fix switching to static address with lwip2 (#6026)

fix switching to static address with lwip2

For some reason, ip address is not propagated in a visible way for lwip2
when switching to static address (wifi.config()) *after* wifi.begin().

This patch calls lwip-v1.4's netif_set_addr() with the new ip address to set
all things up, just like it is done and right when wifi.begin() is called
after wifi.config().

Also tested when IPv6 is enabled.

fix documentation: It is more natural to set an IP address before starting WiFi
(.. and not after dhcp has started)
fixes #5839
fixes #6024

Author: d-a-v

doc/esp8266wifi/station-class.rst

@@ -135,8 +135,8 @@ The following IP configuration may be provided:
       Serial.println();
 
       Serial.printf("Connecting to %s\n", ssid);
-      WiFi.begin(ssid, password);
       WiFi.config(staticIP, gateway, subnet);
+      WiFi.begin(ssid, password);
       while (WiFi.status() != WL_CONNECTED)
       {
         delay(500);

libraries/ESP8266WiFi/src/ESP8266WiFiSTA.cpp

@@ -247,6 +247,27 @@ wl_status_t ESP8266WiFiSTAClass::begin() {
  * @param dns1       Static DNS server 1
  * @param dns2       Static DNS server 2
  */
+
+#if LWIP_VERSION_MAJOR != 1
+/*
+  About the following call in the end of ESP8266WiFiSTAClass::config():
+      netif_set_addr(eagle_lwip_getif(STATION_IF), &info.ip, &info.netmask, &info.gw);
+
+  With lwip2, it is needed to trigger IP address change.
+      Recall: when lwip2 is enabled, lwip1 api is still used by espressif firmware
+          https://github.com/d-a-v/esp82xx-nonos-linklayer/tree/25d5e8186f710a230221021cba97727dbfdfd953#how-it-works
+
+  We need first to disable the lwIP API redirection for netif_set_addr() so lwip1's call will be linked:
+      https://github.com/d-a-v/esp82xx-nonos-linklayer/blob/25d5e8186f710a230221021cba97727dbfdfd953/glue-lwip/arch/cc.h#L122
+
+  We also need to declare its prototype using ip4_addr_t instead of ip_addr_t because lwIP-1.x never has IPv6.
+  No need to worry about this #undef, this call is only needed in lwip2, and never used in arduino core code.
+ */
+#undef netif_set_addr // need to call lwIP-v1.4 netif_set_addr()
+extern "C" struct netif* eagle_lwip_getif (int netif_index);
+extern "C" void netif_set_addr (struct netif* netif, ip4_addr_t* ip, ip4_addr_t* netmask, ip4_addr_t* gw);
+#endif
+
 bool ESP8266WiFiSTAClass::config(IPAddress local_ip, IPAddress arg1, IPAddress arg2, IPAddress arg3, IPAddress dns2) {
 
   if(!WiFi.enableSTA(true)) {
@@ -310,6 +331,12 @@ bool ESP8266WiFiSTAClass::config(IPAddress local_ip, IPAddress arg1, IPAddress a
       dns_setserver(1, dns2);
   }
 
+#if LWIP_VERSION_MAJOR != 1
+  // trigger address change by calling lwIP-v1.4 api
+  // (see explanation above)
+  netif_set_addr(eagle_lwip_getif(STATION_IF), &info.ip, &info.netmask, &info.gw);
+#endif
+
   return true;
 }