[PATCH mptcp-next v6 1/6] selftests: mptcp: connect: retry write() on EAGAIN

[Anil Kaushik <anilkaushikwireless@gmail.com>]

mptcp_connect treats any write() error as fatal. But write() can fail
with EAGAIN even after poll() said the socket was writable: the MPTCP
send buffer shrinks when a subflow is closed.

The tool then exits and the transfer fails on both sides. Instead, go
back to poll() and wait for more space. If a poll timeout is set, still
fail after as many retries, 1ms apart: poll() does not check every
cause of EAGAIN.

Fixes: 048d19d444be ("mptcp: add basic kselftest for mptcp")
Assisted-by: LLM
Signed-off-by: Anil Kaushik <anilkaushikwireless@gmail.com>
---

Notes:
    v6:
     - the comment says the limit only applies with a poll timeout (netdev
       Sashiko)
    v5:
     - no change
    v4:
     - count the EAGAIN retries, not ms (Sashiko)
     - no limit when there is no poll timeout, -t 0 (Sashiko)
    v3:
     - do not wait forever on EAGAIN (Sashiko)
     - wrap lines over 80 columns
    v2:
     - Fixes tag

 .../selftests/net/mptcp/mptcp_connect.c        | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index 178d98d91fea..49017d00ab69 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
@@ -638,6 +638,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 		.events = POLLIN | POLLOUT,
 	};
 	unsigned int total_wlen = 0, total_rlen = 0;
+	int eagain_cnt = 0;
 
 	set_nonblock(peerfd, true);
 
@@ -711,6 +712,22 @@ 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) {
+					/* POLLOUT does not guarantee that
+					 * the next write will not block:
+					 * the MPTCP send buffer can shrink
+					 * in between, e.g. when a subflow
+					 * is closed. Wait for more space,
+					 * but not forever if a poll timeout
+					 * is set: poll() does not check
+					 * every cause of EAGAIN.
+					 */
+					if (errno == EAGAIN &&
+					    (poll_timeout < 0 ||
+					     ++eagain_cnt <= poll_timeout)) {
+						usleep(1000);
+						continue;
+					}
+
 					/* expected reset, continue to read */
 					if (cfg_rcv_trunc &&
 					    (errno == ECONNRESET ||
@@ -723,6 +740,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 					return 111;
 				}
 
+				eagain_cnt = 0;
 				winfo->off += bw;
 				winfo->len -= bw;
 				total_wlen += bw;
-- 
2.25.1