[address-resolver] update EID-to-RLOC cache when attaching MTD child (#4282)

This commits removes deprecated EID-to-RLOC cache if there is entry for
the unicast address registered by MTD child in case this child switches
its parent.

src/core/thread/address_resolver.cpp

@@ -115,6 +115,20 @@ void AddressResolver::Remove(uint16_t aRloc16)
     }
 }
 
+void AddressResolver::Remove(const Ip6::Address &aEid)
+{
+    for (int i = 0; i < kCacheEntries; i++)
+    {
+        if (mCache[i].mState == Cache::kStateInvalid || mCache[i].mTarget != aEid)
+        {
+            continue;
+        }
+
+        InvalidateCacheEntry(mCache[i], kReasonRemovingEid);
+        break;
+    }
+}
+
 AddressResolver::Cache *AddressResolver::NewCacheEntry(void)
 {
     Cache *rval = NULL;
@@ -174,6 +188,10 @@ const char *AddressResolver::ConvertInvalidationReasonToString(InvalidationReaso
     case kReasonEvictingForNewEntry:
         str = "evicting for new entry";
         break;
+
+    case kReasonRemovingEid:
+        str = "removing eid";
+        break;
     }
 
     return str;

src/core/thread/address_resolver.hpp

@@ -102,6 +102,14 @@ public:
      */
     void Remove(uint8_t aRouterId);
 
+    /**
+     * This method removes the cache entry for the EID.
+     *
+     * @param[in]  aEid               A reference to the EID.
+     *
+     */
+    void Remove(const Ip6::Address &aEid);
+
     /**
      * This method updates an existing cache entry for the EID.
      *
@@ -197,6 +205,7 @@ private:
         kReasonRemovingRloc16,
         kReasonReceivedIcmpDstUnreachNoRoute,
         kReasonEvictingForNewEntry,
+        kReasonRemovingEid,
     };
 
     static const char *ConvertInvalidationReasonToString(InvalidationReason aReason);

src/core/thread/mle_router.cpp

@@ -2036,6 +2036,9 @@ otError MleRouter::UpdateChildAddresses(const Message &aMessage, uint16_t aOffse
 
             IgnoreReturnValue(iter.GetChild()->RemoveIp6Address(GetInstance(), address));
         }
+
+        // Clear EID-to-RLOC cache for the unicast address registered by the child.
+        Get<AddressResolver>().Remove(address);
     }
 
     if (registeredCount == 0)