Generic netif status callback and mDNS (#8705)

* sprinkle IPAddress(...).isSet() across our loops to avoid polling on a
  stopped interface. status callback and netif_is_up **does not
  guarantee and we could use the interface**!
* register *one* status callback per instance, e.g. when begin() is called
  multiple times (also notice a subtle issue with schedule function when
  instance is delete'ed)
* consistent LwipIntf callback signature. no need for rvalue, just pass
  stdfunc as-is and let the compiler figure it out

cores/esp8266/LwipIntf.h

@@ -34,8 +34,6 @@ class LwipIntf
 public:
     using CBType = std::function<void(netif*)>;
 
-    static bool stateUpCB(LwipIntf::CBType&& cb);
-
     // reorder WiFi.config() parameters for a esp8266/official Arduino dual-compatibility API
     // args     | esp order  arduino order
     // ----     + ---------  -------------
@@ -67,8 +65,11 @@ public:
     // ESP32 API compatibility
     const char* getHostname();
 
-protected:
-    static bool stateChangeSysCB(LwipIntf::CBType&& cb);
+    // whenever netif status callback is called
+    static bool statusChangeCB(LwipIntf::CBType);
+
+    static bool stateUpCB(LwipIntf::CBType);
+    static bool stateDownCB(LwipIntf::CBType);
 };
 
 #endif  // _LWIPINTF_H

cores/esp8266/LwipIntfCB.cpp

@@ -25,44 +25,54 @@
 #include <Schedule.h>
 #include <debug.h>
 
-#define NETIF_STATUS_CB_SIZE 3
+static constexpr size_t LwipIntfCallbacks = 3;
 
-static int       netifStatusChangeListLength = 0;
-LwipIntf::CBType netifStatusChangeList[NETIF_STATUS_CB_SIZE];
+static LwipIntf::CBType callbacks[LwipIntfCallbacks];
+static size_t           size = 0;
 
+// override empty weak function from glue-lwip
 extern "C" void netif_status_changed(struct netif* netif)
 {
-    // override the default empty weak function
-    for (int i = 0; i < netifStatusChangeListLength; i++)
+    for (size_t index = 0; index < size; ++index)
     {
-        netifStatusChangeList[i](netif);
+        callbacks[index](netif);
     }
 }
 
-bool LwipIntf::stateChangeSysCB(LwipIntf::CBType&& cb)
+bool LwipIntf::statusChangeCB(LwipIntf::CBType cb)
 {
-    if (netifStatusChangeListLength >= NETIF_STATUS_CB_SIZE)
+    if (size < LwipIntfCallbacks)
     {
+        callbacks[size++] = std::move(cb);
+        return true;
+    }
 #if defined(DEBUG_ESP_CORE)
-        DEBUGV("NETIF_STATUS_CB_SIZE is too low\n");
+    DEBUGV("LwipIntf::CB %zu/%zu, cannot add more!\n", size, size);
 #endif
-        return false;
-    }
 
-    netifStatusChangeList[netifStatusChangeListLength++] = cb;
-    return true;
+    return false;
 }
 
-bool LwipIntf::stateUpCB(LwipIntf::CBType&& cb)
+bool LwipIntf::stateUpCB(LwipIntf::CBType cb)
 {
-    return stateChangeSysCB(
-        [cb](netif* nif)
+    return statusChangeCB(
+        [cb](netif* interface)
         {
-            if (netif_is_up(nif))
-                schedule_function(
-                    [cb, nif]()
-                    {
-                        cb(nif);
-                    });
+            if (netif_is_up(interface))
+            {
+                cb(interface);
+            }
+        });
+}
+
+bool LwipIntf::stateDownCB(LwipIntf::CBType cb)
+{
+    return statusChangeCB(
+        [cb](netif* interface)
+        {
+            if (!netif_is_up(interface))
+            {
+                cb(interface);
+            }
         });
 }