Add cid len check to enable multipath

quinn-proto/src/connection/mod.rs

@@ -1437,9 +1437,9 @@ impl Connection {
     /// Multipath is only enabled after the handshake is completed and if it was negotiated
     /// by both peers.
     pub fn is_multipath_enabled(&self) -> bool {
-        // TODO(flub): Multipath can only be enabled if both sides use non-zero length CIDs.
-        //    Also check that.
         !self.is_handshaking()
+            && self.handshake_cid.len() > 0
+            && self.rem_handshake_cid.len() > 0
             && self.config.initial_max_path_id.is_some()
             && self.peer_params.initial_max_path_id.is_some()
     }

quinn-proto/src/shared.rs

@@ -98,6 +98,10 @@ impl ConnectionId {
         res
     }
 
+    pub(crate) fn len(&self) -> usize {
+        self.len as usize
+    }
+
     /// Decode from long header format
     pub(crate) fn decode_long(buf: &mut impl Buf) -> Option<Self> {
         let len = buf.get::<u8>().ok()? as usize;