selftests/bpf: test_progs: fix client/server race in tcp_rtt

This is the same problem I found earlier in test_sockopt_inherit:
there is a race between server thread doing accept() and client
thread doing connect(). Let's explicitly synchronize them via
pthread conditional variable.

v2:
* don't exit from server_thread without signaling condvar,
  fixes possible issue where main() would wait forever (Andrii Nakryiko)

Fixes: b558739 ("selftests/bpf: test BPF_SOCK_OPS_RTT_CB")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

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

@@ -203,14 +203,24 @@ static int start_server(void)
 	return fd;
 }
 
+static pthread_mutex_t server_started_mtx = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t server_started = PTHREAD_COND_INITIALIZER;
+
 static void *server_thread(void *arg)
 {
 	struct sockaddr_storage addr;
 	socklen_t len = sizeof(addr);
 	int fd = *(int *)arg;
 	int client_fd;
+	int err;
+
+	err = listen(fd, 1);
+
+	pthread_mutex_lock(&server_started_mtx);
+	pthread_cond_signal(&server_started);
+	pthread_mutex_unlock(&server_started_mtx);
 
-	if (CHECK_FAIL(listen(fd, 1)) < 0) {
+	if (CHECK_FAIL(err < 0)) {
 		perror("Failed to listed on socket");
 		return NULL;
 	}
@@ -248,7 +258,14 @@ void test_tcp_rtt(void)
 	if (CHECK_FAIL(server_fd < 0))
 		goto close_cgroup_fd;
 
-	pthread_create(&tid, NULL, server_thread, (void *)&server_fd);
+	if (CHECK_FAIL(pthread_create(&tid, NULL, server_thread,
+				      (void *)&server_fd)))
+		goto close_cgroup_fd;
+
+	pthread_mutex_lock(&server_started_mtx);
+	pthread_cond_wait(&server_started, &server_started_mtx);
+	pthread_mutex_unlock(&server_started_mtx);
+
 	CHECK_FAIL(run_test(cgroup_fd, server_fd));
 	close(server_fd);
 close_cgroup_fd: