Allow to overrid the check for p->ref==1 in TX packets (there *are* netif drivers that can handle this)

src/core/ipv4/ip4.c

@@ -710,9 +710,7 @@ err_t ip_output_if_opt_src(struct pbuf *p, ip_addr_t *src, ip_addr_t *dest,
   u32_t chk_sum = 0;
 #endif /* CHECKSUM_GEN_IP_INLINE */
 
-  /* pbufs passed to IP must have a ref-count of 1 as their payload pointer
-     gets altered as the packet is passed down the stack */
-  LWIP_ASSERT("p->ref == 1", p->ref == 1);
+  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
 
   snmp_inc_ipoutrequests();
 
@@ -867,9 +865,7 @@ ip_output(struct pbuf *p, ip_addr_t *src, ip_addr_t *dest,
 {
   struct netif *netif;
 
-  /* pbufs passed to IP must have a ref-count of 1 as their payload pointer
-     gets altered as the packet is passed down the stack */
-  LWIP_ASSERT("p->ref == 1", p->ref == 1);
+  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
 
   if ((netif = ip_route(dest)) == NULL) {
     LWIP_DEBUGF(IP_DEBUG, ("ip_output: No route to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
@@ -907,9 +903,7 @@ ip_output_hinted(struct pbuf *p, ip_addr_t *src, ip_addr_t *dest,
   struct netif *netif;
   err_t err;
 
-  /* pbufs passed to IP must have a ref-count of 1 as their payload pointer
-     gets altered as the packet is passed down the stack */
-  LWIP_ASSERT("p->ref == 1", p->ref == 1);
+  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
 
   if ((netif = ip_route(dest)) == NULL) {
     LWIP_DEBUGF(IP_DEBUG, ("ip_output: No route to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",

src/core/ipv6/ip6.c

@@ -773,9 +773,7 @@ ip6_output_if_src(struct pbuf *p, ip6_addr_t *src, ip6_addr_t *dest,
   struct ip6_hdr *ip6hdr;
   ip6_addr_t dest_addr;
 
-  /* pbufs passed to IP must have a ref-count of 1 as their payload pointer
-     gets altered as the packet is passed down the stack */
-  LWIP_ASSERT("p->ref == 1", p->ref == 1);
+  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
 
   /* Should the IPv6 header be generated or is it already included in p? */
   if (dest != IP_HDRINCL) {
@@ -867,9 +865,7 @@ ip6_output(struct pbuf *p, ip6_addr_t *src, ip6_addr_t *dest,
   struct ip6_hdr *ip6hdr;
   ip6_addr_t src_addr, dest_addr;
 
-  /* pbufs passed to IPv6 must have a ref-count of 1 as their payload pointer
-     gets altered as the packet is passed down the stack */
-  LWIP_ASSERT("p->ref == 1", p->ref == 1);
+  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
 
   if (dest != IP_HDRINCL) {
     netif = ip6_route(src, dest);
@@ -928,9 +924,7 @@ ip6_output_hinted(struct pbuf *p, ip6_addr_t *src, ip6_addr_t *dest,
   ip6_addr_t src_addr, dest_addr;
   err_t err;
 
-  /* pbufs passed to IP must have a ref-count of 1 as their payload pointer
-     gets altered as the packet is passed down the stack */
-  LWIP_ASSERT("p->ref == 1", p->ref == 1);
+  LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
 
   if (dest != IP_HDRINCL) {
     netif = ip6_route(src, dest);

src/include/lwip/ip.h

@@ -54,6 +54,12 @@ extern "C" {
 #endif /* IP_HDRINCL */
 #define IP_HDRINCL  NULL
 
+/** pbufs passed to IP must have a ref-count of 1 as their payload pointer
+    gets altered as the packet is passed down the stack */
+#ifndef LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX
+#define LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p) LWIP_ASSERT("p->ref == 1", (p)->ref == 1)
+#endif
+
 #if LWIP_NETIF_HWADDRHINT
 #define IP_PCB_ADDRHINT ;u8_t addr_hint
 #else