extmod/modlwip: For TCP send keep trying tcp_write if it returns ERR_MEM
If tcp_write returns ERR_MEM then it's not a fatal error but instead means the caller should retry the write later on (and this is what lwIP's netconn API does). This fixes problems where a TCP send would raise OSError(ENOMEM) in situations where the TCP/IP stack is under heavy load. See eg issues #1897 and #1971.
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@ -668,7 +668,25 @@ STATIC mp_uint_t lwip_tcp_send(lwip_socket_obj_t *socket, const byte *buf, mp_ui
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u16_t write_len = MIN(available, len);
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err_t err = tcp_write(socket->pcb.tcp, buf, write_len, TCP_WRITE_FLAG_COPY);
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// If tcp_write returns ERR_MEM then there's currently not enough memory to
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// queue the write, so wait and keep trying until it succeeds (with 10s limit).
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// Note: if the socket is non-blocking then this code will actually block until
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// there's enough memory to do the write, but by this stage we have already
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// committed to being able to write the data.
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err_t err;
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for (int i = 0; i < 200; ++i) {
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err = tcp_write(socket->pcb.tcp, buf, write_len, TCP_WRITE_FLAG_COPY);
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if (err != ERR_MEM) {
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break;
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}
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err = tcp_output(socket->pcb.tcp);
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if (err != ERR_OK) {
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break;
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}
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MICROPY_PY_LWIP_EXIT
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mp_hal_delay_ms(50);
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MICROPY_PY_LWIP_REENTER
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}
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// If the output buffer is getting full then send the data to the lower layers
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if (err == ERR_OK && tcp_sndbuf(socket->pcb.tcp) < TCP_SND_BUF / 4) {
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