netconn/socket api: fixed bug #44225 "closing TCP socket should time out eventually", implemented task #6930 "Implement SO_LINGER": closing TCP sockets times out after 20 seconds or after the configured SND_TIMEOUT or depending on the linger settings; fixed that netconn_close/netconn_delete still used message passing for LWIP_TCPIP_CORE_LOCKING==1

src/include/lwip/api.h

@@ -214,6 +214,10 @@ struct netconn {
       for UDP and RAW, used for FIONREAD */
   int recv_avail;
 #endif /* LWIP_SO_RCVBUF */
+#if LWIP_SO_LINGER
+   /** values <0 mean linger is disabled, values > 0 are seconds to linger */
+  s16_t linger;
+#endif /* LWIP_SO_LINGER */
   /** flags holding more netconn-internal state, see NETCONN_FLAG_* defines */
   u8_t flags;
 #if LWIP_TCP

src/include/lwip/api_msg.h

@@ -113,6 +113,11 @@ struct api_msg_msg {
     /** used for lwip_netconn_do_close (/shutdown) */
     struct {
       u8_t shut;
+#if LWIP_SO_SNDTIMEO || LWIP_SO_LINGER
+      u32_t time_started;
+#else /* LWIP_SO_SNDTIMEO || LWIP_SO_LINGER */
+      u8_t polls_left;
+#endif /* LWIP_SO_SNDTIMEO || LWIP_SO_LINGER */
     } sd;
 #if LWIP_IGMP || (LWIP_IPV6 && LWIP_IPV6_MLD)
     /** used for lwip_netconn_do_join_leave_group */

src/include/lwip/opt.h

@@ -1573,6 +1573,13 @@
 #define LWIP_SO_RCVBUF                  0
 #endif
 
+/**
+ * LWIP_SO_LINGER==1: Enable SO_LINGER processing.
+ */
+#ifndef LWIP_SO_LINGER
+#define LWIP_SO_LINGER                  0
+#endif
+
 /**
  * If LWIP_SO_RCVBUF is used, this is the default value for recv_bufsize.
  */
@@ -1580,6 +1587,13 @@
 #define RECV_BUFSIZE_DEFAULT            INT_MAX
 #endif
 
+/**
+ * By default, TCP socket/netconn close waits 20 seconds max to send the FIN
+ */
+#ifndef LWIP_TCP_CLOSE_TIMEOUT_MS_DEFAULT
+#define LWIP_TCP_CLOSE_TIMEOUT_MS_DEFAULT 20000
+#endif
+
 /**
  * SO_REUSE==1: Enable SO_REUSEADDR option.
  */