In case of poll timeout, it might be too late to print the stats after
the socket closure.

Now, in case of poll timeout, 'ss' and 'nstat' are invoked from
mptcp_connect to print the stats before exiting. This should help
debugging poll timeout issues.

Note: for this "workaround", system() is used for debugging purposes
only. The returned result can then be safely ignored, hence the (void)!
to avoid unused-result warnings.

Also, 'nstat *Tcp*' will print '#kernel' at the top, but that's OK, and
it seems better to avoid using a pipe here.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Changes in v3:
- use double quotes instead of single ones (Sashiko)
- Link to v2: https://patch.msgid.link/20260814-sft-mptcp-stats-b4-close-v2-1-b7a8ab3ddae8@kernel.org

Changes in v2:
- use 'ip netns identify' to avoid a warning from AI reviews
- use 'nstat -s *Tcp*' instead of using grep
- add a note about (void)!
- Link to v1: https://patch.msgid.link/20260814-sft-mptcp-stats-b4-close-v1-1-2945a591be7e@kernel.org
---
 tools/testing/selftests/net/mptcp/mptcp_connect.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index ea4cb6c1bd5e..d6c2237bdba0 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
@@ -210,6 +210,19 @@ static void xgetaddrinfo(const char *node, const char *service,
 	}
 }
 
+static void print_err_stats(void)
+{
+	char cmd[128];
+
+	snprintf(cmd, sizeof(cmd), "ss -Menitam -o '%cport = :%s' >&2",
+		 listen_mode ? 's' : 'd', cfg_port);
+
+	fprintf(stderr, "socket stats before socket closure:\n");
+	(void)!system(cmd);
+	(void)!system("NSTAT_HISTORY=\"/tmp/$(ip netns identify).nstat\" "
+		      "nstat -s '*Tcp*' >&2");
+}
+
 static void set_rcvbuf(int fd, unsigned int size)
 {
 	int err;
@@ -656,6 +669,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 			fprintf(stderr, "%s: poll timed out (events: "
 				"POLLIN %u, POLLOUT %u)\n", __func__,
 				fds.events & POLLIN, fds.events & POLLOUT);
+			print_err_stats();
 			return 2;
 		}
 
@@ -1194,6 +1208,7 @@ int main_loop_s(int listensock)
 		return 1;
 	case 0:
 		fprintf(stderr, "%s: timed out\n", __func__);
+		print_err_stats();
 		close(listensock);
 		return 2;
 	}

---
base-commit: d536472a82d18356d323227213acb5603bb9050c
change-id: 20260814-sft-mptcp-stats-b4-close-20964ff6a6ba

Best regards,
--  
Matthieu Baerts (NGI0) <matttbe@kernel.org>

Hi Matthieu,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Success! ✅
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/31847744292

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/2da643328f05
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1146325


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)