Fix nRF52 AsyncUDP multicast TX/RX race condition causing garbled packets

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 the send
operation. 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.

Made-with: Cursor

Author: PhilipLykov

src/platform/nrf52/AsyncUDP.cpp

@@ -30,10 +30,15 @@ void AsyncUDP::onPacket(const std::function<void(AsyncUDPPacket)> &callback)
 
 bool AsyncUDP::writeTo(const uint8_t *data, size_t len, IPAddress ip, uint16_t port)
 {
-    if (!udp.beginPacket(ip, port))
+    isSending = true;
+    if (!udp.beginPacket(ip, port)) {
+        isSending = false;
         return false;
+    }
     udp.write(data, len);
-    return udp.endPacket();
+    bool ok = udp.endPacket();
+    isSending = false;
+    return ok;
 }
 
 void AsyncUDP::close()
@@ -70,7 +75,7 @@ const uint8_t *AsyncUDPPacket::data()
 
 int32_t AsyncUDP::runOnce()
 {
-    if (_onPacket && udp.parsePacket() > 0) {
+    if (_onPacket && !isSending && udp.parsePacket() > 0) {
         AsyncUDPPacket packet(udp);
         _onPacket(packet);
     }

src/platform/nrf52/AsyncUDP.h

@@ -31,6 +31,7 @@ class AsyncUDP : public Print, private concurrency::OSThread
   private:
     EthernetUDP udp;
     uint16_t localPort;
+    volatile bool isSending = false;
     std::function<void(AsyncUDPPacket)> _onPacket;
     virtual int32_t runOnce() override;
 };