From: Geliang Tang <tanggeliang@kylinos.cn>

Implements FIONREAD ioctl checks to validate available data size, ensuring
synchronization between write and read operations with non-blocking waits.

These codes are from mptcp_inq.c.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
---
 .../selftests/net/mptcp/mptcp_sockopt.c        | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index b8e8fec8979f..7cc4626b7bf3 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -771,6 +771,24 @@ static void process_one_client(int fd, int unixfd)
 	if (expect_len > sizeof(buf))
 		xerror("expect len %zu exceeds buffer size", expect_len);
 
+	for (;;) {
+		struct timespec req;
+		unsigned int queued;
+
+		ret = ioctl(fd, FIONREAD, &queued);
+		if (ret < 0)
+			die_perror("FIONREAD");
+		if (queued > expect_len)
+			xerror("FIONREAD returned %u, but only %zu expected\n",
+			       queued, expect_len);
+		if (queued == expect_len)
+			break;
+
+		req.tv_sec = 0;
+		req.tv_nsec = 1000 * 1000ul;
+		nanosleep(&req, NULL);
+	}
+
 	/* read one byte, expect cmsg to return expected - 1 */
 	ret = recvmsg(fd, &msg, 0);
 	if (ret < 0)
-- 
2.48.1