[dataset] simplify AppendMleDatasetTlv() (#10043)

- Inlines `AppendMleDatasetTlv()` in `DatasetManager` (removing from
  `Dataset` class)
- Updates the locally read 'dataset' directly by removing the
  timestamp before appending it to the message as an MLE TLV value.
- Leverages `Read()` which already updates the `DelayTimerTlv` with the
  remaining delay, eliminating the need to re-calculate and update the
  delay.
- Uses `Tlv::AppendTlv()` to append the MLE TLV with `dataset` bytes
  as value.

src/core/common/tlvs.hpp

@@ -525,6 +525,22 @@ public:
         return FindStringTlv(aMessage, StringTlvType::kType, StringTlvType::kMaxStringLength, aValue);
     }
 
+    /**
+     * Appends a TLV with a given type and value to a message.
+     *
+     * On success this method grows the message by the size of the TLV.
+     *
+     * @param[in]  aMessage      The message to append to.
+     * @param[in]  aType         The TLV type to append.
+     * @param[in]  aValue        A buffer containing the TLV value.
+     * @param[in]  aLength       The value length (in bytes).
+     *
+     * @retval kErrorNone     Successfully appended the TLV to the message.
+     * @retval kErrorNoBufs   Insufficient available buffers to grow the message.
+     *
+     */
+    static Error AppendTlv(Message &aMessage, uint8_t aType, const void *aValue, uint8_t aLength);
+
     /**
      * Appends a TLV with a given type and value to a message.
      *
@@ -687,7 +703,6 @@ private:
     };
 
     static Error FindTlv(const Message &aMessage, uint8_t aType, void *aValue, uint16_t aLength);
-    static Error AppendTlv(Message &aMessage, uint8_t aType, const void *aValue, uint8_t aLength);
     static Error ReadStringTlv(const Message &aMessage, uint16_t aOffset, uint8_t aMaxStringLength, char *aValue);
     static Error FindStringTlv(const Message &aMessage, uint8_t aType, uint8_t aMaxStringLength, char *aValue);
     static Error AppendStringTlv(Message &aMessage, uint8_t aType, uint8_t aMaxStringLength, const char *aValue);

src/core/meshcop/dataset.cpp

@@ -450,43 +450,6 @@ exit:
 
 void Dataset::RemoveTlv(Tlv::Type aType) { RemoveTlv(FindTlv(aType)); }
 
-Error Dataset::AppendMleDatasetTlv(Type aType, Message &aMessage) const
-{
-    Error          error = kErrorNone;
-    Mle::Tlv       tlv;
-    Mle::Tlv::Type type;
-
-    VerifyOrExit(mLength > 0);
-
-    type = (aType == kActive ? Mle::Tlv::kActiveDataset : Mle::Tlv::kPendingDataset);
-
-    tlv.SetType(type);
-    tlv.SetLength(static_cast<uint8_t>(mLength) - sizeof(Tlv) - sizeof(Timestamp));
-    SuccessOrExit(error = aMessage.Append(tlv));
-
-    for (const Tlv *cur = GetTlvsStart(); cur < GetTlvsEnd(); cur = cur->GetNext())
-    {
-        if (((aType == kActive) && (cur->GetType() == Tlv::kActiveTimestamp)) ||
-            ((aType == kPending) && (cur->GetType() == Tlv::kPendingTimestamp)))
-        {
-            ; // skip Active or Pending Timestamp TLV
-        }
-        else if (cur->GetType() == Tlv::kDelayTimer)
-        {
-            uint32_t remainingDelay = DelayTimerTlv::CalculateRemainingDelay(*cur, mUpdateTime);
-
-            SuccessOrExit(error = Tlv::Append<DelayTimerTlv>(aMessage, remainingDelay));
-        }
-        else
-        {
-            SuccessOrExit(error = cur->AppendTo(aMessage));
-        }
-    }
-
-exit:
-    return error;
-}
-
 void Dataset::RemoveTlv(Tlv *aTlv)
 {
     if (aTlv != nullptr)

src/core/meshcop/dataset.hpp

@@ -494,18 +494,6 @@ public:
      */
     void SetFrom(const Tlvs &aTlvs);
 
-    /**
-     * Appends the MLE Dataset TLV but excluding MeshCoP Sub Timestamp TLV.
-     *
-     * @param[in] aType          The type of the dataset, active or pending.
-     * @param[in] aMessage       A message to append to.
-     *
-     * @retval kErrorNone    Successfully append MLE Dataset TLV without MeshCoP Sub Timestamp TLV.
-     * @retval kErrorNoBufs  Insufficient available buffers to append the message with MLE Dataset TLV.
-     *
-     */
-    Error AppendMleDatasetTlv(Type aType, Message &aMessage) const;
-
     /**
      * Applies the Active or Pending Dataset to the Thread interface.
      *

src/core/meshcop/dataset_manager_ftd.cpp

@@ -64,11 +64,18 @@ RegisterLogModule("DatasetManager");
 
 Error DatasetManager::AppendMleDatasetTlv(Message &aMessage) const
 {
-    Dataset dataset;
+    Mle::Tlv::Type mleTlvType = IsActiveDataset() ? Mle::Tlv::kActiveDataset : Mle::Tlv::kPendingDataset;
+    Dataset        dataset;
 
     IgnoreError(Read(dataset));
 
-    return dataset.AppendMleDatasetTlv(GetType(), aMessage);
+    // Remove the Active or Pending Timestamp TLV from Dataset before
+    // appending to the message. The timestamp is appended as its own
+    // MLE TLV to the message.
+
+    dataset.RemoveTlv(IsActiveDataset() ? Tlv::kActiveTimestamp : Tlv::kPendingTimestamp);
+
+    return Tlv::AppendTlv(aMessage, mleTlvType, dataset.GetBytes(), static_cast<uint8_t>(dataset.GetSize()));
 }
 
 Error DatasetManager::HandleSet(Coap::Message &aMessage, const Ip6::MessageInfo &aMessageInfo)