[dataset-manager] simplify Restore() (#10290)

This commit simplifies the `Restore()` methods:

- A private overload `Restore(const Dataset&)` is added, taking the
  read `Dataset` as input to avoid duplicate reads.
- The public `Restore(void)` is updated to use the new private
  `Restore(const Dataset&)`.
- The `HandleDetach()` method is removed, and its use in the `Mle`
  class is replaced with `Restore()`.

src/core/meshcop/dataset_manager.cpp

@@ -80,16 +80,28 @@ Error DatasetManager::Restore(void)
     Error   error;
     Dataset dataset;
 
+    // If `Read()` fails, `dataset` will remain empty. We still call
+    // `Restore(dataset)` to stop timer and clear the timestamp
+    // flags.
+
+    error = Read(dataset);
+    Restore(dataset);
+
+    return error;
+}
+
+void DatasetManager::Restore(const Dataset &aDataset)
+{
     mTimer.Stop();
 
     mNetworkTimestampValid = false;
     mLocalTimestampValid   = false;
 
-    SuccessOrExit(error = Read(dataset));
+    VerifyOrExit(aDataset.GetLength() != 0);
 
     mLocalSaved = true;
 
-    if (dataset.ReadTimestamp(mType, mLocalTimestamp) == kErrorNone)
+    if (aDataset.ReadTimestamp(mType, mLocalTimestamp) == kErrorNone)
     {
         mLocalTimestampValid   = true;
         mNetworkTimestampValid = true;
@@ -98,13 +110,13 @@ Error DatasetManager::Restore(void)
 
     if (IsActiveDataset())
     {
-        IgnoreError(ApplyConfiguration(dataset));
+        IgnoreError(ApplyConfiguration(aDataset));
     }
 
     SignalDatasetChange();
 
 exit:
-    return error;
+    return;
 }
 
 Error DatasetManager::Read(Dataset &aDataset) const
@@ -266,8 +278,6 @@ void DatasetManager::Clear(void)
     SignalDatasetChange();
 }
 
-void DatasetManager::HandleDetach(void) { IgnoreError(Restore()); }
-
 Error DatasetManager::Save(const Timestamp &aTimestamp, const Message &aMessage, uint16_t aOffset, uint16_t aLength)
 {
     Error   error = kErrorNone;
@@ -366,7 +376,7 @@ void DatasetManager::SaveLocal(const Dataset &aDataset)
     switch (Get<Mle::MleRouter>().GetRole())
     {
     case Mle::kRoleDisabled:
-        IgnoreError(Restore());
+        Restore(aDataset);
         break;
 
     case Mle::kRoleChild:
@@ -378,7 +388,7 @@ void DatasetManager::SaveLocal(const Dataset &aDataset)
         break;
 
     case Mle::kRoleLeader:
-        IgnoreError(Restore());
+        Restore(aDataset);
         Get<NetworkData::Leader>().IncrementVersionAndStableVersion();
         break;
 #endif

src/core/meshcop/dataset_manager.hpp

@@ -199,14 +199,6 @@ public:
      */
     Error ApplyConfiguration(void) const;
 
-    /**
-     * Updates the Operational Dataset when detaching from the network.
-     *
-     * On detach, the Operational Dataset is restored from non-volatile memory.
-     *
-     */
-    void HandleDetach(void);
-
     /**
      * Sends a MGMT_SET request to the Leader.
      *
@@ -305,6 +297,7 @@ private:
 
     bool  IsActiveDataset(void) const { return (mType == Dataset::kActive); }
     bool  IsPendingDataset(void) const { return (mType == Dataset::kPending); }
+    void  Restore(const Dataset &aDataset);
     Error ApplyConfiguration(const Dataset &aDataset) const;
     void  HandleGet(const Coap::Message &aMessage, const Ip6::MessageInfo &aMessageInfo) const;
     void  HandleTimer(void);

src/core/meshcop/dataset_manager_ftd.cpp

@@ -383,7 +383,7 @@ Error ActiveDatasetManager::GenerateLocal(void)
     }
 
     LocalSave(dataset);
-    IgnoreError(Restore());
+    Restore(dataset);
 
     LogInfo("Generated local dataset");
 

src/core/thread/mle.cpp

@@ -236,8 +236,8 @@ void Mle::Stop(StopMode aMode)
 {
     if (aMode == kUpdateNetworkDatasets)
     {
-        Get<MeshCoP::ActiveDatasetManager>().HandleDetach();
-        Get<MeshCoP::PendingDatasetManager>().HandleDetach();
+        IgnoreError(Get<MeshCoP::ActiveDatasetManager>().Restore());
+        IgnoreError(Get<MeshCoP::PendingDatasetManager>().Restore());
     }
 
     VerifyOrExit(!IsDisabled());
@@ -530,7 +530,7 @@ Error Mle::BecomeDetached(void)
     // Not in reattach stage after reset
     if (mReattachState == kReattachStop)
     {
-        Get<MeshCoP::PendingDatasetManager>().HandleDetach();
+        IgnoreError(Get<MeshCoP::PendingDatasetManager>().Restore());
     }
 
 #if OPENTHREAD_CONFIG_PARENT_SEARCH_ENABLE