tsnet: fix data race in TestFallbackTCPHandler

Fixes tailscale#9805

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

tsnet/tsnet_test.go

@@ -26,6 +26,7 @@ import (
 	"reflect"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -652,23 +653,23 @@ func TestFallbackTCPHandler(t *testing.T) {
 	}
 	t.Logf("ping success: %#+v", res)
 
-	s1TcpConnCount := 0
+	var s1TcpConnCount atomic.Int32
 	deregister := s1.RegisterFallbackTCPHandler(func(src, dst netip.AddrPort) (handler func(net.Conn), intercept bool) {
-		s1TcpConnCount++
+		s1TcpConnCount.Add(1)
 		return nil, false
 	})
 
-	if _, err = s2.Dial(ctx, "tcp", fmt.Sprintf("%s:8081", s1ip)); err == nil {
+	if _, err := s2.Dial(ctx, "tcp", fmt.Sprintf("%s:8081", s1ip)); err == nil {
 		t.Fatal("Expected dial error because fallback handler did not intercept")
 	}
-	if s1TcpConnCount != 1 {
-		t.Errorf("s1TcpConnCount = %d, want %d", s1TcpConnCount, 1)
+	if got := s1TcpConnCount.Load(); got != 1 {
+		t.Errorf("s1TcpConnCount = %d, want %d", got, 1)
 	}
 	deregister()
-	if _, err = s2.Dial(ctx, "tcp", fmt.Sprintf("%s:8081", s1ip)); err == nil {
+	if _, err := s2.Dial(ctx, "tcp", fmt.Sprintf("%s:8081", s1ip)); err == nil {
 		t.Fatal("Expected dial error because nothing would intercept")
 	}
-	if s1TcpConnCount != 1 {
-		t.Errorf("s1TcpConnCount = %d, want %d", s1TcpConnCount, 1)
+	if got := s1TcpConnCount.Load(); got != 1 {
+		t.Errorf("s1TcpConnCount = %d, want %d", got, 1)
 	}
 }