How to guarantee cancel safety in custom method?

[lz1998]

I want to define a method read_packet(), and use it in tokio::select!, but sometimes I get a negative length.

I have tried FramedRead, but it also doesn't work in tokio::select!.

    pub async fn read_packet(read_half: &mut OwnedReadHalf) -> Bytes {
        let len = read_half.read_i32().await.unwrap();
        if len - 4 < 0 {
            panic!("invalid packet length: {}", len);
        }
        let mut data = vec![0; len as usize - 4];
        read_half.read_exact(&mut data).await.unwrap();
        Bytes::from(data)
    }

[Darksonn]

Taken from the tokio::select! documentation:

To determine whether your own methods are cancellation safe, look for the location of uses of .await. This is because when an asynchronous method is cancelled, that always happens at an .await. If your function behaves correctly even if it is restarted while waiting at an .await, then it is cancellation safe.

In your case, neither the read_i32 or read_exact calls are cancellation safe, so anything built from them wont be either.

I would recommend using a codec from tokio-util. Unlike what you said in the question, the FramedRead type works great with a tokio::select! and its next() method is cancellation safe because it is a Stream, and next() is always cancellation safe on a Stream.