simplex() is unsafe and can cause segfault

[SteveLauC]

For a type that implements unsafe trait Split, when being split into 2 read/write halves, Monoio uses UnsafeCell internally to acquire mutable access to the type instance, this is the reason why trait Split is unsafe. It would be only safe if your type can support read/write at the same time.

SimplexStream is basically a bytes::BytesMut, its read cursor is stored as a pointer, and appending to it can trigger re-allocation, which will invalidate the stored pointer, then a segfault would happen if such an invalidated pointer is accessed. So SimplexStream should not implement the unsafe trait Split.

Tokio's split() and read/write halves do not use UnsafeCell, instead, it uses std::sync::Mutex to ensure read and write to the type instance won't happen at the same time, this explains why it can blindly do the split operation to anything that has AsyncRead + AsyncWrite implemented:

pub fn split<T>(stream: T) -> ReadHalf<T>, WriteHalf<T>
where
    T: AsyncRead + AsyncWrite