[dns] add MatchesKey() to Dns::TxtEntry (#12579)

This commit adds `MatchesKey()` method to `Dns::TxtEntry` class to
check if the entry's key matches a given key string using a
case-insensitive string comparison.

The new method properly handles the case where the entry's key is
`nullptr` (which can happen when the key is longer than the
recommended max length). It simplifies the TXT entry parsing logic in
`BorderAgent` and `Trel::PeerDiscoverer`.

The unit tests for `Dns::TxtEntry` are also updated to verify the new
method.

src/core/meshcop/border_agent_txt_data.cpp

@@ -303,28 +303,21 @@ Error TxtData::Info::ParseFrom(const uint8_t *aTxtData, uint16_t aTxtDataLength)
 
 void TxtData::Info::ProcessTxtEntry(const Dns::TxtEntry &aEntry)
 {
-    if (aEntry.mKey == nullptr)
-    {
-        // If the TXT data happens to have entries with key longer
-        // than `kMaxIterKeyLength`, `mKey` would be `nullptr` and full
-        // entry would be placed in `mValue`. We skip over such
-        // entries.
-    }
-    else if (StringMatch(aEntry.mKey, Key::kRecordVersion))
+    if (aEntry.MatchesKey(Key::kRecordVersion))
     {
         ReadStringValue(aEntry, mRecordVersion);
         mHasRecordVersion = true;
     }
-    else if (StringMatch(aEntry.mKey, Key::kAgentId))
+    else if (aEntry.MatchesKey(Key::kAgentId))
     {
         mHasAgentId = ReadValue(aEntry, mAgentId);
     }
-    else if (StringMatch(aEntry.mKey, Key::kThreadVersion))
+    else if (aEntry.MatchesKey(Key::kThreadVersion))
     {
         ReadStringValue(aEntry, mThreadVersion);
         mHasThreadVersion = true;
     }
-    else if (StringMatch(aEntry.mKey, Key::kStateBitmap))
+    else if (aEntry.MatchesKey(Key::kStateBitmap))
     {
         uint32_t bitmap;
 
@@ -334,16 +327,16 @@ void TxtData::Info::ProcessTxtEntry(const Dns::TxtEntry &aEntry)
             mHasStateBitmap = true;
         }
     }
-    else if (StringMatch(aEntry.mKey, Key::kNetworkName))
+    else if (aEntry.MatchesKey(Key::kNetworkName))
     {
         ReadStringValue(aEntry, mNetworkName.m8);
         mHasNetworkName = true;
     }
-    else if (StringMatch(aEntry.mKey, Key::kExtendedPanId))
+    else if (aEntry.MatchesKey(Key::kExtendedPanId))
     {
         mHasExtendedPanId = ReadValue(aEntry, mExtendedPanId);
     }
-    else if (StringMatch(aEntry.mKey, Key::kActiveTimestamp))
+    else if (aEntry.MatchesKey(Key::kActiveTimestamp))
     {
         Timestamp timestamp;
 
@@ -353,37 +346,37 @@ void TxtData::Info::ProcessTxtEntry(const Dns::TxtEntry &aEntry)
             mHasActiveTimestamp = true;
         }
     }
-    else if (StringMatch(aEntry.mKey, Key::kPartitionId))
+    else if (aEntry.MatchesKey(Key::kPartitionId))
     {
         mHasPartitionId = ReadBigEndianUintValue(aEntry, mPartitionId);
     }
-    else if (StringMatch(aEntry.mKey, Key::kDomainName))
+    else if (aEntry.MatchesKey(Key::kDomainName))
     {
         ReadStringValue(aEntry, mDomainName.m8);
         mHasDomainName = true;
     }
-    else if (StringMatch(aEntry.mKey, Key::kBbrSeqNum))
+    else if (aEntry.MatchesKey(Key::kBbrSeqNum))
     {
         mHasBbrSeqNum = ReadBigEndianUintValue(aEntry, mBbrSeqNum);
     }
-    else if (StringMatch(aEntry.mKey, Key::kBbrPort))
+    else if (aEntry.MatchesKey(Key::kBbrPort))
     {
         mHasBbrPort = ReadBigEndianUintValue(aEntry, mBbrPort);
     }
-    else if (StringMatch(aEntry.mKey, Key::kOmrPrefix))
+    else if (aEntry.MatchesKey(Key::kOmrPrefix))
     {
         mHasOmrPrefix = ReadOmrPrefix(aEntry, AsCoreType(&mOmrPrefix));
     }
-    else if (StringMatch(aEntry.mKey, Key::kExtAddress))
+    else if (aEntry.MatchesKey(Key::kExtAddress))
     {
         mHasExtAddress = ReadValue(aEntry, mExtAddress);
     }
-    else if (StringMatch(aEntry.mKey, Key::kVendorName))
+    else if (aEntry.MatchesKey(Key::kVendorName))
     {
         ReadStringValue(aEntry, mVendorName);
         mHasVendorName = true;
     }
-    else if (StringMatch(aEntry.mKey, Key::kModelName))
+    else if (aEntry.MatchesKey(Key::kModelName))
     {
         ReadStringValue(aEntry, mModelName);
         mHasModelName = true;

src/core/net/dns_types.cpp

@@ -1380,6 +1380,11 @@ exit:
     return error;
 }
 
+bool TxtEntry::MatchesKey(const char *aKey) const
+{
+    return (mKey != nullptr) && StringMatch(mKey, aKey, kStringCaseInsensitiveMatch);
+}
+
 Error TxtEntry::AppendTo(Message &aMessage) const
 {
     Appender appender(aMessage);

src/core/net/dns_types.hpp

@@ -1223,6 +1223,16 @@ public:
         mValueLength = aValueLength;
     }
 
+    /**
+     * Indicates whether the entry's key (`mKey`) matches a given key string using a case-insensitive comparison.
+     *
+     * @param[in] aKey  A pointer to a key string to compare with.
+     *
+     * @retval TRUE   The entry's key matches the given @p aKey.
+     * @retval FALSE  The entry's key does not match or `mKey` is `nullptr`.
+     */
+    bool MatchesKey(const char *aKey) const;
+
     /**
      * Encodes and appends the `TxtEntry` to a message.
      *

src/core/radio/trel_peer_discoverer.cpp

@@ -750,23 +750,14 @@ Error PeerDiscoverer::TxtData::Decode(Info &aInfo)
 
     while ((error = iterator.GetNextEntry(entry)) == kErrorNone)
     {
-        // If the TXT data happens to have entries with key longer
-        // than `kMaxIterKeyLength`, `mKey` would be `nullptr` and full
-        // entry would be placed in `mValue`. We skip over such
-        // entries.
-        if (entry.mKey == nullptr)
-        {
-            continue;
-        }
-
-        if (StringMatch(entry.mKey, kExtAddressKey))
+        if (entry.MatchesKey(kExtAddressKey))
         {
             VerifyOrExit(!parsedExtAddress, error = kErrorParse);
             VerifyOrExit(entry.mValueLength >= sizeof(Mac::ExtAddress), error = kErrorParse);
             aInfo.mExtAddress.Set(entry.mValue);
             parsedExtAddress = true;
         }
-        else if (StringMatch(entry.mKey, kExtPanIdKey))
+        else if (entry.MatchesKey(kExtPanIdKey))
         {
             VerifyOrExit(!parsedExtPanId, error = kErrorParse);
             VerifyOrExit(entry.mValueLength >= sizeof(MeshCoP::ExtendedPanId), error = kErrorParse);

tests/unit/test_dns.cpp

@@ -34,6 +34,7 @@
 #include "test_util.hpp"
 
 #include "common/array.hpp"
+#include "common/string.hpp"
 #include "instance/instance.hpp"
 #include "net/dns_types.hpp"
 
@@ -1632,10 +1633,8 @@ void TestHeaderAndResourceRecords(void)
 
 void TestDnsTxtEntry(void)
 {
-    enum
-    {
-        kMaxTxtDataSize = 255,
-    };
+    static constexpr uint16_t kMaxTxtDataSize   = 255;
+    static constexpr uint16_t kMaxKeyStringSize = Dns::TxtEntry::kMaxIterKeyLength;
 
     struct EncodedTxtData
     {
@@ -1718,6 +1717,7 @@ void TestDnsTxtEntry(void)
     Dns::TxtEntry                  txtEntry;
     Dns::TxtEntry::Iterator        iterator;
     MutableData<kWithUint16Length> data;
+    char                           keyString[kMaxKeyStringSize];
 
     printf("================================================================\n");
     printf("TestDnsTxtEntry()\n");
@@ -1772,6 +1772,14 @@ void TestDnsTxtEntry(void)
         }
 
         VerifyOrQuit(strcmp(txtEntry.mKey, expectedTxtEntry.mKey) == 0);
+
+        SuccessOrQuit(StringCopy(keyString, expectedTxtEntry.mKey));
+        VerifyOrQuit(txtEntry.MatchesKey(keyString));
+        StringConvertToLowercase(keyString);
+        VerifyOrQuit(txtEntry.MatchesKey(keyString));
+        StringConvertToUppercase(keyString);
+        VerifyOrQuit(txtEntry.MatchesKey(keyString));
+
         VerifyOrQuit(txtEntry.mValueLength == expectedTxtEntry.mValueLength);
 
         if (txtEntry.mValueLength != 0)