04-login.md

Lesson 4 - Login to the chat server! (15-20 minutes)

Goals

When a user logs in to our chat server, we need to know who they are. And they will want to know who we are.

In this exercise, we will:

• Print a banner when the user connects.
• Implement ChatUser.Login (and some related functions) to be able to read a username from the user.

Reminder: To test your server, you can use nc localhost 6677 or telnet localhost 6677.

Steps

1. In chat.go, view the main function and notice how ChatRoom.Join is called on each connection. Find the section of the code ChatRoom.Join

// This is what we want to modify
func (cr *ChatRoom) Join(conn net.Conn)     {}

1. In ChatRoom.Join, write the code that does the following: * 🌟 Creates a new ChatUser object using NewChatUser * 🌟 Calls ChatUser.Login on this object (and verify there is no error) * 🌟 Notifies of a new user by putting the newly created ChatUser object on the ChatRoom.joins channel. (Don't worry about how this is used for now, I'll show you how we consume it later.)

[Stuck on any of the steps above? See the solution!](code/04-login/chat.go#L39-L44)

1. Great! Now let's start implementing ChatUser.Login. First, let's create a helpful banner that says "Welcome to [foo's] server", where foo is your name.

2. 🌟 Find ChatUser.Login and edit it to call cu.WriteString with your banner message. Make sure you also write the newline.

Here's what it should look like: 
```go
func (cu *ChatUser) Login(chatroom *ChatRoom) error {
	// TODO: login the user
	cu.WriteString("Welcome to Jen's chat server!\n")
	return nil
}
```

1. Find the function ChatUser.WriteString

```go
func (cu *ChatUser) WriteString(msg string) error {
  // TODO: write a line to the socket
  return nil
}
```

1. :star2: Implement the code in `WriteString` that will write the `msg` to the `writer`.
*Make sure you call `writer.Flush`*.

 [Stuck on any of the steps above? See the solution!](code/04-login/chat.go##L115-L121)

1. Start the server using go run chat.go. Test this using the telnet or nc tool to connect to port 6677.

```bash
$ telnet localhost 6677                                                                                                                      ~ 1 ↵
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Welcome to Jen's chat server!
```

1. Now we are going to read from the socket. We want to ask for the person's username and store it on the ChatUser.username field.

2. First, let's implement the ChatUser.ReadLine function.

Find this code:

```go
func (cu *ChatUser) ReadLine() (string, error) {
	// TODO: read a line from the socket
	return "", nil
}
```

1. :star2: Implement the code that calls `cu.reader.ReadLine` and returns the results as a string.

[Stuck on any of the steps above? See the solution!](code/04-login/chat.go#L109-L113)

1. Go back to the ChatUser.Login function. After you print the banner, write some code that:

1. :star2: Will print to the socket "Please enter your username:";
1. :star2: Read the username from the socket using `cu.ReadLine`;
1. :star2: Store the read username on the `cu.username` field;
1. :star2: And write back to the socket "Welcome, [cu.username]".

[Stuck on any of the steps above? See the solution!](code/04-login/chat.go#L91-L107)

Here is what is should look like when you connect via  `telnet localhost 6677`: 

   ```bash
  $ telnet localhost 6677                                                                                                                      ~ 1 ↵
  Trying ::1...
  telnet: connect to address ::1: Connection refused
  Trying 127.0.0.1...
  Connected to localhost.
  Escape character is '^]'.
  Welcome to Jen's chat server!
  Please enter your username: funcuddles
  Welcome, funcuddles
  ```

1. One more thing! when you call chatroom.Join in main, what happens if more than one client tries to connect?

for {
		conn, _ := listener.Accept()
		log.Println("Connection joined.", conn.RemoteAddr())
		chatroom.Join(conn)
	}

Hint: only one thing can be connecting at a time!

🌟 How can you fix this? Update main accordingly (View solution).

Proceed to Lesson 5