fix bug #2595: "loopif results

in NULL reference for incoming TCP packets". Loopif has to be configured
(using LWIP_LOOPIF_MULTITHREADING) to directly call netif->input()
(multithreading environments, e.g. netif->input() = tcpip_input()) or
putting packets on a list that is fed to the stack by calling loopif_poll()
(single-thread / NO_SYS / polling environment where e.g.
netif->input() = ip_input).

src/include/lwip/opt.h

@@ -408,6 +408,17 @@ a lot of data that needs to be copied, this should be set high. */
 #define LWIP_HAVE_LOOPIF                0
 #endif
 
+/* This switches between directly calling netif->input() (=1 for multithreading
+ * environments like tcpip.c) or putting the packets on a list and calling
+ * loopif_poll() in the main application loop (=0 for polling / NO_SYS
+ * environments).
+ * Setting this is needed to avoid reentering non-reentrant functions like
+ * tcp_input().
+ */
+#ifndef LWIP_LOOPIF_MULTITHREADING
+#define LWIP_LOOPIF_MULTITHREADING      1
+#endif
+
 #ifndef LWIP_EVENT_API
 #define LWIP_EVENT_API                  0
 #define LWIP_CALLBACK_API               1

src/include/netif/loopif.h

@@ -34,6 +34,10 @@
 
 #include "lwip/netif.h"
 
+#if !LWIP_LOOPIF_MULTITHREADING
+void loopif_poll(struct netif *netif);
+#endif
+
 err_t loopif_init(struct netif *netif);
 
 #endif /* __NETIF_LOOPIF_H__ */