fix(mmserver): handle EAGAIN during sendmmsg

This is not actually sufficient for error handling in sendmmsg. The
following cases are unconsidered:

 - Only some of the buffer are sent
 - Only some of one of the iovecs is sent

We'll handle this when we migrate to rustix's sendmmsg, which is waiting
on the following PRs:

 - bytecodealliance/rustix#1171
 - bytecodealliance/rustix#1409

Note that those cases should be gracefully-ish handled by FEC and the
protocol (they're equivalent to dropped or biffed packets).

Author: colinmarc

mm-server/src/server/sendmmsg.rs

@@ -42,14 +42,20 @@ impl<'a> SendMmsg<'a> {
             .map(ControlMessage::TxTime)
             .collect::<Vec<_>>();
 
-        nix::sys::socket::sendmmsg(
-            fd.as_raw_fd(),
-            &mut data,
-            &self.iovs,
-            &self.addrs,
-            &cmsgs,
-            MsgFlags::empty(),
-        )?;
+        loop {
+            match nix::sys::socket::sendmmsg(
+                fd.as_raw_fd(),
+                &mut data,
+                &self.iovs,
+                &self.addrs,
+                &cmsgs,
+                MsgFlags::empty(),
+            ) {
+                Ok(_) => break,
+                Err(nix::errno::Errno::EAGAIN) => continue,
+                Err(e) => return Err(e),
+            };
+        }
 
         Ok(())
     }