fix(udp): retry send on first EINVAL

Android API level < 26 does not support the `libc::IP_TOS` control message.
`sendmsg` calls with `libc::IP_TOS` return `libc::EINVAL`.

quinn-rs#1516 added a fallback, not setting
`libc::IP_TOS` on consecutive calls to `sendmsg` after a failure with
`libc::EINVAL`. The current datagram would be dropped. Consecutive datagrams
passed to `sendmsg` would succeed as they would be sent without `libc::IP_TOS`
through the fallback.

Instead of dropping the first datagram on `libc::EINVAL`, this commit adds a
retry for it without `libc::IP_TOS`.

This is e.g. relevant for Neqo. When establishing a QUIC connection, dropping
the first datagram [delays connection establishment by
100ms](https://github.com/mozilla/neqo/blob/3001a3a56f2274eaafaa956fb394f0817f526ae7/neqo-transport/src/rtt.rs#L28).
With the retry introduced in this commit, delay due to unsupported
`libc::IP_TOS` should be negligeable.

Closes quinn-rs#1975.

quinn-udp/src/unix.rs

@@ -332,8 +332,20 @@ fn send(
 
                     if e.raw_os_error() == Some(libc::EINVAL) {
                         // Some arguments to `sendmsg` are not supported.
-                        // Switch to fallback mode.
-                        state.set_sendmsg_einval();
+                        if !state.sendmsg_einval() {
+                            // Switch to fallback mode.
+                            state.set_sendmsg_einval();
+                            prepare_msg(
+                                transmit,
+                                &dst_addr,
+                                &mut msg_hdr,
+                                &mut iovec,
+                                &mut cmsgs,
+                                encode_src_ip,
+                                state.sendmsg_einval(),
+                            );
+                            continue;
+                        }
                     }
 
                     // - EMSGSIZE is expected for MTU probes. Future work might be able to avoid

quinn-udp/tests/tests.rs

@@ -241,7 +241,10 @@ fn test_send_recv(send: &Socket, recv: &Socket, transmit: Transmit) {
             }
         }
 
-        if cfg!(target_os = "android")
+        if match transmit.destination.ip() {
+            IpAddr::V4(_) => true,
+            IpAddr::V6(a) => a.to_ipv4_mapped().is_some(),
+        } && cfg!(target_os = "android")
             && std::env::var("API_LEVEL")
                 .ok()
                 .and_then(|v| v.parse::<u32>().ok())