lwip: ipv6: Fix incorrect peer address.

[greezybacon]

Summary

For IPv6 UDP sockets, the peer address was previously defined as only the first four bytes of the IP address. For IPv6 addresses, this resulted in an incorrect address expressed as IPv4. For instance, receiving a packet via ::recvfrom from 'fe80::87:e7ff:fe48:629a' is returned as having a peer address of '254.128.0.0'

Testing

I have tested this with a UDP-based IPv6 service using ::sendto and ::recvfrom, and the address are now returned correctly to Python.

The underlying sockets function works correctly if provided the correct IPv6 addresses, so this doesn't affect the underlying socket interface. Here is an example echo service which doesn't work with the current implementation:

s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
s.bind(('::', 4000))
while True:
    msg, addr = s.recvfrom(512)
    # NOTE: `addr` is incorrect
    s.sendto(msg, addr)

Trade-offs and Alternatives

This uses slightly more memory for IPv4 UDP socket servers as memory is allocated based on the largest supported address family. However, it's only a few bytes per function call.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.58%. Comparing base (03fe9c5) to head (f29bd5a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #16613   +/-   ##
=======================================
  Coverage   98.58%   98.58%           
=======================================
  Files         167      167           
  Lines       21590    21590           
=======================================
  Hits        21285    21285           
  Misses        305      305           

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:   +16 +0.002% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

[dpgeorge]

Thanks for the fix, this is definitely needed.

I think the #include "shared/netutils/netutils.h" line can now be removed as well.

[dpgeorge]

Thanks for updating, now merged.