tcp.h, opt.h, api.h, api_msg.h, tcp.c, tcp_in.c, api_lib.c, api_msg.c, sockets.c, init.c: task #7252: Implement TCP listen backlog: Warning: raw API applications have to call 'tcp_accepted(pcb)' in their accept callback to keep accepting new connections.

src/core/init.c

@@ -152,6 +152,10 @@
 #if (MEM_USE_POOLS && !MEMP_USE_CUSTOM_POOLS)
   #error "MEM_USE_POOLS requires custom pools (MEMP_USE_CUSTOM_POOLS) to be enabled in your lwipopts.h"
 #endif
+#if (LWIP_DEFAULT_LISTEN_BACKLOG < 0) || (LWIP_DEFAULT_LISTEN_BACKLOG > 255)
+  #error "LWIP_DEFAULT_LISTEN_BACKLOG must fit into an u8_t"
+#endif
+
 
 /* Compile-time checks for deprecated options.
  */

src/core/tcp.c

@@ -368,6 +368,10 @@ tcp_listen(struct tcp_pcb *pcb)
 #if LWIP_CALLBACK_API
   lpcb->accept = tcp_accept_null;
 #endif /* LWIP_CALLBACK_API */
+#if LWIP_LISTEN_BACKLOG
+  lpcb->accepts_pending = 0;
+  lpcb->backlog = LWIP_DEFAULT_LISTEN_BACKLOG;
+#endif /* LWIP_LISTEN_BACKLOG */
   TCP_REG(&tcp_listen_pcbs.listen_pcbs, lpcb);
   return (struct tcp_pcb *)lpcb;
 }

src/core/tcp_in.c

@@ -381,6 +381,11 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
       tcphdr->dest, tcphdr->src);
   } else if (flags & TCP_SYN) {
     LWIP_DEBUGF(TCP_DEBUG, ("TCP connection request %"U16_F" -> %"U16_F".\n", tcphdr->src, tcphdr->dest));
+#if LWIP_LISTEN_BACKLOG
+    if (pcb->accepts_pending >= pcb->backlog) {
+      return ERR_ABRT;
+    }
+#endif /* LWIP_LISTEN_BACKLOG */
     npcb = tcp_alloc(pcb->prio);
     /* If a new PCB could not be created (probably due to lack of memory),
        we don't do anything, but rely on the sender will retransmit the
@@ -390,6 +395,9 @@ tcp_listen_input(struct tcp_pcb_listen *pcb)
       TCP_STATS_INC(tcp.memerr);
       return ERR_MEM;
     }
+#if LWIP_LISTEN_BACKLOG
+    pcb->accepts_pending++;
+#endif /* LWIP_LISTEN_BACKLOG */
     /* Set up the new PCB. */
     ip_addr_set(&(npcb->local_ip), &(iphdr->dest));
     npcb->local_port = pcb->local_port;