Fix recvmmsg(2) implementation

There were two problems discovered with the `recvmmsg(2)` implementation
that this changeset attempts to fix:

1. As mentioned in nix-rust/issues/1325, `recvmmsg(2)` can return fewer
   messages than requested, and
2. Passing the return value of `recvmmsg(2)` as the number of bytes in
   the messages received is incorrect.

This changeset incorporates the proposed fix from nix-rust/issues/1325,
as well as passing the correct value (`mmsghdr.msg_len`) for the number
of bytes in a given message.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/sys/socket/mod.rs b/src/sys/socket/mod.rs
index 3bb96074..a4f599c1 100644
--- a/src/sys/socket/mod.rs
+++ b/src/sys/socket/mod.rs
@@ -1217,17 +1217,18 @@ pub fn recvmmsg<'a, I>(
 
     let ret = unsafe { libc::recvmmsg(fd, output.as_mut_ptr(), output.len() as _, flags.bits() as _, timeout) };
 
-    let r = Errno::result(ret)?;
+    let _ = Errno::result(ret)?;
 
     Ok(output
         .into_iter()
+        .take(ret as usize)
         .zip(addresses.iter().map(|addr| unsafe{addr.assume_init()}))
         .zip(results.into_iter())
         .map(|((mmsghdr, address), (msg_controllen, cmsg_buffer))| {
             unsafe {
                 read_mhdr(
                     mmsghdr.msg_hdr,
-                    r as isize,
+                    mmsghdr.msg_len as isize,
                     msg_controllen,
                     address,
                     cmsg_buffer