vsock/test: Fix occasional failure in SIOCOUTQ tests

These tests:
    "SOCK_STREAM ioctl(SIOCOUTQ) 0 unsent bytes"
    "SOCK_SEQPACKET ioctl(SIOCOUTQ) 0 unsent bytes"
output: "Unexpected 'SIOCOUTQ' value, expected 0, got 64 (CLIENT)".

They test that the SIOCOUTQ ioctl reports 0 unsent bytes after the data
have been received by the other side. However, sometimes there is a delay
in updating this "unsent bytes" counter, and the test fails even though
the counter properly goes to 0 several milliseconds later.

The delay occurs in the kernel because the used buffer notification
callback virtio_vsock_tx_done(), called upon receipt of the data by the
other side, doesn't update the counter itself. It delegates that to
a kernel thread (via vsock->tx_work). Sometimes that thread is delayed
more than the test expects.

Change the test to poll SIOCOUTQ until it returns 0 or a timeout occurs.

Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Fixes: 18ee44c ("test/vsock: add ioctl unsent bytes test")
Link: https://patch.msgid.link/20250507151456.2577061-1-kshk@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/vsock/vsock_test.c

@@ -1264,21 +1264,25 @@ static void test_unsent_bytes_client(const struct test_opts *opts, int type)
 	send_buf(fd, buf, sizeof(buf), 0, sizeof(buf));
 	control_expectln("RECEIVED");
 
-	ret = ioctl(fd, SIOCOUTQ, &sock_bytes_unsent);
-	if (ret < 0) {
-		if (errno == EOPNOTSUPP) {
-			fprintf(stderr, "Test skipped, SIOCOUTQ not supported.\n");
-		} else {
+	/* SIOCOUTQ isn't guaranteed to instantly track sent data. Even though
+	 * the "RECEIVED" message means that the other side has received the
+	 * data, there can be a delay in our kernel before updating the "unsent
+	 * bytes" counter. Repeat SIOCOUTQ until it returns 0.
+	 */
+	timeout_begin(TIMEOUT);
+	do {
+		ret = ioctl(fd, SIOCOUTQ, &sock_bytes_unsent);
+		if (ret < 0) {
+			if (errno == EOPNOTSUPP) {
+				fprintf(stderr, "Test skipped, SIOCOUTQ not supported.\n");
+				break;
+			}
 			perror("ioctl");
 			exit(EXIT_FAILURE);
 		}
-	} else if (ret == 0 && sock_bytes_unsent != 0) {
-		fprintf(stderr,
-			"Unexpected 'SIOCOUTQ' value, expected 0, got %i\n",
-			sock_bytes_unsent);
-		exit(EXIT_FAILURE);
-	}
-
+		timeout_check("SIOCOUTQ");
+	} while (sock_bytes_unsent != 0);
+	timeout_end();
 	close(fd);
 }