[lwip-users] Assertion "pcb->snd_queuelen >= pbuf_clen(next->p)" failed after TCP handshake

[Tomas Mudrunka]

Hello,
i have written my own netif driver on FreeRTOS+LWIP.
Pings work reliably, TCP handshake works reliably,
but once i start sending data to the established TCP connection it 
crashes on this:

assertion "pcb->snd_queuelen >= pbuf_clen(next->p)" failed: file 
"/opt/Espressif/esp-idf/components/lwip/lwip/src/core/tcp_in.c", line 
1112, function: tcp_free_acked_segments

My netif has FreeRTOS task which handles communication and then passes 
received L2 frames to LWIP like this:

p = pbuf_alloc(PBUF_RAW,rx_len,PBUF_POOL);
pbuf_take(p, rx_buf, rx_len);
if(netif->input(p, netif) != ERR_OK) {
  ESP_LOGE(TAG, "Input failed!");
  pbuf_free(p);
}

Netif task runs with same priority as LWIP task.
(Both run at ESP_TASK_TCPIP_PRIO defined in esp_task.h, but that is 
ESP32 specific)
When i was running task with higher priority it was causing deadlocks.
Now when i run both task on the same priority it gives me assertion 
mentioned before...

Any idea what might be causing this?
I am not sure how should i handle the case with netif having its own 
task sending data to netif->input()
Is there some explicit locking required?

Thanks for your help.

--
Best regards
     Tomáš Mudruňka