wgengine/bench: ignore "engine closing" errors

On benchmark completion, we shut down the wgengine.
If we happen to poll for status during shutdown,
we get an "engine closing" error.
It doesn't hurt anything; ignore it.

Fixes tailscale/corp#1776

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/bench/wg.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"errors"
 	"io"
 	"log"
 	"os"
@@ -89,6 +90,9 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netadd
 
 	var e1waitDoneOnce sync.Once
 	e1.SetStatusCallback(func(st *wgengine.Status, err error) {
+		if errors.Is(err, wgengine.ErrEngineClosing) {
+			return
+		}
 		if err != nil {
 			log.Fatalf("e1 status err: %v", err)
 		}
@@ -124,6 +128,9 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netadd
 
 	var e2waitDoneOnce sync.Once
 	e2.SetStatusCallback(func(st *wgengine.Status, err error) {
+		if errors.Is(err, wgengine.ErrEngineClosing) {
+			return
+		}
 		if err != nil {
 			log.Fatalf("e2 status err: %v", err)
 		}

wgengine/userspace.go

@@ -880,6 +880,8 @@ func (e *userspaceEngine) getStatusCallback() StatusCallback {
 
 var singleNewline = []byte{'\n'}
 
+var ErrEngineClosing = errors.New("engine closing; no status")
+
 func (e *userspaceEngine) getStatus() (*Status, error) {
 	// Grab derpConns before acquiring wgLock to not violate lock ordering;
 	// the DERPs method acquires magicsock.Conn.mu.
@@ -893,7 +895,7 @@ func (e *userspaceEngine) getStatus() (*Status, error) {
 	closing := e.closing
 	e.mu.Unlock()
 	if closing {
-		return nil, errors.New("engine closing; no status")
+		return nil, ErrEngineClosing
 	}
 
 	if e.wgdev == nil {