Add recvmmsg fallback

quinn-udp/src/unix.rs

@@ -117,6 +117,58 @@ unsafe fn sendmmsg_with_fallback(
     */
 }
 
+/// Fallback implementation of `recvmmsg` using `recvmsg`
+/// for systems which do not support `recvmmsg`
+/// such as Linux <2.6.33.
+#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+unsafe fn recvmmsg_fallback(
+    sockfd: libc::c_int,
+    msgvec: *mut libc::mmsghdr,
+    vlen: libc::c_uint,
+) -> libc::c_int {
+    let flags = 0;
+    if vlen == 0 {
+        return 0;
+    }
+
+    let n = libc::recvmsg(sockfd, &mut (*msgvec).msg_hdr, flags);
+    if n == -1 {
+        -1
+    } else {
+        (*msgvec).msg_len = n as libc::c_uint;
+        1
+    }
+}
+
+#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+unsafe fn recvmmsg_with_fallback(
+    sockfd: libc::c_int,
+    msgvec: *mut libc::mmsghdr,
+    vlen: libc::c_uint,
+) -> libc::c_int {
+    if true {
+        recvmmsg_fallback(sockfd, msgvec, vlen)
+    } else {
+        let flags = 0;
+        let timeout = ptr::null_mut::<libc::timespec>();
+        let ret =
+            libc::syscall(libc::SYS_recvmmsg, sockfd, msgvec, vlen, flags, timeout) as libc::c_int;
+
+        if ret == -1 {
+            let e = io::Error::last_os_error();
+            match e.raw_os_error() {
+                Some(libc::ENOSYS) => {
+                    // Fallback to `sendmsg`.
+                    recvmmsg_fallback(sockfd, msgvec, vlen)
+                }
+                _ => -1,
+            }
+        } else {
+            ret
+        }
+    }
+}
+
 fn init(io: SockRef<'_>) -> io::Result<()> {
     let mut cmsg_platform_space = 0;
     if cfg!(target_os = "linux") || cfg!(target_os = "freebsd") || cfg!(target_os = "macos") {
@@ -358,12 +410,10 @@ fn recv(io: SockRef<'_>, bufs: &mut [IoSliceMut<'_>], meta: &mut [RecvMeta]) ->
     }
     let msg_count = loop {
         let n = unsafe {
-            libc::recvmmsg(
+            recvmmsg_with_fallback(
                 io.as_raw_fd(),
                 hdrs.as_mut_ptr(),
                 bufs.len().min(BATCH_SIZE) as _,
-                0,
-                ptr::null_mut(),
             )
         };
         if n == -1 {