Allow try_clone on TcpStream

[stackjohn]

Is it possible to implement the try_clone method for the tokio TcpStream type?

I'm using the echo server example, eventually I am hoping to be able to iterate through all the currently connected sockets and broadcast a message to them one by one.

Lets say I have the following code:

   // Bind the server's socket
    let addr = "127.0.0.1:12345".parse().unwrap();
    let tcp = TcpListener::bind(&addr, &handle).unwrap();
    
    let mut connections = Arc::new((Mutex::new(Vec::new())));
   
    let conn = connections.clone(); 
    thread::spawn(move || {
        let mut i;
        loop {
            i = conn.lock().unwrap().len();
            //Here I am hoping to eventually be able to write to each one of the sockets in the vector. 
            println!("There are {} connections", i);
             stdout().flush();
            thread::sleep_ms(10000);
        }
    });

    // Iterate incoming connections
    let server = tcp.incoming().for_each(|(tcp, _)| {

        let connection = Arc::new(Mutex::new(tcp));   
 
        connections.lock().unwrap().push(connection.lock().try_clone());
        
        // Split up the read and write halves
        let (reader, writer) = connection.lock().unwrap().split();

        // Future of the copy
        let bytes_copied = io::copy(reader, writer);

        // ... after which we'll print what happened
        let handle_conn = bytes_copied.map(|(n, _, _)| {
            println!("wrote {} bytes", n)
        }).map_err(|err| {
            println!("IO error {:?}", err)
        });

        // Spawn the future as a concurrent task
        handle.spawn(handle_conn);

        Ok(())
    });

The code fails to compile as try_clone is not implemented.

        connections.lock().unwrap().push(connection.lock().try_clone());

It appears that TcpStream in the standard library does allow the cloning of TcpStream objects.

Is there a way to work around this? Apologies I am very new to Rust, so what I am trying to do might not make any sense at all.

[alexcrichton]

Thanks for the report! Unfortunately this is a pretty difficult notion on the backend, however, as the semantics of duplicate file descriptors and epoll notifications tends to get... interesting. (much less the Windows semantics as well). I don't think it's impossible but I don't think it's easy either, and we currently haven't put a huge amount of thought into how to do this.

For now could you use something like Arc<TcpStream>? That way you can read/write and still clone it to get multiple references.

[stackjohn]

Thanks for your reply, much appreciated. I will take a look at Arc<TcpStream>.

I see there is a good example here.

[carllerche]

Relates to tokio-rs/mio#361

[pavlix]

A few notes…

Arc<TcpStream> is not usable for reader/writer clones passed to concurrent async blocks.

TcpStream::split() is not usable either because reader and writer are not independent and cannot be both moved each to its own async block;

The use case of the standard TcpStream::try_clone() is to create two completely independent channels (the dependency is moved to the operating system internals) for reading and writing. The provided example doesn't perform concurrent operation.

However, there is TcpStream::into_split() (that possibly didn't exist when @stackjohn asked) that does the job. It returns a reader/writer pair that can be moved and split into two separate tasks. It looks like it's not an OS-level solution based on dup() but rather the TcpStream seems to be moved to a dynamically allocated memory and shared by the reader/writer pair.

[ramiroaisen]

You can create a tokio broadcast channel and subscribe to it in the TcpStream handler fn, then when you get new messages you send them back to the TcpStream, no need to clone it.

[SofwanCoder]

Another good option is to convert the tokio TcpStream into std TcpStream using TcpStream::into_std() then clone the std TcpStream using it's TcpStream::try_clone() method.

Then you can eventually do TcpStream::from_std

// 
let std_tcp_stream = tokio_tcp_stream.into_std()?;
let std_tcp_stream_clone_one =  std_tcp_stream.try_clone()?;
let std_tcp_stream_clone_two = std_tcp_stream.try_clone()?;

// Both pointing to the same file descriptors
let new_tokio_tcp_stream _one= TcpStream::from_std(std_tcp_stream_clone_one)?;
let new_tokio_tcp_stream _two= TcpStream::from_std(std_tcp_stream_clone_two)?;

[QyInvoLing]

Another good option is to convert the tokio TcpStream into std TcpStream using TcpStream::into_std() then clone the std TcpStream using it's TcpStream::try_clone() method.

Then you can eventually do TcpStream::from_std

// 
let std_tcp_stream = tokio_tcp_stream.into_std()?;
let std_tcp_stream_clone_one =  std_tcp_stream.try_clone()?;
let std_tcp_stream_clone_two = std_tcp_stream.try_clone()?;

// Both pointing to the same file descriptors
let new_tokio_tcp_stream _one= TcpStream::from_std(std_tcp_stream_clone_one)?;
let new_tokio_tcp_stream _two= TcpStream::from_std(std_tcp_stream_clone_two)?;

This method takes the ownership of the origin tokio's tcpstream object, which can cause problem somewhere (for example, the stream itself is a field of another struct)