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J. Nick Koston
a88e9b8146
On RP2040 (Pico W), arduino-pico sets PICO_CYW43_ARCH_THREADSAFE_BACKGROUND=1, which means lwip callbacks (recv_fn, accept_fn, err_fn) run from a PendSV interrupt — not the main loop. This allows them to preempt read(), write(), close(), and accept() at any point, causing race conditions on shared state like the rx_buf_ pbuf chain. The most critical race: recv_fn calls pbuf_cat(rx_buf_, pb) while read() is freeing nodes in the same chain, leading to use-after-free and lwip's "Creating an infinite loop" assertion panic. This is the root cause of #10681. Fix: implement RP2040's LwIPLock (previously a no-op) to call cyw43_arch_lwip_begin/end, which acquires the pico-sdk async_context recursive mutex. Add LWIP_LOCK() guards to all main-loop lwip API call sites in the socket layer. On ESP8266, lwip callbacks run cooperatively from the main loop, so LwIPLock remains a no-op. Closes #10681
