MPTCP Join "fastclose server" selftest is sometimes failing because the
client output file doesn't have the expected size, e.g. 296B instead of
1024B.

When looking at a packet trace when this happens, the server sent the
expected 1024B in two parts -- 100B, then 924B -- then the MP_FASTCLOSE.
It is then strange to see the client only receiving 296B, which would
mean it only got a part of the second packet. The problem is then not on
the networking side, but rather on the data reception side.

When mptcp_connect is launched with '-f -1', it means the connection
might stop before having sent everything, because a reset has been
received. When this happens, the program was directly stopped. But it is
also possible there are still some data to read, simply because the
previous 'read' step was done with a buffer smaller than the pending
data, see do_rnd_read(). In this case, it is important to read what's
left in the kernel buffers before stopping without error like before.

Fixes: 6bf41020b72b ("selftests: mptcp: update and extend fastclose test-cases")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 tools/testing/selftests/net/mptcp/mptcp_connect.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index c030b08a7195..6ad2126b08a7 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
@@ -710,8 +710,12 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 
 				bw = do_rnd_write(peerfd, winfo->buf + winfo->off, winfo->len);
 				if (bw < 0) {
-					if (cfg_rcv_trunc)
-						return 0;
+					/* expected reset, continue to read */
+					if (cfg_rcv_trunc && errno == ECONNRESET) {
+						fds.events &= ~POLLOUT;
+						continue;
+					}
+
 					perror("write");
 					return 111;
 				}
@@ -737,8 +741,10 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 		}
 
 		if (fds.revents & (POLLERR | POLLNVAL)) {
-			if (cfg_rcv_trunc)
-				return 0;
+			if (cfg_rcv_trunc) {
+				fds.events &= ~(POLLERR | POLLNVAL);
+				continue;
+			}
 			fprintf(stderr, "Unexpected revents: "
 				"POLLERR/POLLNVAL(%x)\n", fds.revents);
 			return 5;

-- 
2.51.0