[RFC PATCH mptcp-next v3 1/3] 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 EAGAIN lasts longer than the
poll timeout, still fail: 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>
---
 .../testing/selftests/net/mptcp/mptcp_connect.c  | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index 178d98d91fea..03fff859e4d1 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_ms = 0;
 
 	set_nonblock(peerfd, true);
 
@@ -711,6 +712,20 @@ 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: poll() does not
+					 * check every cause of EAGAIN.
+					 */
+					if (errno == EAGAIN &&
+					    ++eagain_ms <= poll_timeout) {
+						usleep(1000);
+						continue;
+					}
+
 					/* expected reset, continue to read */
 					if (cfg_rcv_trunc &&
 					    (errno == ECONNRESET ||
@@ -723,6 +738,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 					return 111;
 				}
 
+				eagain_ms = 0;
 				winfo->off += bw;
 				winfo->len -= bw;
 				total_wlen += bw;
-- 
2.25.1