STM32 - fix bug where sockets stop receiving data

[c1728p9]

The function _eth_arch_low_level_input() is meant to pass data into LWIP and to prepare the ethernet buffers to receive more data. If the LWIP heap is empty and the call to pbuf_alloc() in _eth_arch_low_level_input returns null, the ethernet receive buffers are not updated to receive data. Because of this the ethernet RX interrupt will not fire. Since the RX interrupt is the only thing that triggers a call to _eth_arch_low_level_input(), the receive buffers will never get cleared, and the device stops receiving data.

To prevent this from happening, this patch ensures that the function _eth_arch_low_level_input() clears the receive buffers even if a new pbuf for the data couldn't be allocated.

This issue can be reproduce by running the test
"features-feature_lwip-tests-mbedmicro-net-udp_echo_parallel"
and on the same machine running the below python script to flood the
device with UDP broadcast packets:

MY_IP = #ADD your local IP here
from socket import *
s = socket(AF_INET, SOCK_DGRAM)
s.bind((MY_IP, 1234))
s.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
s.setsockopt(SOL_SOCKET, SO_BROADCAST, 1)
for _ in range(1000):
    s.sendto("test data", ('255.255.255.255', 1234))
print("Message sent")

[c1728p9]

CC: @bridadan @studavekar

[bridadan]

/morph test-nightly

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test-nightly

Output

mbed Build Number: 540

All builds and test passed!