dangernoodle-io / breadboard / 26202226332

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fix(bb_http): one-shot serialize JSON responses to dodge lwip chunked race (#291)

bb_http_resp_send_json with an object/array root has been emitting via
json_stream_node, which calls bb_http_resp_send_chunk once per field /
comma / brace. A 4 KB JSON tree easily becomes 200+ chunks. Each chunk
routes through tcpip_send_msg_wait_sem, which calls LWIP_ASSERT on the
netconn's op_completed_sem. If the peer closes the socket and the
netconn is freed between two chunks, the next chunk's assert fires and
the device aborts.

Reproduced on bitaxe-650 with the TaipanMiner dashboard tab + SSE log
stream open: backtrace ends in vPortTaskWrapper -> fwrite_r ->
__sfvwrite_r -> httpd_default_send -> lwip_send -> netconn_write_partly
-> tcpip_send_msg_wait_sem (tcpip.c:461) -> abort. TaipanMiner's
/api/pool grew from ~80 to ~200 chunks per response after the per-pool
stats array landed, widening the race window enough to make this a
consistent crash under SSE + 5s polling.

Try one-shot serialize first; fall back to streaming only when
cJSON_PrintUnformatted returns NULL (very large trees, e.g. a fully-
materialized /api/openapi.json — that endpoint already streams via its
own path). For typical /api responses this collapses ~200 lwip_send
calls to one and effectively eliminates the race window.

Resolves B1-223.

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