A finger client June 24, 2021 on Drew DeVault's blog

This is a short follow-up to the io_uring finger server article posted about a month ago. In the time since, we have expanded our language with a more complete networking stack, most importantly by adding a DNS resolver. I have used these improvements to write a small client implementation of the finger protocol.

use fmt;
use io;
use net::dial;
use os;
use strings;

@init fn registersvc() void = dial::registersvc("tcp", "finger", [], 79);
@noreturn fn usage() void = fmt::fatal("Usage: {} <user>[@<host>]", os::args[0]);

export fn main() void = {
	if (len(os::args) != 2) usage();

	const items = strings::split(os::args[1], "@");
	defer free(items);
	if (len(items) == 0) usage();

	const user = items[0];
	const host = if (len(items) == 1) "localhost"
		else if (len(items) == 2) items[1]
		else usage();

	match (execute(user, host)) {
		err: dial::error => fmt::fatal(dial::strerror(err)),
		err: io::error => fmt::fatal(io::strerror(err)),
		void => void,
	};
};

fn execute(user: str, host: str) (void | dial::error | io::error) = {
	const conn = dial::dial("tcp", host, "finger")?;
	defer io::close(conn);
	fmt::fprintf(conn, "{}\r\n", user)?;
	io::copy(os::stdout, conn)?;
};

Technically, we could do more, but I chose to just address the most common use-case for finger servers in active use today: querying a specific user. Expanding this with full support for all finger requests would probably only grow this code by 2 or 3 times.

Our language now provides a net::dial module, inspired by Go’s net.Dial and the Plan 9 dial function Go is derived from. Our dial actually comes a bit closer to Plan 9 by re-introducing the service parameter — Plan 9’s “tcp!example.org!http” becomes net::dial(“tcp”, “example.org”, “http”) in our language — which we use to find the port (unless you specify it in the address). The service parameter is tested against a small internal list of known services, and against /etc/services. We also automatically perform an SRV lookup for “_finger._tcp.example.org”, so most programs written in our language will support SRV records with no additional effort.

In our client code, we can see that the @init function adds “finger” to the list of known internal services. @init functions run on start-up, and this one just lets dial know about our protocol. Our network stack is open to extension in other respects, too — unlike Go, third-party libraries can define new protocol handlers for dial as well, perhaps opening it up in the future to networks like AF_BLUETOOTH, AF_AX25, and so on, complete with support for network-appropriate addresses and resolver functionality.

The rest is pretty straightforward! We just parse the command line, dial the server, write the username to it, and splice the connection into stdout. Much simpler than the server. Future improvements might rewrite the CRLF to LF, but that’s not particularly important.