Won't get to upstream

--- a/src/core/init.c
+++ b/src/core/init.c
@@ -175,6 +175,9 @@
 #if ((LWIP_SOCKET || LWIP_NETCONN) && (NO_SYS==1))
 #error "If you want to use Sequential API, you have to define NO_SYS=0 in your lwipopts.h"
 #endif
+#if (LWIP_SOCKET_OPEN_COUNT && LWIP_SOCKET_SELECT)
+#error "If you want to use LWIP_SOCKET_OPEN_COUNT, you must define LWIP_SOCKET_SELECT=0 in your lwipopts.h"
+#endif
 #if (LWIP_PPP_API && (NO_SYS==1))
 #error "If you want to use PPP API, you have to define NO_SYS=0 in your lwipopts.h"
 #endif
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -1989,6 +1989,15 @@
 #endif
 
 /**
+ * LWIP_SOCKET_OPEN_COUNT==1: Number of sockets is not limited to MEMP_NUM_NETCONN.
+ * When enabled, sockets are allocated in the heap and the amount of sockets is
+ * only limited by the heap size. Handle with care regarding execution speed.
+ */
+#if !defined LWIP_SOCKET_OPEN_COUNT || defined __DOXYGEN__
+#define LWIP_SOCKET_OPEN_COUNT          0
+#endif
+
+/**
  * LWIP_TCP_KEEPALIVE==1: Enable TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT
  * options processing. Note that TCP_KEEPIDLE and TCP_KEEPINTVL have to be set
  * in seconds. (does not require sockets.c, and will affect tcp.c)
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -256,8 +256,13 @@
 static void lwip_socket_drop_registered_mld6_memberships(int s);
 #endif /* LWIP_IPV6_MLD */
 
+#if LWIP_SOCKET_OPEN_COUNT
+/** The global linked list of available sockets */
+static struct lwip_sock *sockets = NULL;
+#else /* LWIP_SOCKET_OPEN_COUNT */
 /** The global array of available sockets */
 static struct lwip_sock sockets[NUM_SOCKETS];
+#endif /* LWIP_SOCKET_OPEN_COUNT */
 
 #if LWIP_SOCKET_SELECT || LWIP_SOCKET_POLL
 #if LWIP_TCPIP_CORE_LOCKING
@@ -406,16 +411,47 @@
 #define done_socket(sock)
 #endif /* LWIP_NETCONN_FULLDUPLEX */
 
+/* Translate a socket 'int' into a pointer (doesn't check the validity of the index) */
+static struct lwip_sock *
+translate_socket(int s)
+{
+#if LWIP_SOCKET_OPEN_COUNT
+  struct lwip_sock *sock;
+
+  /*
+   * We need to protect the integrity of the list itself,
+   * since elements may be dynamically inserted or deleted.
+   */
+  SYS_ARCH_DECL_PROTECT(lev);
+  SYS_ARCH_PROTECT(lev);
+  for(sock = sockets; sock != NULL; sock = sock->next) {
+    if(sock->count == s) {
+      break;
+    }
+  }
+  SYS_ARCH_UNPROTECT(lev);
+  return sock;
+#else /* LWIP_SOCKET_OPEN_COUNT */
+  return &sockets[s];
+#endif /* LWIP_SOCKET_OPEN_COUNT */
+}
+
 /* Translate a socket 'int' into a pointer (only fails if the index is invalid) */
 static struct lwip_sock *
 tryget_socket_unconn_nouse(int fd)
 {
   int s = fd - LWIP_SOCKET_OFFSET;
-  if ((s < 0) || (s >= NUM_SOCKETS)) {
+
+  if ((s < 0)
+#if !LWIP_SOCKET_OPEN_COUNT
+      || (s >= NUM_SOCKETS)
+#endif /* !LWIP_SOCKET_OPEN_COUNT */
+      ) {
     LWIP_DEBUGF(SOCKETS_DEBUG, ("tryget_socket_unconn(%d): invalid\n", fd));
     return NULL;
   }
-  return &sockets[s];
+
+  return translate_socket(s);
 }
 
 struct lwip_sock *
@@ -480,8 +516,11 @@
 {
   struct lwip_sock *sock = tryget_socket(fd);
   if (!sock) {
-    if ((fd < LWIP_SOCKET_OFFSET) || (fd >= (LWIP_SOCKET_OFFSET + NUM_SOCKETS))) {
-      LWIP_DEBUGF(SOCKETS_DEBUG, ("get_socket(%d): invalid\n", fd));
+    if ((fd < LWIP_SOCKET_OFFSET)
+#if !LWIP_SOCKET_OPEN_COUNT
+          || (fd >= (LWIP_SOCKET_OFFSET + NUM_SOCKETS))
+#endif /* !LWIP_SOCKET_OPEN_COUNT */
+        ) {      LWIP_DEBUGF(SOCKETS_DEBUG, ("get_socket(%d): invalid\n", fd));
     }
     set_errno(EBADF);
     return NULL;
@@ -501,40 +540,87 @@
 alloc_socket(struct netconn *newconn, int accepted)
 {
   int i;
+  struct lwip_sock *newsock = NULL;
+#if LWIP_SOCKET_OPEN_COUNT
+  struct lwip_sock **it;
+#endif /* LWIP_SOCKET_OPEN_COUNT */
+
   SYS_ARCH_DECL_PROTECT(lev);
   LWIP_UNUSED_ARG(accepted);
 
+#if LWIP_SOCKET_OPEN_COUNT
+  newsock = (struct lwip_sock*)mem_calloc(1, sizeof(struct lwip_sock));
+  if(!newsock) {
+    /* No free memory */
+    return -1;
+  }
+
+  it = &sockets;
+  i = 0;
+  /* Protect socket list */
+  SYS_ARCH_PROTECT(lev);
+  while(*it) {
+    if((*it)->count != i) {
+      /* There's a gap in the list, fill it */
+      break;
+    }
+    i++;
+    it = &(*it)->next;
+  }
+  /* Add the new socket in the first gap found or in the end */
+  newsock->count = i;
+  newsock->next = (*it);
+  (*it) = newsock;
+  newsock->conn = newconn;
+#else /* LWIP_SOCKET_OPEN_COUNT */
   /* allocate a new socket identifier */
   for (i = 0; i < NUM_SOCKETS; ++i) {
     /* Protect socket array */
     SYS_ARCH_PROTECT(lev);
-    if (!sockets[i].conn) {
+    newsock = &sockets[i];
+    if (!newsock->conn) {
 #if LWIP_NETCONN_FULLDUPLEX
-      if (sockets[i].fd_used) {
+      if (newsock->fd_used) {
         SYS_ARCH_UNPROTECT(lev);
         continue;
       }
-      sockets[i].fd_used    = 1;
-      sockets[i].fd_free_pending = 0;
-#endif
-      sockets[i].conn       = newconn;
-      /* The socket is not yet known to anyone, so no need to protect
-         after having marked it as used. */
-      SYS_ARCH_UNPROTECT(lev);
-      sockets[i].lastdata.pbuf = NULL;
-#if LWIP_SOCKET_SELECT || LWIP_SOCKET_POLL
-      LWIP_ASSERT("sockets[i].select_waiting == 0", sockets[i].select_waiting == 0);
-      sockets[i].rcvevent   = 0;
-      /* TCP sendbuf is empty, but the socket is not yet writable until connected
-       * (unless it has been created by accept()). */
-      sockets[i].sendevent  = (NETCONNTYPE_GROUP(newconn->type) == NETCONN_TCP ? (accepted != 0) : 1);
-      sockets[i].errevent   = 0;
-#endif /* LWIP_SOCKET_SELECT || LWIP_SOCKET_POLL */
-      return i + LWIP_SOCKET_OFFSET;
+#endif /* LWIP_NETCONN_FULLDUPLEX */
+      newsock->conn = newconn;
+      break;
     }
     SYS_ARCH_UNPROTECT(lev);
   }
-  return -1;
+
+  if(!newsock->conn) {
+    /* No free sockets available */
+    SYS_ARCH_UNPROTECT(lev);
+    return -1;
+  }
+#endif /* LWIP_SOCKET_OPEN_COUNT */
+
+#if LWIP_NETCONN_FULLDUPLEX
+  newsock->fd_used    = 1;
+  newsock->fd_free_pending = 0;
+#endif /* LWIP_NETCONN_FULLDUPLEX */
+
+  /* The socket is not yet known to anyone, so no need to protect
+     after having marked it as used. */
+  SYS_ARCH_UNPROTECT(lev);
+  newsock->lastdata.pbuf = NULL;
+#if LWIP_SOCKET_SELECT || LWIP_SOCKET_POLL
+  LWIP_ASSERT("newsock->select_waiting == 0", newsock->select_waiting == 0);
+  newsock->rcvevent   = 0;
+  /* TCP sendbuf is empty, but the socket is not yet writable until connected
+   * (unless it has been created by accept()). */
+  newsock->sendevent  = (NETCONNTYPE_GROUP(newconn->type) == NETCONN_TCP ? (accepted != 0) : 1);
+  newsock->errevent   = 0;
+#endif /* LWIP_SOCKET_SELECT || LWIP_SOCKET_POLL */
+
+#if LWIP_SOCKET_OPEN_COUNT
+  return newsock->count + LWIP_SOCKET_OFFSET;
+#else /* LWIP_SOCKET_OPEN_COUNT */
+  return i + LWIP_SOCKET_OFFSET;
+#endif /* LWIP_SOCKET_OPEN_COUNT */
 }
 
 /** Free a socket (under lock)
@@ -548,6 +634,10 @@
 free_socket_locked(struct lwip_sock *sock, int is_tcp, struct netconn **conn,
                    union lwip_sock_lastdata *lastdata)
 {
+#if LWIP_SOCKET_OPEN_COUNT
+  struct lwip_sock **it = &sockets;
+#endif /* LWIP_SOCKET_OPEN_COUNT */
+
 #if LWIP_NETCONN_FULLDUPLEX
   LWIP_ASSERT("sock->fd_used > 0", sock->fd_used > 0);
   sock->fd_used--;
@@ -560,9 +650,18 @@
 #endif /* LWIP_NETCONN_FULLDUPLEX */
 
   *lastdata = sock->lastdata;
-  sock->lastdata.pbuf = NULL;
   *conn = sock->conn;
   sock->conn = NULL;
+
+#if LWIP_SOCKET_OPEN_COUNT
+  while(*it != sock) it = &(*it)->next;
+  *it = (*it)->next;
+  mem_free(sock);
+#else /* LWIP_SOCKET_OPEN_COUNT */
+  sock->lastdata.pbuf = NULL;
+  sock->conn = NULL;
+#endif /* LWIP_SOCKET_OPEN_COUNT */
+
   return 1;
 }
 
@@ -657,9 +756,14 @@
     done_socket(sock);
     return -1;
   }
+
+#if !LWIP_SOCKET_OPEN_COUNT
   LWIP_ASSERT("invalid socket index", (newsock >= LWIP_SOCKET_OFFSET) && (newsock < NUM_SOCKETS + LWIP_SOCKET_OFFSET));
-  nsock = &sockets[newsock - LWIP_SOCKET_OFFSET];
+#else /* !LWIP_SOCKET_OPEN_COUNT */
+  LWIP_ASSERT("invalid socket index", (newsock >= LWIP_SOCKET_OFFSET));
+#endif /* !LWIP_SOCKET_OPEN_COUNT */
 
+  nsock = translate_socket(newsock - LWIP_SOCKET_OFFSET);
   /* See event_callback: If data comes in right away after an accept, even
    * though the server task might not have created a new socket yet.
    * In that case, newconn->socket is counted down (newconn->socket--),
@@ -1735,7 +1839,7 @@
     return -1;
   }
   conn->socket = i;
-  done_socket(&sockets[i - LWIP_SOCKET_OFFSET]);
+  done_socket(translate_socket(i - LWIP_SOCKET_OFFSET));
   LWIP_DEBUGF(SOCKETS_DEBUG, ("%d\n", i));
   set_errno(0);
   return i;
--- a/src/include/lwip/priv/sockets_priv.h
+++ b/src/include/lwip/priv/sockets_priv.h
@@ -65,6 +65,12 @@
 
 /** Contains all internal pointers and states used for a socket */
 struct lwip_sock {
+#if LWIP_SOCKET_OPEN_COUNT
+  /** Next element in the linked list */
+  struct lwip_sock *next;
+  /** Socket number*/
+  int count;
+#endif /* LWIP_SOCKET_OPEN_COUNT */
   /** sockets currently are built on netconns, each socket has one netconn */
   struct netconn *conn;
   /** data that was left from the previous read */
--- a/port/include/lwipopts.h
+++ b/port/include/lwipopts.h
@@ -27,6 +27,8 @@
 #define LWIP_COMPAT_SOCKETS       0
 #define LWIP_SOCKET_OFFSET        1
 #define LWIP_POLL                 1
+#define LWIP_SOCKET_OPEN_COUNT    1
+#define LWIP_SOCKET_SELECT        0
 
 /* User posix socket headers */
 #define LWIP_SOCKET_EXTERNAL_HEADERS            1