Add recvmsg implmentation (#252)

Adds the missing `recvmsg` function to the udp socket.

the level of duplication required vs `recv_from` is uncomfortable, but
required due to the lack of specialization.

src/io/mod.rs

@@ -23,6 +23,8 @@ mod readv;
 
 mod recv_from;
 
+mod recvmsg;
+
 mod rename_at;
 
 mod send_to;

src/io/recvmsg.rs

@@ -0,0 +1,97 @@
+use crate::runtime::driver::op::{Completable, CqeResult, Op};
+use crate::runtime::CONTEXT;
+use crate::{buf::BoundedBufMut, io::SharedFd, BufResult};
+use socket2::SockAddr;
+use std::{
+    io::IoSliceMut,
+    {boxed::Box, io, net::SocketAddr},
+};
+
+pub(crate) struct RecvMsg<T> {
+    #[allow(dead_code)]
+    fd: SharedFd,
+    pub(crate) buf: Vec<T>,
+    #[allow(dead_code)]
+    io_slices: Vec<IoSliceMut<'static>>,
+    pub(crate) socket_addr: Box<SockAddr>,
+    pub(crate) msghdr: Box<libc::msghdr>,
+}
+
+impl<T: BoundedBufMut> Op<RecvMsg<T>> {
+    pub(crate) fn recvmsg(fd: &SharedFd, mut bufs: Vec<T>) -> io::Result<Op<RecvMsg<T>>> {
+        use io_uring::{opcode, types};
+
+        let mut io_slices = Vec::with_capacity(bufs.len());
+        for buf in &mut bufs {
+            io_slices.push(IoSliceMut::new(unsafe {
+                std::slice::from_raw_parts_mut(buf.stable_mut_ptr(), buf.bytes_total())
+            }));
+        }
+
+        let socket_addr = Box::new(unsafe { SockAddr::init(|_, _| Ok(()))?.1 });
+
+        let mut msghdr: Box<libc::msghdr> = Box::new(unsafe { std::mem::zeroed() });
+        msghdr.msg_iov = io_slices.as_mut_ptr().cast();
+        msghdr.msg_iovlen = io_slices.len() as _;
+        msghdr.msg_name = socket_addr.as_ptr() as *mut libc::c_void;
+        msghdr.msg_namelen = socket_addr.len();
+
+        CONTEXT.with(|x| {
+            x.handle().expect("Not in a runtime context").submit_op(
+                RecvMsg {
+                    fd: fd.clone(),
+                    buf: bufs,
+                    io_slices,
+                    socket_addr,
+                    msghdr,
+                },
+                |recv_from| {
+                    opcode::RecvMsg::new(
+                        types::Fd(recv_from.fd.raw_fd()),
+                        recv_from.msghdr.as_mut() as *mut _,
+                    )
+                    .build()
+                },
+            )
+        })
+    }
+}
+
+impl<T> Completable for RecvMsg<T>
+where
+    T: BoundedBufMut,
+{
+    type Output = BufResult<(usize, SocketAddr), Vec<T>>;
+
+    fn complete(self, cqe: CqeResult) -> Self::Output {
+        // Convert the operation result to `usize`
+        let res = cqe.result.map(|v| v as usize);
+        // Recover the buffers
+        let mut bufs = self.buf;
+
+        let socket_addr = (*self.socket_addr).as_socket();
+
+        let res = res.map(|n| {
+            let socket_addr: SocketAddr = socket_addr.unwrap();
+
+            let mut bytes = n;
+            for buf in &mut bufs {
+                // Safety: the kernel wrote `n` bytes to the buffer.
+                unsafe {
+                    buf.set_init(bytes);
+                }
+                let total = buf.bytes_total();
+                if bytes > total {
+                    bytes -= total;
+                } else {
+                    // In the current API bytes_init is a watermark,
+                    // so remaining don't need zeroing.
+                    break;
+                }
+            }
+            (n, socket_addr)
+        });
+
+        (res, bufs)
+    }
+}

src/io/socket.rs

@@ -179,6 +179,14 @@ impl Socket {
         op.await
     }
 
+    pub(crate) async fn recvmsg<T: BoundedBufMut>(
+        &self,
+        buf: Vec<T>,
+    ) -> crate::BufResult<(usize, SocketAddr), Vec<T>> {
+        let op = Op::recvmsg(&self.fd, buf).unwrap();
+        op.await
+    }
+
     pub(crate) async fn accept(&self) -> io::Result<(Socket, Option<SocketAddr>)> {
         let op = Op::accept(&self.fd)?;
         op.await

src/net/udp.rs

@@ -283,6 +283,16 @@ impl UdpSocket {
         self.inner.recv_from(buf).await
     }
 
+    /// Receives a single datagram message on the socket, into multiple buffers
+    ///
+    /// On success, returns the number of bytes read and the origin.
+    pub async fn recvmsg<T: BoundedBufMut>(
+        &self,
+        buf: Vec<T>,
+    ) -> crate::BufResult<(usize, SocketAddr), Vec<T>> {
+        self.inner.recvmsg(buf).await
+    }
+
     /// Reads a packet of data from the socket into the buffer.
     ///
     /// Returns the original buffer and quantity of data read.