chat.go/chatserver-telnet.go

package main

import (
	"fmt"
	"io"
	"os"
	"strings"
	"sync"
	"time"
)

type telnetUser struct {
	bufConn   bufferedConn
	userCount chan int
	email     string
	newMsg    chan string
}

// Trying to keep it slim with just one goroutine per client for each reads and writes.
// Initially I was spawning a goroutine per write in the main select, but my guess is that
// constantly allocating and cleaning up 4k of memory (or perhaps less these days
// https://blog.nindalf.com/posts/how-goroutines-work/) is probably not very efficient for
// small tweet-sized network writes. Also, I like this style better
// TODO: Learn if it matters at all to have fewer long-lived vs more short-lived goroutines

// Auth & Reads
func handleTelnetConn(bufConn bufferedConn) {
	// Used as a reference: https://jameshfisher.com/2017/04/18/golang-tcp-server.html

	var email string
	var code string
	var authn bool

	// Handle all subsequent packets
	buffer := make([]byte, 1024)
	var u *telnetUser
	for {
		//fmt.Fprintf(os.Stdout, "[raw] Waiting for message...\n")
		count, err := bufConn.Read(buffer)
		if nil != err {
			if io.EOF != err {
				fmt.Fprintf(os.Stderr, "Non-EOF socket error: %s\n", err)
			} else {
				broadcastMsg <- chatMsg{
					sender:     nil,
					Message:    fmt.Sprintf("<%s> left #general\r\n", u.email),
					ReceivedAt: time.Now(),
					Channel:    "general",
					User:       "system",
				}
			}

			if nil != u {
				cleanTelnet <- *u
			}
			break
		}
		msg := string(buffer[:count])
		if "" == strings.TrimSpace(msg) {
			continue
		}

		// Rate Limit: Reasonable poor man's DoS prevention (Part 1)
		// A human does not send messages super fast and blocking the
		// writes of other incoming messages to this client for this long
		// won't hinder the user experience (and may in fact enhance it)
		// TODO: should do this for HTTP as well (or, better yet, implement hashcash)
		time.Sleep(150 * time.Millisecond)

		// Fun fact: if the buffer's current length (not capacity) is 0
		// then the Read returns 0 without error
		if 0 == count {
			fmt.Fprintf(os.Stdout, "[SANITY FAIL] using a 0-length buffer")
			break
		}

		if !authn {
			if "" == email {
				// Indeed telnet sends CRLF as part of the message
				//fmt.Fprintf(os.Stdout, "buf{%s}\n", buf[:count])

				// TODO use safer email testing
				email = strings.TrimSpace(msg)
				emailParts := strings.Split(email, "@")
				if 2 != len(emailParts) {
					fmt.Fprintf(bufConn, "Email: ")
					continue
				}

				// Debugging any weird characters as part of the message (just CRLF)
				//fmt.Fprintf(os.Stdout, "email: '%v'\n", []byte(email))

				// Just for a fun little bit of puzzah
				// Note: Reaction times are about 100ms
				//       Procesing times are about 250ms
				//       Right around 300ms is about when a person literally begins to get bored (begin context switching)
				//       Therefore any interaction should take longer than 100ms (time to register)
				//       and either engage the user or complete before reaching 300ms (not yet bored)
				//       This little ditty is meant to act as a psuedo-progress bar to engage the user
				//       Aside: a keystroke typically takes >=50s to type (probably closer to 200ms between words)
				//       https://stackoverflow.com/questions/22505698/what-is-a-typical-keypress-duration
				wg := sync.WaitGroup{}
				wg.Add(1)
				go func() {
					time.Sleep(50 * time.Millisecond)
					const msg = "Mailing auth code..."
					for _, r := range msg {
						time.Sleep(20 * time.Millisecond)
						fmt.Fprintf(bufConn, string(r))
					}
					time.Sleep(50 * time.Millisecond)
					wg.Done()
				}()
				if "" != config.Mailer.ApiKey {
					wg.Add(1)
					go func() {
						code, err = sendAuthCode(config.Mailer, strings.TrimSpace(email))
						wg.Done()
					}()
				} else {
					code, err = genAuthCode()
				}
				wg.Wait()
				if nil != err {
					// TODO handle better
					// (not sure why a random number would fail,
					//  but on a machine without internet the calls
					//  to mailgun APIs would fail)
					panic(err)
				}
				// so I don't have to actually go check my email
				fmt.Fprintf(os.Stdout, "\n== TELNET AUTHORIZATION ==\n[cheat code for %s]: %s\n", email, code)
				time.Sleep(150 * time.Millisecond)
				fmt.Fprintf(bufConn, " done\n")
				time.Sleep(150 * time.Millisecond)
				fmt.Fprintf(bufConn, "Auth Code: ")
				continue
			}

			if code != strings.TrimSpace(msg) {
				fmt.Fprintf(bufConn, "Incorrect Code\nAuth Code: ")
			} else {
				authn = true
				time.Sleep(150 * time.Millisecond)
				fmt.Fprintf(bufConn, "\n")
				u = &telnetUser{
					bufConn:   bufConn,
					email:     email,
					userCount: make(chan int, 1),
					newMsg:    make(chan string, 10), // reasonably sized
				}
				authTelnet <- *u
				// prevent data race on len(telnetConns)
				count := <-u.userCount
				close(u.userCount)
				u.userCount = nil

				// Note: There's a 500ms gap between when we accept the client
				// and when it can start receiving messages and when it begins
				// to handle them, however, it's unlikely that >= 10 messages
				// will simultaneously flood in during that time

				time.Sleep(50 * time.Millisecond)
				fmt.Fprintf(bufConn, "\n")
				time.Sleep(50 * time.Millisecond)
				// It turns out that ANSI characters work in Telnet just fine
				fmt.Fprintf(bufConn, "\033[1;32m"+"Welcome to #general (%d users)!"+"\033[22;39m", count)
				time.Sleep(50 * time.Millisecond)
				fmt.Fprintf(bufConn, "\n")
				time.Sleep(50 * time.Millisecond)
				// TODO /help /join <room> /users /channels /block <user> /upgrade <http/ws>
				//fmt.Fprintf(bufConn, "(TODO `/help' for list of commands)")
				time.Sleep(100 * time.Millisecond)
				fmt.Fprintf(bufConn, "\n")

				// Would be cool to write a prompt...
				// I wonder if I could send fudge some ANSI codes to keep the prompt
				// even when new messages come in, but not overwrite what he user typed...
				//fmt.Fprintf(bufConn, "\n%s> ", email)

				go handleTelnetBroadcast(u)
			}
			continue
		}

		broadcastMsg <- chatMsg{
			ReceivedAt: time.Now(),
			sender:     bufConn,
			Message:    strings.TrimRight(msg, "\r\n"),
			Channel:    "general",
			User:       email,
		}
	}
}

// Writes (post Auth)
func handleTelnetBroadcast(u *telnetUser) {
	for {
		msg, more := <-u.newMsg
		if !more {
			// channel was closed
			break
		}

		// Disallow Reverse Rate Limit: Reasonable poor man's DoS prevention (Part 3)
		// https://blog.cloudflare.com/the-complete-guide-to-golang-net-http-timeouts/
		timeoutDuration := 2 * time.Second
		u.bufConn.SetWriteDeadline(time.Now().Add(timeoutDuration))
		_, err := fmt.Fprintf(u.bufConn, msg+"\r\n")
		if nil != err {
			cleanTelnet <- *u
			break
		}
	}
}