[routing-manager] fix small OMR prefix election issue (#7405)

This commit fixes two small issues regarding OMR prefix election:
- This commit handles the case where `mLocalOmrPrefix` is already in
  Leader's Network Data, but not locally added. It is possible when
  the BR restores the Network Data from Leader. In such case the BR
  should re-add `mLocalOmrPrefix` to local Network Data so that it
  keeps advertising OMR Prefix. Otherwise, the OMR Prefix will be
  removed from Leader's Network Data because of Network Data
  inconsistentency, and then all BRs start to re-elect OMR Prefix. So,
  the change will make the OMR Prefix more stable in some corner
  cases.
- The change here checks if `mLocalOmrPrefix` was added to the Local
  Network Data, instead of checking if `mLocalOmrPrefix` is added to
  the Leader's Network Data. Here the BR has already decided that it
  should not be advertising `mLocalOmrPrefix`. Whether or not
  `mLocalOmrPrefix` is in Leader's Network Data is relevant. The BR
  only needs to remove `mLocalOmrPrefix` from Local Network Data and
  eventually Local Network Data will synchronize with Leader's Network
  Data. The original code also has a potential issue that the BR may
  send `NET_DATA.ntf` to Leader indefinitely when these conditions are
  met:
  - `mLocalOmrPrefix` was added to Local Network Data
  - `mLocalOmrPrefix` is not added to Leader' Network Data because
    there is no more space in Leader's Network Data

src/core/border_router/routing_manager.cpp

@@ -462,18 +462,29 @@ void RoutingManager::EvaluateOmrPrefix(OmrPrefixArray &aNewOmrPrefixes)
         // The `aNewOmrPrefixes` remains empty if we fail to publish
         // the local OMR prefix.
     }
-    else if (publishedLocalOmrPrefix != nullptr && smallestOmrPrefix != publishedLocalOmrPrefix)
+    else
     {
-        LogInfo("EvaluateOmrPrefix: There is already a smaller OMR prefix %s in the Thread network",
-                smallestOmrPrefix->ToString().AsCString());
+        OT_ASSERT(smallestOmrPrefix != nullptr);
 
-        UnpublishLocalOmrPrefix();
+        if (*smallestOmrPrefix == mLocalOmrPrefix)
+        {
+            IgnoreError(PublishLocalOmrPrefix());
+        }
+        else if (IsOmrPrefixAddedToLocalNetworkData())
+        {
+            LogInfo("EvaluateOmrPrefix: There is already a smaller OMR prefix %s in the Thread network",
+                    smallestOmrPrefix->ToString().AsCString());
 
-        // Remove the local OMR prefix from the list by overwriting it
-        // with the last element and then popping it from the list.
+            UnpublishLocalOmrPrefix();
 
-        *publishedLocalOmrPrefix = *aNewOmrPrefixes.Back();
-        aNewOmrPrefixes.PopBack();
+            // Remove the local OMR prefix from the list by overwriting it
+            // with the last element and then popping it from the list.
+            if (publishedLocalOmrPrefix != nullptr)
+            {
+                *publishedLocalOmrPrefix = *aNewOmrPrefixes.Back();
+                aNewOmrPrefixes.PopBack();
+            }
+        }
     }
 }
 
@@ -484,6 +495,8 @@ Error RoutingManager::PublishLocalOmrPrefix(void)
 
     OT_ASSERT(mIsRunning);
 
+    VerifyOrExit(!IsOmrPrefixAddedToLocalNetworkData());
+
     omrPrefixConfig.Clear();
     omrPrefixConfig.mPrefix       = mLocalOmrPrefix;
     omrPrefixConfig.mStable       = true;
@@ -505,6 +518,7 @@ Error RoutingManager::PublishLocalOmrPrefix(void)
         LogInfo("Publishing local OMR prefix %s in Thread network", mLocalOmrPrefix.ToString().AsCString());
     }
 
+exit:
     return error;
 }
 
@@ -514,6 +528,8 @@ void RoutingManager::UnpublishLocalOmrPrefix(void)
 
     VerifyOrExit(mIsRunning);
 
+    VerifyOrExit(IsOmrPrefixAddedToLocalNetworkData());
+
     SuccessOrExit(error = Get<NetworkData::Local>().RemoveOnMeshPrefix(mLocalOmrPrefix));
 
     Get<NetworkData::Notifier>().HandleServerDataUpdated();
@@ -527,6 +543,11 @@ exit:
     }
 }
 
+bool RoutingManager::IsOmrPrefixAddedToLocalNetworkData(void) const
+{
+    return Get<NetworkData::Local>().ContainsOnMeshPrefix(mLocalOmrPrefix);
+}
+
 Error RoutingManager::AddExternalRoute(const Ip6::Prefix &aPrefix, RoutePreference aRoutePreference, bool aNat64)
 {
     Error                            error;

src/core/border_router/routing_manager.hpp

@@ -305,6 +305,7 @@ private:
     void  EvaluateOmrPrefix(OmrPrefixArray &aNewOmrPrefixes);
     Error PublishLocalOmrPrefix(void);
     void  UnpublishLocalOmrPrefix(void);
+    bool  IsOmrPrefixAddedToLocalNetworkData(void) const;
     Error AddExternalRoute(const Ip6::Prefix &aPrefix, RoutePreference aRoutePreference, bool aNat64 = false);
     void  RemoveExternalRoute(const Ip6::Prefix &aPrefix);
     void  StartRouterSolicitationDelay(void);

src/core/thread/network_data_local.cpp

@@ -69,6 +69,20 @@ Error Local::RemoveOnMeshPrefix(const Ip6::Prefix &aPrefix)
     return RemovePrefix(aPrefix, NetworkDataTlv::kTypeBorderRouter);
 }
 
+bool Local::ContainsOnMeshPrefix(const Ip6::Prefix &aPrefix) const
+{
+    const PrefixTlv *tlv;
+    bool             contains = false;
+
+    VerifyOrExit((tlv = FindPrefix(aPrefix)) != nullptr);
+    VerifyOrExit(tlv->FindSubTlv(NetworkDataTlv::kTypeBorderRouter) != nullptr);
+
+    contains = true;
+
+exit:
+    return contains;
+}
+
 Error Local::AddHasRoutePrefix(const ExternalRouteConfig &aConfig)
 {
     Error error = kErrorInvalidArgs;

src/core/thread/network_data_local.hpp

@@ -97,6 +97,17 @@ public:
      */
     Error RemoveOnMeshPrefix(const Ip6::Prefix &aPrefix);
 
+    /**
+     * This method indicates whether or not the Thread Network Data contains a given on mesh prefix.
+     *
+     * @param[in]  aPrefix   The on mesh prefix to check.
+     *
+     * @retval TRUE  if Network Data contains mesh prefix @p aPrefix.
+     * @retval FALSE if Network Data does not contain mesh prefix @p aPrefix.
+     *
+     */
+    bool ContainsOnMeshPrefix(const Ip6::Prefix &aPrefix) const;
+
     /**
      * This method adds a Has Route entry to the Thread Network data.
      *