net: a smart way to check internal/poll ErrTimeout

[detailyang]

Hello guys.

It looks like we have no smart way to check the internal/poll ErrTimeout when we set the deadline on the connection.

The net.Error interface is not good enough as following

An Error represents a network error.

type Error interface {
        error
        Timeout() bool   // Is the error a timeout?
        Temporary() bool // Is the error temporary?
}

The Timeout() method may mislead caller think it's deadline error if the error is actually caused by sycall.Errno (e == EAGAIN || e == EWOULDBLOCK || e == ETIMEDOUT) rather than internal/poll ErrTimeout

Maybe we need a smart way to check the deadline error?

Many thanks in advance:)

[detailyang]

#31449 is a good example which the TCP keepalive probe failure cause the ETIMEDOUT

[agnivade]

I have removed the proposal tag and made it a feature request because you have not proposed anything concrete (which a proposal should be), but you would like something like this to happen.

/cc @mikioh

[ianlancetaylor]

Can you explain further why checking for and calling the Timeout method is not sufficient? Note that network descriptors will never return EAGAIN or EWOULDBLOCK in practice.

[detailyang]

@agnivade @ianlancetaylor

I wrote a gist in Linux to trigger the keepalive probe ETIMEDOUT in https://gist.github.com/detailyang/2f5ce65321c6f97ab66babedc8d92f16

The code above is that using iptables to drop TCP keepalive probe (dport is 12345) and see kernel code in https://elixir.bootlin.com/linux/v5.2-rc5/source/net/ipv4/tcp_timer.c#L642, we see when the TCP keepalive timer failed then kernel will set the error to ETIMEDOUT

/**
 *  tcp_write_err() - close socket and save error info
 *  @sk:  The socket the error has appeared on.
 *
 *  Returns: Nothing (void)
 */

static void tcp_write_err(struct sock *sk)
{
	sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
	sk->sk_error_report(sk);

	tcp_write_queue_purge(sk);
	tcp_done(sk);
	__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
}

[ianlancetaylor]

@detailyang Thanks, but I don't understand how that is an answer to my question. After all, you can already test for ETIMEDOUT for the cases where you need to distinguish that case.

[detailyang]

@ianlancetaylor Yep, because I know the error is ETIMEDOUT but for some guys who expect the net.Timeout means deadline timeout, it will be confused. See #31449

[detailyang]

People always check the deadline timeout error as the following

if err := conn.SetDeadline(time.Now().Add(1 * time.Hour)); err != nil {
    panic(err)
}

if _, err := conn.Write([]byte("test")); err != nil {
    if ne, ok := err.(net.Error); ok && ne.Timeout() {
        fmt.Println("deadline timeout)
    }
}