extmod/modlwip: Implement a queue of incoming UDP/raw packets

[dpgeorge]

Summary

The bare-metal lwIP socket interface is currently quite limited when used for UDP streams, because it only allows one outstanding incoming UDP packet. If one UDP packet is waiting to be socket.recv'd and another one comes along, then the second one is simply dropped.

This PR implements a queue for incoming UDP and raw packets. The queue depth is fixed at compile time, and is currently 4.

This allows better use of UDP connections, eg more efficient. It also makes DTLS work better which sometimes has a queue of UDP packets (eg during the connection phase).

Testing

A new network multi-test is added which sends a burst of UDP packets from the client, and the server doesn't recv them until they have all been sent.

Tested on PYBD-SF2 and RPI_PICO2_W. The new test passes (it fails without the UDP queue added here).

Also tested on unix and ESP32_GENERIC, both of which pass (they already have a UDP queue).

Trade-offs and Alternatives

• Some memory is needed for the queue, and is allocated on the MP heap. That memory holds a ring buffer of pbufs. Unfortunately pbufs don't support "packet queue" mode in lwIP yet, or maybe we could just link-link the pubfs together (also need to store the peer address and port, which probably requires additional memory anyway).
• The queue depth is fixed at compile time. That's probably adequate.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (61eedbb) to head (a05766f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #17261   +/-   ##
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  Coverage   98.54%   98.54%           
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  Files         169      169           
  Lines       21896    21896           
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  Hits        21578    21578           
  Misses        318      318           

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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:  +144 +0.016% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

[projectgus]

Implementation and test look solid to me. I have a couple of minor comment suggestions for future reference, but that's all.

[projectgus]

Suggested change

	// Total queue length for UDP/raw connections.
	// Total queue length for buffered UDP/raw incoming packets.

(That's right, isn't it?)

[dpgeorge]

Yes, that's right. Comment updated.

[projectgus]

Suggested change

	// UDP and raw sockets have a queue of incoming pbuf's.
	// UDP and raw sockets have a queue of incoming pbuf's, implemented as a ringbuffer

[dpgeorge]

Updated.

[projectgus]

Suggested change

	uint8_t iget;
	uint8_t iget; // Ringbuffer read index

(and same for iput)

[dpgeorge]

Updated for both lines.