Failing to write an async version of recvmsg for Tokio's UnixDatagram

[krasenyp]

Hello! I'm a Rust beginner and I'm trying to write an extension trait for Tokio's UnixDatagram with two functions - try_recvmsg and recvmsg. I'm using nix's safe wrapper for recvmsg. The trait is as follows.

trait TokioRecvmsgExt {
    fn try_recvmsg<'a, 'outer, 'inner, S>(
        &'a self,
        iov: &'outer mut [IoSliceMut<'inner>],
        cmsg_buf: Option<&'a mut Vec<u8>>,
        flags: MsgFlags,
    ) -> io::Result<RecvMsg<'a, 'outer, S>>
    where
        S: SockaddrLike + 'a,
        'inner: 'outer;

    async fn recvmsg<'a, 'outer, 'inner, S>(
        &'a self,
        iov: &'outer mut [IoSliceMut<'inner>],
        cmsg_buf: Option<&'a mut Vec<u8>>,
        flags: MsgFlags,
    ) -> io::Result<RecvMsg<'a, 'outer, S>>
    where
        S: SockaddrLike + 'a,
        'inner: 'outer;
}

I used the same lifetime annotations as the nix's recvmsg and try_recvmsg's implementation works without problems:

fn try_recvmsg<'a, 'outer, 'inner, S>(
    &'a self,
    iov: &'outer mut [IoSliceMut<'inner>],
    cmsg_buf: Option<&'a mut Vec<u8>>,
    flags: MsgFlags,
) -> io::Result<RecvMsg<'a, 'outer, S>> 
where
    S: SockaddrLike + 'a,
    'inner: 'outer,
{
    self.try_io(Interest::READABLE, || {
        recvmsg(self.as_raw_fd(), iov, cmsg_buf, flags).map_err(|e| io::Error::from(e))
    })
}

Unfortunately, trying to implement the asynchronous function isn't that easy. The following code returns a couple of errors:

async fn recvmsg<'a, 'outer, 'inner, S>(
    &'a self,
    iov: &'outer mut [IoSliceMut<'inner>],
    cmsg_buf: Option<&'a mut Vec<u8>>,
    flags: MsgFlags,
) -> io::Result<RecvMsg<'a, 'outer, S>>
where
    S: SockaddrLike + 'a,
    'inner: 'outer,
{
    self.async_io(Interest::READABLE, || {
        recvmsg(self.as_raw_fd(), iov, cmsg_buf, flags).map_err(|e| io::Error::from(e))
    })
    .await
}

error: captured variable cannot escape `FnMut` closure body
  --> src/main.rs:46:13
   |
37 |         iov: &'outer mut [IoSliceMut<'inner>],
   |         --- variable defined here
...
45 |         self.async_io(Interest::READABLE, || {
   |                                            - inferred to be a `FnMut` closure
46 |             recvmsg(self.as_raw_fd(), iov, cmsg_buf, flags).map_err(|e| io::Error::from(e))
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^---^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |             |                         |
   |             |                         variable captured here
   |             returns a reference to a captured variable which escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape

error[E0507]: cannot move out of `cmsg_buf`, a captured variable in an `FnMut` closure
  --> src/main.rs:46:44
   |
38 |         cmsg_buf: Option<&'a mut Vec<u8>>,
   |         -------- captured outer variable
...
45 |         self.async_io(Interest::READABLE, || {
   |                                           -- captured by this `FnMut` closure
46 |             recvmsg(self.as_raw_fd(), iov, cmsg_buf, flags).map_err(|e| io::Error::from(e))
   |                                            ^^^^^^^^ move occurs because `cmsg_buf` has type `std::option::Option<&mut std::vec::Vec<u8>>`, which does not implement the `Copy` trait

Part of the problem is that async_io calls the closure argument in a loop and iov is a mutable reference and cmsg_buf is an Option of mutable reference to a vector. Maybe interior mutability can help to make the closure Fn and not FnMut. I don't know how to approach the errors. My intuition can be completely wrong so I'm asking for help.

[alice]

If you have a working try_io, then I recommend that you just combine it with writable like this:

loop {
    stream.writable().await?;
    match stream.try_recvmsg(...) {
        Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => continue,
        output => return output,
    }
}

[krasenyp]

Thank you! It works but I'm wondering how to make it work with async_io, mostly for learning purposes? If I use Option<Rc<RefCell<Vec<u8>>>> for cmsg_buf and borrowing it "by hand" but the compiler still complains about the closure being FnOnce because of moving.