[dataset-local] cache Active/Pending Timestamp value (#2599)

This commit caches the Active/Pending Timestamp value in memory so that
subsequent calls to `DatasetLocal::Compare()` does not require reading
from non-volatile settings and allocating another dataset buffer on the
stack to do so.

src/core/meshcop/dataset_local.cpp

@@ -55,14 +55,42 @@ DatasetLocal::DatasetLocal(Instance &aInstance, const Tlv::Type aType)
     : InstanceLocator(aInstance)
     , mUpdateTime(0)
     , mType(aType)
+    , mTimestampPresent(false)
 {
+    mTimestamp.Init();
 }
 
 void DatasetLocal::Clear(void)
 {
+    mTimestamp.Init();
+    mTimestampPresent = false;
     otPlatSettingsDelete(&GetInstance(), GetSettingsKey(), -1);
 }
 
+otError DatasetLocal::Restore(Dataset &aDataset)
+{
+    const Timestamp *timestamp;
+    otError          error;
+
+    error = Get(aDataset);
+    SuccessOrExit(error);
+
+    timestamp = aDataset.GetTimestamp();
+
+    if (timestamp != NULL)
+    {
+        mTimestamp        = *timestamp;
+        mTimestampPresent = true;
+    }
+    else
+    {
+        mTimestampPresent = false;
+    }
+
+exit:
+    return error;
+}
+
 otError DatasetLocal::Get(Dataset &aDataset) const
 {
     DelayTimerTlv *delayTimer;
@@ -131,8 +159,9 @@ exit:
 
 otError DatasetLocal::Set(const Dataset &aDataset)
 {
-    Dataset dataset(aDataset);
-    otError error;
+    Dataset          dataset(aDataset);
+    const Timestamp *timestamp;
+    otError          error;
 
     if (mType == Tlv::kActiveTimestamp)
     {
@@ -153,6 +182,18 @@ otError DatasetLocal::Set(const Dataset &aDataset)
 
     SuccessOrExit(error);
 
+    timestamp = dataset.GetTimestamp();
+
+    if (timestamp != NULL)
+    {
+        mTimestamp        = *timestamp;
+        mTimestampPresent = true;
+    }
+    else
+    {
+        mTimestampPresent = false;
+    }
+
     mUpdateTime = TimerMilli::GetNow();
 
 exit:
@@ -177,32 +218,31 @@ uint16_t DatasetLocal::GetSettingsKey(void) const
 
 int DatasetLocal::Compare(const Timestamp *aCompareTimestamp)
 {
-    const Timestamp *thisTimestamp;
-    Dataset          dataset(mType);
-    int              rval = 1;
+    int rval = 1;
 
-    SuccessOrExit(Get(dataset));
-
-    thisTimestamp = dataset.GetTimestamp();
-
-    if (aCompareTimestamp == NULL && thisTimestamp == NULL)
+    if (aCompareTimestamp == NULL)
     {
-        rval = 0;
-    }
-    else if (aCompareTimestamp == NULL && thisTimestamp != NULL)
-    {
-        rval = -1;
-    }
-    else if (aCompareTimestamp != NULL && thisTimestamp == NULL)
-    {
-        rval = 1;
+        if (!mTimestampPresent)
+        {
+            rval = 0;
+        }
+        else
+        {
+            rval = -1;
+        }
     }
     else
     {
-        rval = thisTimestamp->Compare(*aCompareTimestamp);
+        if (!mTimestampPresent)
+        {
+            rval = 1;
+        }
+        else
+        {
+            rval = mTimestamp.Compare(*aCompareTimestamp);
+        }
     }
 
-exit:
     return rval;
 }
 

src/core/meshcop/dataset_local.hpp

@@ -71,6 +71,19 @@ public:
      */
     void Clear(void);
 
+    /**
+     * This method restores and retrieves the dataset from non-volatile memory.
+     *
+     * This method also sets the memory-cached timestamp for subsequent calls to `Compare()`.
+     *
+     * @param[out]  aDataset  Where to place the dataset.
+     *
+     * @retval OT_ERROR_NONE       Successfully retrieved the dataset.
+     * @retval OT_ERROR_NOT_FOUND  There is no corresponding dataset stored in non-volatile memory.
+     *
+     */
+    otError Restore(Dataset &aDataset);
+
     /**
      * This method retrieves the dataset from non-volatile memory.
      *
@@ -135,8 +148,10 @@ private:
     uint16_t GetSettingsKey(void) const;
     void     SetTimestamp(const Dataset &aDataset);
 
-    uint32_t  mUpdateTime; ///< Local time last updated
-    Tlv::Type mType;       ///< Active or Pending
+    Timestamp mTimestamp;            ///< Active or Pending Timestamp
+    uint32_t  mUpdateTime;           ///< Local time last updated
+    Tlv::Type mType;                 ///< Active or Pending
+    bool      mTimestampPresent : 1; ///< Whether or not timestamp is present
 };
 
 } // namespace MeshCoP

src/core/meshcop/dataset_manager.cpp

@@ -111,7 +111,7 @@ otError DatasetManager::Restore(void)
 
     mTimer.Stop();
 
-    SuccessOrExit(error = mLocal.Get(dataset));
+    SuccessOrExit(error = mLocal.Restore(dataset));
 
     timestamp = dataset.GetTimestamp();