[lwip-devel] [bug #56147] lwip_select() does not always wakup on received data

[Joel Palsson]

URL:
  <https://savannah.nongnu.org/bugs/?56147>

                 Summary: lwip_select() does not always wakup on received data
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: jpa
            Submitted on: Wed 17 Apr 2019 02:42:23 PM UTC
                Category: sockets/netconn
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: Other

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Details:

This pseudocode would result in lwip_select() waking up for data received once
(if less than 1000 bytes), but not second time data is received:

while( 1 )
{
   lwip_select( maxfdp1, &readset, NULL, NULL, NULL);
   lwip_read( s, pData, 1000 );
}


The reason is that when first data is read with lwip_read(), the loop in
lwip_recv_tcp() is not exited resulting in calling
netconn_recv_tcp_pbuf_flags() twice which will call:
   ->netconn_recv_data_tcp()
      ->netconn_recv_data()
          ->API_EVENT(conn, NETCONN_EVT_RCVMINUS, len);

That results in decrementing the sock->rcvevent twice which, if only one
receive event has occured, will then be -1. So next receive event will restore
it to 0, but select() won't wake since it should be >0.

A fix is to change the exit condition for the loop in lwip_recv_tcp() in
socket.c at line 1025 from

} while ((recv_left > 0) && !(flags & MSG_PEEK));

to

} while ((recv_left > 0) && !(flags & MSG_PEEK) && (sock->rcvevent > 0) );






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