Reading a line in a BufReader creates an infinite loop - rust

Question

I'm trying to receive a message (server side) on the network using TcpListener in Rust.
Here's the server code:
// Instanciate TcpListener
let server = TcpListener::bind("0.0.0.0:52001").expect("Could not bind");
match server.accept() { // Waiting new connection from a client
Ok((stream, addr)) => {
println!("New connection from {}", addr);
// Wrapping the stream in a BufReader
let mut reader = BufReader::new(&stream);
let mut message = String::new();
loop { // Entering the loop when a client is connected
reader.read_line(&mut message).expect("Cannot read new line");
println!("message received: {}", message);
message.clear();
}
}
Err(e) => {
println!("Fail: {:?}", e)
}
}
Here's my Kotlin client:
Socket("192.168.134.138", 52001).use { client ->
client.getOutputStream().use { out ->
out.write("test\n".toByteArray())
}
}
while(true) {
Thread.sleep(15_000)
}
The client send the following line: test\n and it ends with a linebreak for the server to read.
The intended behaviours would be that on the server side it prints message received: test and then the server waits at the read_line() instruction for the next line
It works because I receive the test but the read_line() method does not seem to block nor wait for another message. So it creates an infinite loop. So in the terminal I'm getting:
New connection from 192.168.134.123:7869
message received: test
message received:
message received:
message received:
message received:
Process finished with exit code 130 (interrupted by signal 2: SIGINT)
And I have to stop the program forcefully.
Any ideas?

Answer 1

To detect the end of the stream, you need to check if read_line() returned Ok(0):
From the docs:
If this function returns Ok(0), the stream has reached EOF.
loop { // Entering the loop when a client is connected
let mut message = String::new();
if reader.read_line(&mut message).expect("Cannot read new line") == 0 {
break;
}
println!("message received: {}", message);
}
Another way option is to use BufReader::lines() iterator:
for line in reader.lines() {
let message = line.expect("Cannot read new line");
println!("message received: {}", message);
}
This approach is a bit inefficient as it allocates a new String on every iteration. For best performance, you should allocate a single String and reuse it like #BlackBeans pointed out in a comment:
let mut message = String::new();
loop { // Entering the loop when a client is connected
message.clear();
if reader.read_line(&mut message).expect("Cannot read new line") == 0 {
break;
}
println!("message received: {}", message);
}