] wgengine/magicsock: fix data race with sync.Pool in
 error+logging path

Fixes #3122

Change-Id: Ib52e84f9bd5813d6cf2e80ce5b2296912a48e064
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit c759fcc7d3ead9ddd188e980ee3c13e865fcd185)
---
 wgengine/magicsock/magicsock.go | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/wgengine/magicsock/magicsock.go b/wgengine/magicsock/magicsock.go
index db9cfe00a..7f6fb90e3 100644
--- a/wgengine/magicsock/magicsock.go
+++ b/wgengine/magicsock/magicsock.go
@@ -1149,8 +1149,12 @@ func (c *Conn) Send(b []byte, ep conn.Endpoint) error {
 // See sendAddr's docs on the return value meanings.
 func (c *Conn) sendUDP(ipp netaddr.IPPort, b []byte) (sent bool, err error) {
 	ua := udpAddrPool.Get().(*net.UDPAddr)
-	defer udpAddrPool.Put(ua)
-	return c.sendUDPStd(ipp.UDPAddrAt(ua), b)
+	sent, err = c.sendUDPStd(ipp.UDPAddrAt(ua), b)
+	if err == nil {
+		// Only return it to the pool on success; Issue 3122.
+		udpAddrPool.Put(ua)
+	}
+	return
 }
 
 // sendUDP sends UDP packet b to addr.