[operational-dataset] only generate active dataset once (#4304)

Add check to see if Active Timestamp already exists in the Active
Operational Dataset. If Active Timestamp exists, do not generate
Active Operational Dataset again.

src/core/meshcop/dataset_local.hpp

@@ -80,6 +80,15 @@ public:
      */
     bool IsSaved(void) const { return mSaved; }
 
+    /**
+     * This method indicates whether an Active (Pending) Timestamp is present in the Active (Pending) Dataset.
+     *
+     * @retval TRUE  if an Active/Pending Timestamp is present.
+     * @retval FALSE if an Active/Pending Timestamp is not present.
+     *
+     */
+    bool IsTimestampPresent(void) const { return mTimestampPresent; }
+
     /**
      * This method restores and retrieves the dataset from non-volatile memory.
      *

src/core/meshcop/dataset_manager.hpp

@@ -391,7 +391,8 @@ public:
     /**
      * This method generate a default Active Operational Dataset.
      *
-     * @retval OT_ERROR_NONE  Successfully generated an Active Operational Dataset.
+     * @retval OT_ERROR_NONE           Successfully generated an Active Operational Dataset.
+     * @retval OT_ERROR_ALREADY        A valid Active Operational Dataset already exists.
      * @retval OT_ERROR_INVALID_STATE  Device is not currently attached to a network.
      *
      */

src/core/meshcop/dataset_manager_ftd.cpp

@@ -370,6 +370,7 @@ otError ActiveDataset::GenerateLocal(void)
     Dataset dataset(mLocal.GetType());
 
     VerifyOrExit(Get<Mle::MleRouter>().IsAttached(), error = OT_ERROR_INVALID_STATE);
+    VerifyOrExit(!mLocal.IsTimestampPresent(), error = OT_ERROR_ALREADY);
 
     mLocal.Read(dataset);