Minor fixes to socket support when used with LWIP

 - LWIP UDP sockets are always writable and they are not reported as
   such by select() (bug? featuyre?). TCP sockets are ok.
 - Always deliver MG_EV_POLL, whether we have other fd_events or not.
   Otherwise MG_EV_POLL would not be delivered at all to perma-writable
   UDP sockets (as they are on LWIP).
 - Ignore EAGAIN/EWOULDBLOCK errors while connecting, this happens
   during SSL handshake.

PUBLISHED_FROM=3b1ee0d2b0c672512da5c9d6141aba1112983426

mongoose.c

@@ -3163,7 +3163,8 @@ void mg_socket_if_connect_tcp(struct mg_connection *nc,
 #endif
   rc = connect(nc->sock, &sa->sa, sizeof(sa->sin));
   nc->err = mg_is_error(rc) ? mg_get_errno() : 0;
-  LOG(LL_INFO, ("%p sock %d err %d", nc, nc->sock, nc->err));
+  DBG(("%p sock %d rc %d errno %d err %d", nc, nc->sock, rc, mg_get_errno(),
+       nc->err));
 }
 
 void mg_socket_if_connect_udp(struct mg_connection *nc) {
@@ -3482,8 +3483,7 @@ static void mg_ssl_begin(struct mg_connection *nc) {
 
 void mg_mgr_handle_conn(struct mg_connection *nc, int fd_flags, double now) {
   int worth_logging =
-      fd_flags != 0 || (nc->flags & (MG_F_WANT_READ | MG_F_SSL_HANDSHAKE_DONE |
-                                     MG_F_WANT_WRITE));
+      fd_flags != 0 || (nc->flags & (MG_F_WANT_READ | MG_F_WANT_WRITE));
   if (worth_logging) {
     DBG(("%p fd=%d fd_flags=%d nc_flags=%lu rmbl=%d smbl=%d", nc, nc->sock,
          fd_flags, nc->flags, (int) nc->recv_mbuf.len,
@@ -3498,7 +3498,11 @@ void mg_mgr_handle_conn(struct mg_connection *nc, int fd_flags, double now) {
         socklen_t len = sizeof(err);
         int ret =
             getsockopt(nc->sock, SOL_SOCKET, SO_ERROR, (char *) &err, &len);
-        if (ret != 0) err = 1;
+        if (ret != 0) {
+          err = 1;
+        } else if (err == EAGAIN || err == EWOULDBLOCK) {
+          err = 0;
+        }
       }
 #else
       /*
@@ -3541,10 +3545,7 @@ void mg_mgr_handle_conn(struct mg_connection *nc, int fd_flags, double now) {
     if ((fd_flags & _MG_F_FD_CAN_WRITE) && nc->send_mbuf.len > 0) {
       mg_write_to_socket(nc);
     }
-
-    if (!(fd_flags & (_MG_F_FD_CAN_READ | _MG_F_FD_CAN_WRITE))) {
-      mg_if_poll(nc, (time_t) now);
-    }
+    mg_if_poll(nc, (time_t) now);
     mg_if_timer(nc, now);
   }
 
@@ -3721,8 +3722,10 @@ time_t mg_socket_if_poll(struct mg_iface *iface, int timeout_ms) {
                    (FD_ISSET(nc->sock, &err_set) ? _MG_F_FD_ERROR : 0);
       }
 #if MG_LWIP
-      /* With LWIP socket emulation layer, we don't get write events */
-      fd_flags |= _MG_F_FD_CAN_WRITE;
+      /* With LWIP socket emulation layer, we don't get write events for UDP */
+      if ((nc->flags & MG_F_UDP) && nc->listener == NULL) {
+        fd_flags |= _MG_F_FD_CAN_WRITE;
+      }
 #endif
     }
     tmp = nc->next;