selftests/bpf: Fix a CI failure caused by vsock write

While commit 90f0074c ("selftests/bpf: fix a CI failure caused by vsock sockmap test")
fixes a receive failure of vsock sockmap test, there is still a write failure:

Error: #211/79 sockmap_listen/sockmap VSOCK test_vsock_redir
Error: #211/79 sockmap_listen/sockmap VSOCK test_vsock_redir
  ./test_progs:vsock_unix_redir_connectible:1501: egress: write: Transport endpoint is not connected
  vsock_unix_redir_connectible:FAIL:1501
  ./test_progs:vsock_unix_redir_connectible:1501: ingress: write: Transport endpoint is not connected
  vsock_unix_redir_connectible:FAIL:1501
  ./test_progs:vsock_unix_redir_connectible:1501: egress: write: Transport endpoint is not connected
  vsock_unix_redir_connectible:FAIL:1501

The reason is that the vsock connection in the test is set to ESTABLISHED state
by function virtio_transport_recv_pkt, which is executed in a workqueue thread,
so when the user space test thread runs before the workqueue thread, this
problem occurs.

To fix it, before writing the connection, wait for it to be connected.

Fixes: d61bd8c1 ("selftests/bpf: add a test case for vsock sockmap")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230901031037.3314007-1-xukuohai@huaweicloud.com

tools/testing/selftests/bpf/prog_tests/sockmap_helpers.h

@@ -179,6 +179,32 @@
 		__ret;                                                         \
 	})
 
+static inline int poll_connect(int fd, unsigned int timeout_sec)
+{
+	struct timeval timeout = { .tv_sec = timeout_sec };
+	fd_set wfds;
+	int r, eval;
+	socklen_t esize = sizeof(eval);
+
+	FD_ZERO(&wfds);
+	FD_SET(fd, &wfds);
+
+	r = select(fd + 1, NULL, &wfds, NULL, &timeout);
+	if (r == 0)
+		errno = ETIME;
+	if (r != 1)
+		return -1;
+
+	if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &eval, &esize) < 0)
+		return -1;
+	if (eval != 0) {
+		errno = eval;
+		return -1;
+	}
+
+	return 0;
+}
+
 static inline int poll_read(int fd, unsigned int timeout_sec)
 {
 	struct timeval timeout = { .tv_sec = timeout_sec };

tools/testing/selftests/bpf/prog_tests/sockmap_listen.c

@@ -1452,11 +1452,18 @@ static int vsock_socketpair_connectible(int sotype, int *v0, int *v1)
 	if (p < 0)
 		goto close_cli;
 
+	if (poll_connect(c, IO_TIMEOUT_SEC) < 0) {
+		FAIL_ERRNO("poll_connect");
+		goto close_acc;
+	}
+
 	*v0 = p;
 	*v1 = c;
 
 	return 0;
 
+close_acc:
+	close(p);
 close_cli:
 	close(c);
 close_srv: