[child] adding Child::GetMeshLocalIp6Address() (#2511)

This commit adds a new method `Child::GetMeshLocalIp6Address()`
to get the mesh-local IPv6 address registered by a child. The
unit test `test_child` is also updated to verify the new method.

src/core/thread/topology.cpp

@@ -78,6 +78,21 @@ exit:
     return retval;
 }
 
+otError Child::GetMeshLocalIp6Address(Instance &aInstance, Ip6::Address &aAddress) const
+{
+    otError error = OT_ERROR_NONE;
+
+    VerifyOrExit(!IsAllZero(mMeshLocalIid, sizeof(mMeshLocalIid)), error = OT_ERROR_NOT_FOUND);
+
+    memcpy(aAddress.mFields.m8, aInstance.GetThreadNetif().GetMle().GetMeshLocalPrefix(),
+           Ip6::Address::kMeshLocalPrefixSize);
+
+    aAddress.SetIid(mMeshLocalIid);
+
+exit:
+    return error;
+}
+
 otError Child::GetNextIp6Address(Instance &aInstance, Ip6AddressIterator &aIterator, Ip6::Address &aAddress) const
 {
     otError error = OT_ERROR_NONE;
@@ -88,14 +103,7 @@ otError Child::GetNextIp6Address(Instance &aInstance, Ip6AddressIterator &aItera
     if (aIterator.Get() == 0)
     {
         aIterator.Increment();
-
-        if (!IsAllZero(mMeshLocalIid, sizeof(mMeshLocalIid)))
-        {
-            memcpy(aAddress.mFields.m8, aInstance.GetThreadNetif().GetMle().GetMeshLocalPrefix(),
-                   Ip6::Address::kMeshLocalPrefixSize);
-            aAddress.SetIid(mMeshLocalIid);
-            ExitNow();
-        }
+        VerifyOrExit(GetMeshLocalIp6Address(aInstance, aAddress) == OT_ERROR_NOT_FOUND);
     }
 
     index = aIterator.Get() - 1;

src/core/thread/topology.hpp

@@ -420,6 +420,18 @@ public:
      */
     void ClearIp6Addresses(void);
 
+    /**
+     * This method gets the mesh-local IPv6 address.
+     *
+     * @param[in]    aInstance           A reference to the OpenThread instance.
+     * @param[out]   aAddress            A reference to an IPv6 address to provide address (if any).
+     *
+     * @retval       OT_ERROR_NONE       Successfully found the mesh-local address and updated @p aAddress.
+     * @retval       OT_ERROR_NOT_FOUND  No mesh-local IPv6 address in the IPv6 address list.
+     *
+     */
+    otError GetMeshLocalIp6Address(Instance &aInstance, Ip6::Address &aAddress) const;
+
     /**
      * This method gets the next IPv6 address in the list.
      *

tests/unit/test_child.cpp

@@ -49,6 +49,8 @@ void VerifyChildIp6Addresses(const Child &aChild, uint8_t aAddressListLength, co
     Child::Ip6AddressIterator iterator;
     Ip6::Address address;
     bool addressObserved[kMaxChildIp6Addresses];
+    bool addressIsMeshLocal[kMaxChildIp6Addresses];
+    bool hasMeshLocal = false;
 
     for (uint8_t index = 0; index < aAddressListLength; index++)
     {
@@ -56,6 +58,14 @@ void VerifyChildIp6Addresses(const Child &aChild, uint8_t aAddressListLength, co
     }
 
     memset(addressObserved, 0, sizeof(addressObserved));
+    memset(addressIsMeshLocal, 0, sizeof(addressObserved));
+
+    for (uint8_t index = 0; index < aAddressListLength; index++)
+    {
+        {
+            addressIsMeshLocal[index] = true;
+        }
+    }
 
     while (aChild.GetNextIp6Address(*sInstance, iterator, address) == OT_ERROR_NONE)
     {
@@ -77,6 +87,20 @@ void VerifyChildIp6Addresses(const Child &aChild, uint8_t aAddressListLength, co
     for (uint8_t index = 0; index < aAddressListLength; index++)
     {
         VerifyOrQuit(addressObserved[index], "Child::GetNextIp6Address() missed an entry from the expected list\n");
+
+        if (sInstance->GetThreadNetif().GetMle().IsMeshLocalAddress(aAddressList[index]))
+        {
+            SuccessOrQuit(aChild.GetMeshLocalIp6Address(*sInstance, address),
+                          "Child::GetMeshLocalIp6Address() failed\n");
+            VerifyOrQuit(address == aAddressList[index], "GetMeshLocalIp6Address() did not return expected address\n");
+            hasMeshLocal = true;
+        }
+    }
+
+    if (!hasMeshLocal)
+    {
+        VerifyOrQuit(aChild.GetMeshLocalIp6Address(*sInstance, address) == OT_ERROR_NOT_FOUND,
+                     "Child::GetMeshLocalIp6Address() returned an address not in the exptect list\n");
     }
 }