enforce timeout on initial socks5 proxy connection

[conduition]

This PR fixes a bug which excepted initial SOCKS5 proxy connection attempts from the punctual enforcement of timeouts.

Before this change, invoking Socks5Stream::connect (or Socks5Stream::connect_with_password) could block for much longer than the configured timeout. In practice, this manifested as Client::from_config apparently failing to respect the timeout specified in the Config passed to it. AFAICT this only applied to SOCKS proxy connections.

Example

To demonstrate, here is a simple example program which attempts to connect to an unreachable electrum server with a 10 second timeout.

use electrum_client::{Client, ConfigBuilder, Socks5Config};

fn main() {
    let proxy = Socks5Config::new("127.0.0.1:9050");
    let config = ConfigBuilder::new()
        .socks5(Some(proxy))
        .timeout(Some(10))
        .build();

    let start = std::time::SystemTime::now();
    let result = Client::from_config(
        "tcp://bejqtnc64qttdempkczylydg7l3ordwugbdar7yqbndck53ukx7wnwad.onion:50001",
        config,
    );
    match result {
        Ok(_) => {
            println!("Successfully connected")
        }
        Err(e) => {
            println!(
                "failed to connect after {:.2}s: {e}",
                start.elapsed().unwrap().as_secs_f64()
            );
        }
    };
}

You'd expect the connection attempt to always fail at around 10 seconds, but in fact most attempts take considerably longer.

$ for i in {1..10} ; do cargo run -q ; done
failed to connect after 7.65s: host unreachable
failed to connect after 47.78s: host unreachable
failed to connect after 18.17s: host unreachable
failed to connect after 29.24s: host unreachable
failed to connect after 16.15s: host unreachable
failed to connect after 14.40s: host unreachable
failed to connect after 16.89s: host unreachable
failed to connect after 9.93s: host unreachable
failed to connect after 8.81s: host unreachable
failed to connect after 17.80s: host unreachable

Cause and Fix

This was happening because the private method Socks5Stream::connect_raw only respected the timeout parameter for the initial connection to the proxy address.

Once that TCP socket is established, the SOCKS5 client code must exchange a couple of messages with the proxy itself: One request/response cycle to authenticate, and then another request/response cycle to configure the forward proxy to the ultimate destination address. The latter of these two request/response cycles could block for long periods of time, in the case where the proxy was responsive but the ultimate destination was unresponsive.

Since no timeout was set on the socket at this stage, the Socks5Stream code would wait for an indefinite amount of time for a reply from the proxy, usually only once the proxy itself times out and finally sends a reply.

My suggested fix in this PR is to set the read/write timeouts immediately on the socket connecting to the proxy, so that if the proxy doesn't reply in time, we return an error to the caller promptly.

[notmandatory]

@conduition thanks for the fix, and detailed analysis.

@RCasatta do you have any concerns with this fix? otherwise I'll merge it.

[notmandatory]

@conduition can you repush this change with a signed commit? it's a rule on our repo that commits be signed.

[conduition]

@notmandatory My pleasure. Re-pushed with GPG signature. :heavy_check_mark:

[RCasatta]

utACK d8554fb550c87bbbcb3c129b2679a22d5f6dc70b