Re: [PATCH mptcp-net v2 2/2] selftests: mptcp: join: retry connect after early fallback

[Matthieu Baerts <matttbe@kernel.org>]

Hi Quanye,

On 11/09/2026 17:42, Quanye Yang via B4 Relay wrote:
> From: Quanye Yang <quanyeyang@proton.me>
> 
> Cover leftover msk state when connect() fails before
> SS_CONNECTING. Existing mptcp_connect tests always use a fresh
> socket, so they cannot see a sticky fallback on the same fd.
> 
> Add a small helper that optionally issues a wrong-family connect()
> on an MPTCP socket, then connects to a real IPv4 destination and
> checks MPTCP_INFO. A new mptcp_join.sh group (-n) runs two cases:
> 
> - a failed connect without early fallback must not poison the retry
> - after a blackhole-driven early fallback plus a failed connect,
>   clearing blackhole_timeout, the same fd must complete MP_CAPABLE
>   again

Thank you for this test, but we cannot accept this: it is good to have a
test linked to a fix or a feature, but it has to be maintainable. If
each fix/feature adds 300+ LoC, that's unmaintainable (or LLM become
mandatory for that, but that's not what we want: we still need to be
able to read the test).

Ideally:

- create a small packetdrill test instead, using the MPTCP version [1]

- not everything can be tested with packetdrill (even if it can also be
  extended) and adding code in the selftests is OK, but, if possible, it
  should reuse the existing tools and helpers

So in this case here: can you have a packetdrill test instead? It should
be possible, no? If not, can you only use 'mptcp_connect' and the
existing helpers from mptcp_join.sh?

Also, mptcp_join.sh is to validate cases with multiple subflows. I don't
think you need that, right?

[1] https://github.com/multipath-tcp/packetdrill/

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.