[netdiag] define AnswerTlvValue to allow reuse (#12792)

This commit defines `AnswerTlvValue` to represent the value of an
Answer TLV, allowing it to be reused across different modules,
specifically `NetworkDiagnostic` and `HistoryTracker`.

The `AnswerTlv` implementation is also updated to use the
template-based `SimpleTlvInfo` pattern. This enables the use of
generic `Tlv::Append<AnswerTlv>()` and `Tlv::Find<AnswerTlv>()`
methods, which improves type safety and reduces manual TLV handling.

src/core/thread/network_diagnostic.cpp

@@ -511,7 +511,7 @@ void Server::SendAnswer(const Ip6::Address &aDestination, const Message &aReques
 {
     Error          error  = kErrorNone;
     Coap::Message *answer = nullptr;
-    AnswerTlv      answerTlv;
+    AnswerTlvValue answerTlvValue;
     uint16_t       queryId;
 
     answer = Get<Tmf::Agent>().AllocateAndInitConfirmablePostMessage(kUriDiagnosticGetAnswer);
@@ -526,8 +526,8 @@ void Server::SendAnswer(const Ip6::Address &aDestination, const Message &aReques
 
     SuccessOrExit(error = AppendRequestedTlvs(aRequest, *answer));
 
-    answerTlv.Init(0, AnswerTlv::kIsLast);
-    SuccessOrExit(answer->Append(answerTlv));
+    answerTlvValue.Init(0, AnswerTlvValue::kIsLast);
+    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*answer, answerTlvValue));
 
     error = Get<Tmf::Agent>().SendMessageAllowMulticastLoop(*answer, aDestination);
 
@@ -572,13 +572,13 @@ bool Server::IsLastAnswer(const Coap::Message &aAnswer) const
     // Indicates whether `aAnswer` is the last one associated with
     // the same query.
 
-    bool      isLast = true;
-    AnswerTlv answerTlv;
+    bool           isLast = true;
+    AnswerTlvValue answerTlvValue;
 
     // If there is no Answer TLV, we assume it is the last answer.
 
-    SuccessOrExit(Tlv::FindTlv(aAnswer, answerTlv));
-    isLast = answerTlv.IsLast();
+    SuccessOrExit(Tlv::Find<AnswerTlv>(aAnswer, answerTlvValue));
+    isLast = answerTlvValue.IsLast();
 
 exit:
     return isLast;
@@ -608,7 +608,7 @@ void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Messa
     AnswerInfo          info;
     uint8_t             tlvType;
     TlvTypeListIterator iterator;
-    AnswerTlv           answerTlv;
+    AnswerTlvValue      answerTlvValue;
 
     if (Tlv::Find<QueryIdTlv>(aRequest, info.mQueryId) == kErrorNone)
     {
@@ -642,8 +642,8 @@ void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Messa
         SuccessOrExit(error = CheckAnswerLength(answer, info));
     }
 
-    answerTlv.Init(info.mAnswerIndex, AnswerTlv::kIsLast);
-    SuccessOrExit(error = answer->Append(answerTlv));
+    answerTlvValue.Init(info.mAnswerIndex, AnswerTlvValue::kIsLast);
+    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*answer, answerTlvValue));
 
     SendNextAnswer(*info.mFirstAnswer, aDestination);
 
@@ -662,13 +662,13 @@ Error Server::CheckAnswerLength(Coap::Message *&aAnswer, AnswerInfo &aInfo)
     // message. In this case, it will also allocate a new answer
     // message.
 
-    Error     error = kErrorNone;
-    AnswerTlv answerTlv;
+    Error          error = kErrorNone;
+    AnswerTlvValue answerTlvValue;
 
     VerifyOrExit(aAnswer->GetLength() >= kAnswerMessageLengthThreshold);
 
-    answerTlv.Init(aInfo.mAnswerIndex++, AnswerTlv::kMoreToFollow);
-    SuccessOrExit(error = aAnswer->Append(answerTlv));
+    answerTlvValue.Init(aInfo.mAnswerIndex++, AnswerTlvValue::kMoreToFollow);
+    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*aAnswer, answerTlvValue));
 
     error = AllocateAnswer(aAnswer, aInfo);
 

src/core/thread/network_diagnostic_tlvs.cpp

@@ -197,13 +197,10 @@ void RouterNeighborTlv::InitFrom(const Router &aRouter)
 #endif // OPENTHREAD_FTD
 
 //---------------------------------------------------------------------------------------------------------------------
-// AnswerTlv
+// AnswerTlvValue
 
-void AnswerTlv::Init(uint16_t aIndex, IsLastFlag aIsLastFlag)
+void AnswerTlvValue::Init(uint16_t aIndex, IsLastFlag aIsLastFlag)
 {
-    SetType(kAnswer);
-    SetLength(sizeof(*this) - sizeof(Tlv));
-
     SetFlagsIndex((aIndex & kIndexMask) | (aIsLastFlag == kIsLast ? kIsLastFlag : 0));
 }
 

src/core/thread/network_diagnostic_tlvs.hpp

@@ -802,10 +802,10 @@ private:
 } OT_TOOL_PACKED_END;
 
 /**
- * Implements Answer TLV generation and parsing.
+ * Represents an Answer TLV value.
  */
 OT_TOOL_PACKED_BEGIN
-class AnswerTlv : public Tlv, public TlvInfo<Tlv::kAnswer>
+class AnswerTlvValue
 {
 public:
     enum IsLastFlag : uint8_t
@@ -815,7 +815,7 @@ public:
     };
 
     /**
-     * Initializes the TLV.
+     * Initializes the TLV value.
      *
      * @param[in] aIndex       The index value.
      * @param[in] aIsLastFlag  Indicates the `IsLastFlag` value.
@@ -847,6 +847,11 @@ private:
     uint16_t mFlagsIndex;
 } OT_TOOL_PACKED_END;
 
+/**
+ * Defines Answer TLV constants and types.
+ */
+typedef SimpleTlvInfo<Tlv::kAnswer, AnswerTlvValue> AnswerTlv;
+
 /**
  * Represents the MLE Counters.
  */

src/core/utils/history_tracker_client.cpp

@@ -133,9 +133,9 @@ exit:
 
 Error Client::ProcessAnswer(const Coap::Msg &aMsg)
 {
-    Error     error = kErrorFailed;
-    AnswerTlv answerTlv;
-    uint16_t  queryId;
+    Error          error = kErrorFailed;
+    AnswerTlvValue answerTlvValue;
+    uint16_t       queryId;
 
     VerifyOrExit(mActive);
     VerifyOrExit(Get<Mle::Mle>().IsRoutingLocator(aMsg.mMessageInfo.GetPeerAddr()));
@@ -144,9 +144,9 @@ Error Client::ProcessAnswer(const Coap::Msg &aMsg)
     SuccessOrExit(Tlv::Find<QueryIdTlv>(aMsg.mMessage, queryId));
     VerifyOrExit(queryId == mQueryId);
 
-    SuccessOrExit(Tlv::FindTlv(aMsg.mMessage, answerTlv));
+    SuccessOrExit(Tlv::Find<AnswerTlv>(aMsg.mMessage, answerTlvValue));
 
-    if (answerTlv.GetIndex() != mAnswerIndex)
+    if (answerTlvValue.GetIndex() != mAnswerIndex)
     {
         Finalize(kErrorResponseTimeout);
         ExitNow();

src/core/utils/history_tracker_server.cpp

@@ -93,13 +93,13 @@ bool Server::IsLastAnswer(const Coap::Message &aAnswer) const
     // Indicates whether `aAnswer` is the last one associated with
     // the same query.
 
-    bool      isLast = true;
-    AnswerTlv answerTlv;
+    bool           isLast = true;
+    AnswerTlvValue answerTlvValue;
 
     // If there is no Answer TLV, we assume it is the last answer.
 
-    SuccessOrExit(Tlv::FindTlv(aAnswer, answerTlv));
-    isLast = answerTlv.IsLast();
+    SuccessOrExit(Tlv::Find<AnswerTlv>(aAnswer, answerTlvValue));
+    isLast = answerTlvValue.IsLast();
 
 exit:
     return isLast;
@@ -130,7 +130,7 @@ void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Messa
     OffsetRange    offsetRange;
     Tlv::Info      tlvInfo;
     RequestTlv     requestTlv;
-    AnswerTlv      answerTlv;
+    AnswerTlvValue answerTlvValue;
 
     if (Tlv::Find<QueryIdTlv>(aRequest, info.mQueryId) == kErrorNone)
     {
@@ -171,8 +171,8 @@ void Server::PrepareAndSendAnswers(const Ip6::Address &aDestination, const Messa
         }
     }
 
-    answerTlv.Init(info.mAnswerIndex, /* aIsLast */ true);
-    SuccessOrExit(error = answer->Append(answerTlv));
+    answerTlvValue.Init(info.mAnswerIndex, AnswerTlvValue::kIsLast);
+    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*answer, answerTlvValue));
 
     SendNextAnswer(*info.mFirstAnswer, aDestination);
 
@@ -190,13 +190,13 @@ Error Server::CheckAnswerLength(Coap::Message *&aAnswer, AnswerInfo &aInfo)
     // appending an Answer TLV with the current index to the message.
     // In this case, it will also allocate a new answer message.
 
-    Error     error = kErrorNone;
-    AnswerTlv answerTlv;
+    Error          error = kErrorNone;
+    AnswerTlvValue answerTlvValue;
 
     VerifyOrExit(aAnswer->GetLength() >= kAnswerMessageLengthThreshold);
 
-    answerTlv.Init(aInfo.mAnswerIndex++, /* aIsLast */ false);
-    SuccessOrExit(error = aAnswer->Append(answerTlv));
+    answerTlvValue.Init(aInfo.mAnswerIndex++, AnswerTlvValue::kMoreToFollow);
+    SuccessOrExit(error = Tlv::Append<AnswerTlv>(*aAnswer, answerTlvValue));
 
     error = AllocateAnswer(aAnswer, aInfo);
 

src/core/utils/history_tracker_tlvs.cpp

@@ -38,14 +38,6 @@
 namespace ot {
 namespace HistoryTracker {
 
-void AnswerTlv::Init(uint16_t aIndex, bool aIsLast)
-{
-    SetType(kAnswer);
-    SetLength(sizeof(*this) - sizeof(Tlv));
-
-    SetFlagsIndex((aIndex & kIndexMask) | (aIsLast ? kIsLastFlag : 0));
-}
-
 void RequestTlv::Init(uint8_t aTlvType, uint16_t aNumEntries, uint32_t aMaxEntryAge)
 {
     SetType(kRequest);

src/core/utils/history_tracker_tlvs.hpp

@@ -42,6 +42,7 @@
 
 #include "common/encoding.hpp"
 #include "common/tlvs.hpp"
+#include "thread/network_diagnostic_tlvs.hpp"
 #include "utils/history_tracker.hpp"
 
 namespace ot {
@@ -72,44 +73,14 @@ public:
 typedef UintTlvInfo<Tlv::kQueryId, uint16_t> QueryIdTlv;
 
 /**
- * Implements Answer TLV generation and parsing.
+ * Represents an Answer TLV value.
  */
-OT_TOOL_PACKED_BEGIN
-class AnswerTlv : public Tlv, public TlvInfo<Tlv::kAnswer>
-{
-public:
-    /**
-     * Initializes the TLV.
-     *
-     * @param[in] aIndex   The index value.
-     * @param[in] aIsLast  The "IsLast" flag value.
-     */
-    void Init(uint16_t aIndex, bool aIsLast);
+typedef NetworkDiagnostic::AnswerTlvValue AnswerTlvValue;
 
-    /**
-     * Indicates whether or not the "IsLast" flag is set
-     *
-     * @retval TRUE   "IsLast" flag is set (this is the last answer for this query).
-     * @retval FALSE  "IsLast" flag is not set (more answer messages are expected for this query).
-     */
-    bool IsLast(void) const { return GetFlagsIndex() & kIsLastFlag; }
-
-    /**
-     * Gets the index.
-     *
-     * @returns The index.
-     */
-    uint16_t GetIndex(void) const { return GetFlagsIndex() & kIndexMask; }
-
-private:
-    static constexpr uint16_t kIsLastFlag = 1 << 15;
-    static constexpr uint16_t kIndexMask  = 0x7fff;
-
-    uint16_t GetFlagsIndex(void) const { return BigEndian::HostSwap16(mFlagsIndex); }
-    void     SetFlagsIndex(uint16_t aFlagsIndex) { mFlagsIndex = BigEndian::HostSwap16(aFlagsIndex); }
-
-    uint16_t mFlagsIndex;
-} OT_TOOL_PACKED_END;
+/**
+ * Defines Answer TLV constants and types.
+ */
+typedef SimpleTlvInfo<Tlv::kAnswer, AnswerTlvValue> AnswerTlv;
 
 /**
  * Implements Request TLV generation and parsing.

src/core/utils/mesh_diag.cpp

@@ -258,9 +258,9 @@ Error MeshDiag::ProcessMessage(Coap::Message &aMessage, const Ip6::MessageInfo &
     // check whether it is from the intended sender and matches
     // the expected query ID and answer index.
 
-    Error     error = kErrorFailed;
-    AnswerTlv answerTlv;
-    uint16_t  queryId;
+    Error          error = kErrorFailed;
+    AnswerTlvValue answerTlvValue;
+    uint16_t       queryId;
 
     VerifyOrExit(Get<Mle::Mle>().IsRoutingLocator(aMessageInfo.GetPeerAddr()));
     VerifyOrExit(aMessageInfo.GetPeerAddr().GetIid().GetLocator() == aSenderRloc16);
@@ -268,9 +268,9 @@ Error MeshDiag::ProcessMessage(Coap::Message &aMessage, const Ip6::MessageInfo &
     SuccessOrExit(Tlv::Find<QueryIdTlv>(aMessage, queryId));
     VerifyOrExit(queryId == mExpectedQueryId);
 
-    SuccessOrExit(Tlv::FindTlv(aMessage, answerTlv));
+    SuccessOrExit(Tlv::Find<AnswerTlv>(aMessage, answerTlvValue));
 
-    if (answerTlv.GetIndex() != mExpectedAnswerIndex)
+    if (answerTlvValue.GetIndex() != mExpectedAnswerIndex)
     {
         Finalize(kErrorResponseTimeout);
         ExitNow();