On shutdown, smtpd waits for one more message before shutting down

[cuu508]

Hello, I'm looking at implementing a graceful shutdown but can't quite get it to work. Here's my code that starts the SMTP listener:

func listenEmail(ctx context.Context, port string) {
    srv := &smtpd.Server{
        Addr: "0.0.0.0:" + port,
        Handler: mailHandler,
        Appname: "...",
        Hostname: "...",
    }

    go func() {
        <- ctx.Done()

        log.Println("We should stop the SMTP server now...")
        srv.Close()
        log.Println("We closed it")
    }()

    err := srv.ListenAndServe()
    if err != nil {
        log.Fatal("Unable to start smtp server: ", err)
        os.Exit(1)
    }

    log.Println("listenEmail goroutine quits")
}

Not shown here, but on Ctrl+C, ctx gets cancelled, so the inner goroutine runs and calls srv.Close(). When I run this, the "We closed it" message from the inner goroutine gets printed, but the final "listenEmail goroutine quits" does not. I assume that's because ListenAndServe still keeps blocking.

I was looking at the smtpd source code:

// Serve creates a new SMTP session after a network connection is established.
func (srv *Server) Serve(ln net.Listener) error {
	if atomic.LoadInt32(&srv.inShutdown) != 0 {
		return ErrServerClosed
	}

	defer ln.Close()
	for {

		// if we are shutting down, don't accept new connections
		select {
		case <-srv.getShutdownChan():
			return ErrServerClosed
		default:
		}

		conn, err := ln.Accept()
		if err != nil {
			if netErr, ok := err.(net.Error); ok && netErr.Temporary() {
				continue
			}
			return err
		}

		session := srv.newSession(conn)
		atomic.AddInt32(&srv.openSessions, 1)
		go session.serve()
	}
}

To me it looks like it checks the shutdown condition at the start of the for loop, and then blocks at ln.Accept(). So, I'm guessing, it needs to receive one extra email for the loop to advance and the Serve call to return. Does that make sense? I'm a Go novice and am stepping on rakes and making wrong guesses left and right :-)

I also run a HTTP listener (http.Server) in a similar way, and it is able to finish up immediately, without waiting for one extra HTTP request. Perhaps there's a viable way to make smtpd work the same?

Originally posted by @cuu508 in #27 (comment)