socketSendUDP() calls yield() while waiting for SEND_OK, allowing the
cooperative scheduler to run AsyncUDP::runOnce(). When runOnce() calls
parsePacket() on the same socket during an active send, the multicast
loopback packet arriving from the switch produces a garbled 8-byte RX
header (wrong source IP, wrong payload length), leading to protobuf
decode errors in UdpMulticastHandler.
Add a volatile isSending flag that writeTo() sets around endPacket()
(the only yield point). runOnce() checks this flag and skips
parsePacket() while a send is in progress. Loopback packets stay
buffered in the W5100S RX buffer and are read cleanly on the next
poll cycle.
Also propagate writeTo() return value in UdpMulticastHandler::onSend()
instead of unconditionally returning true.
Made-with: Cursor
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>
