go: net: DNS broken on darwin without cgo (1.13 regression)

I was testing some new code for the Go build system and found that a simple TCP dial doesn’t work on Mac anymore, at least when the binary is cross-compiled.

Code is just:

var coordDialer = &net.Dialer{
        Timeout:   10 * time.Second,
        KeepAlive: 15 * time.Second,
}       

// dialCoordinatorTCP returns a TCP connection to the coordinator, making                                                                                                                                                                                          
// a CONNECT request to a proxy as a fallback.                                                                                                                                                                                                                     
func dialCoordinatorTCP(ctx context.Context, addr string) (net.Conn, error) {
        tcpConn, err := coordDialer.DialContext(ctx, "tcp", addr)

… with a context.Background() for ctx.

It always times out after 10 seconds.

But if I redeploy the same code but built with Go 1.12.x, it works fine.