fix bug for nonblocking udp is zero

src/api/sockets.c

@@ -2577,13 +2577,16 @@ event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len)
 static void select_check_waiters(int s, int has_recvevent, int has_sendevent, int has_errevent)
 {
   struct lwip_select_cb *scb;
+  struct lwip_sock *sock;
 #if !LWIP_TCPIP_CORE_LOCKING
   int last_select_cb_ctr;
   SYS_ARCH_DECL_PROTECT(lev);
 #endif /* !LWIP_TCPIP_CORE_LOCKING */
 
   LWIP_ASSERT_CORE_LOCKED();
-
+#if ESP_LWIP_SELECT  
+  sock = tryget_socket_unconn(s);
+#endif/* ESP_LWIP_SELECT */ 
 #if !LWIP_TCPIP_CORE_LOCKING
   SYS_ARCH_PROTECT(lev);
 again:
@@ -2605,17 +2608,29 @@ again:
 #if LWIP_SOCKET_SELECT
       {
         /* Test this select call for our socket */
+#if ESP_LWIP_SELECT         
+        if (sock->rcvevent) {
+#else            
         if (has_recvevent) {
+#endif/* ESP_LWIP_SELECT */                 
           if (scb->readset && FD_ISSET(s, scb->readset)) {
             do_signal = 1;
           }
         }
+#if ESP_LWIP_SELECT         
+        if (sock->sendevent) {
+#else        
         if (has_sendevent) {
+#endif/* ESP_LWIP_SELECT */            
           if (!do_signal && scb->writeset && FD_ISSET(s, scb->writeset)) {
             do_signal = 1;
           }
         }
+#if ESP_LWIP_SELECT         
+        if (sock->errevent) {
+#else        
         if (has_errevent) {
+#endif/* ESP_LWIP_SELECT */            
           if (!do_signal && scb->exceptset && FD_ISSET(s, scb->exceptset)) {
             do_signal = 1;
           }
@@ -2645,6 +2660,9 @@ again:
     last_select_cb_ctr = select_cb_ctr;
   }
   SYS_ARCH_UNPROTECT(lev);
+#if ESP_LWIP_SELECT  
+  done_socket(sock);
+#endif/* ESP_LWIP_SELECT */  
 #endif
 }
 #endif /* LWIP_SOCKET_SELECT || LWIP_SOCKET_POLL */

test/unit/lwipopts.h

@@ -99,6 +99,6 @@
 #define ESP_LWIP_IGMP_TIMERS_ONDEMAND           1
 #define ESP_LWIP_MLD6_TIMERS_ONDEMAND           1
 #define ESP_GRATUITOUS_ARP                      1
-
+#define ESP_LWIP_SELECT                         1
 
 #endif /* LWIP_HDR_LWIPOPTS_H */