rustls seems to have unexpected behavior when used in tokio::io::copy

[3andero]

Hi, I'm building a proxy. The server side accepts a TlsStream (from the client side) and makes a Tcp connection to the target website. Then we just need to copy everything we get from the TlsStream to the TcpStream and collect the response vice versa.
I was initially using tokio::io::copy for this task. However, I encountered a weird bug that my client never received nearly half of the response from the target website, and the connection just stuck until timeout. I then replace the io::copy with my simplified version:

// (didn't work, same as tokio::io::copy)
pub async fn copy_tcp<R: AsyncRead + Unpin, W: AsyncWrite + Unpin>(
    r: &mut R,
    w: &mut W,
) -> Result<()> {
    let mut buf = [0; 2048];
    loop {
        let len = r.read(&mut buf).await?;
        if len == 0 {
            return Ok(());
        }
        w.write(&buf[..len]).await?;
    }
}

And this didn't work until I added a flush after the write call,

pub async fn copy_tcp<R: AsyncRead + Unpin, W: AsyncWrite + Unpin>(
    r: &mut R,
    w: &mut W,
) -> Result<()> {
    let mut buf = [0; 2048];
    loop {
        let len = r.read(&mut buf).await?;
        if len == 0 {
            return Ok(());
        }
        w.write(&buf[..len]).await?;
    --> w.flush().await?;
    }
}

I found that in tokio-tls/rustls,

tls/tokio-rustls/src/server.rs

Lines 95 to 96 in db01bce

	/// Note: that it does not guarantee the final data to be sent.
	/// To be cautious, you must manually call `flush`.

that we are required to manually call flush.
While in tokio::io::copy,
https://github.com/tokio-rs/tokio/blob/c306bf853a1f8423b154f17fa47926f04eecd9b4/tokio/src/io/util/copy.rs#L69-L74
only when EOF is seen does it call poll_flush.

So, should we avoid using tokio::io::copy for this purpose?

[quininer]

You're right. I think tokio::io::copy can be simply modified to deal with this problem. we only need to call poll_flush when poll_read is pending.

cc @Darksonn

[quininer]

This problem has been fixed in tokio. tokio-rs/tokio#4001