selftests: mptcp: avoid spurious errors on disconnect

selftests: mptcp: avoid spurious errors on disconnect

The disconnect test-case generates spurious errors:

  INFO: disconnect
  INFO: extra options: -I 3 -i /tmp/tmp.r43niviyoI
  01 ns1 MPTCP -> ns1 (10.0.1.1:10000      ) MPTCP (duration 140ms) [FAIL]
  file received by server does not match (in, out):
  Unexpected revents: POLLERR/POLLNVAL(19)
  -rw-r--r-- 1 root root 10028676 Jan 10 10:47 /tmp/tmp.r43niviyoI.disconnect
  Trailing bytes are:
  ��\����R���!8��u2��5N%
  -rw------- 1 root root 9992290 Jan 10 10:47 /tmp/tmp.Os4UbnWbI1
  Trailing bytes are:
  ��\����R���!8��u2��5N%
  02 ns1 MPTCP -> ns1 (dead:beef:1::1:10001) MPTCP (duration 206ms) [ OK ]
  03 ns1 MPTCP -> ns1 (dead:beef:1::1:10002) TCP   (duration  31ms) [ OK ]
  04 ns1 TCP   -> ns1 (dead:beef:1::1:10003) MPTCP (duration  26ms) [ OK ]
  [FAIL] Tests of the full disconnection have failed
  Time: 2 seconds

The root cause is actually in the user-space bits: the test program
currently disconnects as soon as all the pending data has been spooled,
generating an FASTCLOSE. If such option reaches the peer before the
latter has reached the closed status, the msk socket will report an
error to the user-space, as per protocol specification, causing the
above failure.

Address the issue explicitly waiting for all the relevant sockets to
reach a closed status before performing the disconnect.

Fixes: 05be5e273c84 ("selftests: mptcp: add disconnect tests")
Cc: [email protected]
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/20250113-net-mptcp-connect-st-flakes-v1-3-0d986ee7b1b6@kernel.org
Signed-off-by: Jakub Kicinski <[email protected]>