[PATCH mptcp-next v3] selftests: mptcp: print stats before socket closure

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) <[email protected]>
---
Changes in v3:
- use double quotes instead of single ones (Sashiko)
- Link to v2: https://patch.msgid.link/[email protected]

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/[email protected]
---
 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) <[email protected]>