Add cache flush bit to resource records. (#7181)

* Add cache flush bit to resource records. See https://datatracker.ietf.org/doc/html/rfc6762#section-10.2. Currently, this flag is never set by the code. Just adding as an option. * review comments - tests and const function.
project-chip · Jun 24, 2021 · 4147283 · 4147283
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diff --git a/src/lib/mdns/minimal/core/Constants.h b/src/lib/mdns/minimal/core/Constants.h
@@ -45,6 +45,9 @@ enum class QType : uint16_t
 
 /// Flag encoded in QCLASS requesting unicast answers
 constexpr uint16_t kQClassUnicastAnswerFlag = 0x8000;
+// Flag used to indicate receiver should flush cache rather than appending. Used for Response RR's.
+// See https://datatracker.ietf.org/doc/html/rfc6762#section-10.2.
+constexpr uint16_t kQClassResponseFlushBit = 0x8000;
 
 enum class QClass : uint16_t
 {
@@ -53,6 +56,7 @@ enum class QClass : uint16_t
 
     // Unicast version for the class
     IN_UNICAST = IN | kQClassUnicastAnswerFlag,
+    IN_FLUSH   = IN | kQClassResponseFlushBit,
 };
 
 enum class ResourceType

diff --git a/src/lib/mdns/minimal/records/ResourceRecord.h b/src/lib/mdns/minimal/records/ResourceRecord.h
@@ -38,7 +38,7 @@ class ResourceRecord
     ResourceRecord & operator=(const ResourceRecord & other) = default;
 
     const FullQName & GetName() const { return mQName; }
-    QClass GetClass() const { return QClass::IN; }
+    QClass GetClass() const { return mCacheFlush ? QClass::IN_FLUSH : QClass::IN; }
     QType GetType() const { return mType; }
 
     uint32_t GetTtl() const { return mTtl; }
@@ -48,6 +48,13 @@ class ResourceRecord
         return *this;
     }
 
+    ResourceRecord & SetCacheFlush(bool set)
+    {
+        mCacheFlush = set;
+        return *this;
+    }
+    bool GetCacheFlush() const { return mCacheFlush; }
+
     /// Append the given record to the underlying output.
     /// Updates header item count on success, does NOT update header on failure.
     bool Append(HeaderRef & hdr, ResourceType asType, chip::Encoding::BigEndian::BufferWriter & out) const;
@@ -62,6 +69,7 @@ class ResourceRecord
     QType mType;
     uint32_t mTtl = kDefaultTtl;
     FullQName mQName;
+    bool mCacheFlush = false;
 };
 
 } // namespace Minimal

diff --git a/src/lib/mdns/minimal/records/tests/TestResourceRecord.cpp b/src/lib/mdns/minimal/records/tests/TestResourceRecord.cpp
@@ -245,13 +245,31 @@ void RecordCount(nlTestSuite * inSuite, void * inContext)
         NL_TEST_ASSERT(inSuite, header.GetAdditionalCount() == i + 1);
     }
 }
+void CacheFlushBit(nlTestSuite * inSuite, void * inContext)
+{
+    FakeResourceRecord record("somedata");
+    // No cache flush bit by default.
+    NL_TEST_ASSERT(inSuite, record.GetClass() == QClass::IN);
+    NL_TEST_ASSERT(inSuite, record.GetCacheFlush() == false);
+
+    // Check we can set flush bit and the class marker reflects that.
+    record.SetCacheFlush(true);
+    NL_TEST_ASSERT(inSuite, record.GetClass() == QClass::IN_FLUSH);
+    NL_TEST_ASSERT(inSuite, record.GetCacheFlush() == true);
+
+    // Check we can unset.
+    record.SetCacheFlush(false);
+    NL_TEST_ASSERT(inSuite, record.GetClass() == QClass::IN);
+    NL_TEST_ASSERT(inSuite, record.GetCacheFlush() == false);
+}
 
 const nlTest sTests[] = {
     NL_TEST_DEF("CanWriteSimpleRecord", CanWriteSimpleRecord),       //
     NL_TEST_DEF("CanWriteMultipleRecords", CanWriteMultipleRecords), //
     NL_TEST_DEF("RecordOrderIsEnforced", RecordOrderIsEnforced),     //
     NL_TEST_DEF("ErrorsOutOnSmallBuffers", ErrorsOutOnSmallBuffers), //
     NL_TEST_DEF("RecordCount", RecordCount),                         //
+    NL_TEST_DEF("CacheFlushBit", CacheFlushBit),                     //
     NL_TEST_SENTINEL()                                               //
 };