Don't claim minimal mdns is initialized until it's advertising on some IPv6 interface. (#25173)

This fixes two things:

1. We now don't consider advertising properly initialized until we are
   advertising on at least one ipv6 interface.

2. Actually check which sorts of addresses interfaces have, instead of
   just assuming that all interfaces have both IPv4 and IPv6 addresses.

Fixes #25013

Author: bzbarsky-apple

src/lib/dnssd/minimal_mdns/AddressPolicy_DefaultImpl.cpp

@@ -69,23 +69,38 @@ class AllInterfaces : public mdns::Minimal::ListenIterator
 #if INET_CONFIG_ENABLE_IPV4
         if (mState == State::kIpV4)
         {
-            *id    = mIterator.GetInterfaceId();
-            *type  = chip::Inet::IPAddressType::kIPv4;
             mState = State::kIpV6;
-            return true;
+
+            if (CurrentInterfaceHasAddressOfType(chip::Inet::IPAddressType::kIPv4))
+            {
+                *id   = mIterator.GetInterfaceId();
+                *type = chip::Inet::IPAddressType::kIPv4;
+                return true;
+            }
         }
 #endif
 
-        *id   = mIterator.GetInterfaceId();
-        *type = chip::Inet::IPAddressType::kIPv6;
 #if INET_CONFIG_ENABLE_IPV4
         mState = State::kIpV4;
 #endif
 
+        bool haveResult = CurrentInterfaceHasAddressOfType(chip::Inet::IPAddressType::kIPv6);
+        if (haveResult)
+        {
+            *id   = mIterator.GetInterfaceId();
+            *type = chip::Inet::IPAddressType::kIPv6;
+        }
+
         for (mIterator.Next(); SkipCurrentInterface(); mIterator.Next())
         {
         }
-        return true;
+
+        if (haveResult)
+        {
+            return true;
+        }
+
+        return Next(id, type);
     }
 
 private:
@@ -119,6 +134,8 @@ class AllInterfaces : public mdns::Minimal::ListenIterator
 
         return !IsCurrentInterfaceUsable(mIterator);
     }
+
+    bool CurrentInterfaceHasAddressOfType(chip::Inet::IPAddressType type);
 };
 
 class AllAddressesIterator : public mdns::Minimal::IpAddressIterator
@@ -170,6 +187,19 @@ class AllAddressesIterator : public mdns::Minimal::IpAddressIterator
     chip::Inet::InterfaceAddressIterator mIterator;
 };
 
+bool AllInterfaces::CurrentInterfaceHasAddressOfType(chip::Inet::IPAddressType type)
+{
+    // mIterator.HasCurrent() must be true here.
+    AllAddressesIterator addressIter(mIterator.GetInterfaceId(), type);
+    chip::Inet::IPAddress addr;
+    if (addressIter.Next(addr))
+    {
+        return true;
+    }
+
+    return false;
+}
+
 class DefaultAddressPolicy : public AddressPolicy
 {
 public:

src/lib/dnssd/minimal_mdns/Server.cpp

@@ -279,8 +279,8 @@ CHIP_ERROR ServerBase::Listen(chip::Inet::EndPointManager<chip::Inet::UDPEndPoin
         }
 #endif
 
-        // If at least one interface is used by the mDNS server, notify the application that DNS-SD is ready.
-        if (!mIsInitialized)
+        // If at least one IPv6 interface is used by the mDNS server, notify the application that DNS-SD is ready.
+        if (!mIsInitialized && addressType == chip::Inet::IPAddressType::kIPv6)
         {
 #if !CHIP_DEVICE_LAYER_NONE
             chip::DeviceLayer::ChipDeviceEvent event{};