Remove support for PacketBufferTLVReader using chained buffers.

It was unused, and if we can guarantee a contiguous buffer consumers can be safer

src/lib/core/tests/TestCHIPTLV.cpp

@@ -1967,7 +1967,7 @@ void CheckPacketBuffer(nlTestSuite * inSuite, void * inContext)
 
     ReadEncoding1(inSuite, reader);
 
-    reader.Init(buf.Retain(), buf->MaxDataLength());
+    reader.Init(buf.Retain());
     reader.ImplicitProfileId = TestProfile_2;
 
     ReadEncoding1(inSuite, reader);
@@ -2477,33 +2477,41 @@ void CheckCHIPTLVSkipCircular(nlTestSuite * inSuite, void * inContext)
  */
 void CheckBufferOverflow(nlTestSuite * inSuite, void * inContext)
 {
-    System::PacketBufferTLVWriter writer;
     System::PacketBufferTLVReader reader;
 
     System::PacketBufferHandle buf = System::PacketBufferHandle::New(sizeof(Encoding1), 0);
     uint16_t maxDataLen            = buf->MaxDataLength();
     uint16_t reserve = static_cast<uint16_t>((sizeof(Encoding1) < maxDataLen) ? (maxDataLen - sizeof(Encoding1)) + 2 : 0);
 
     // Repeatedly write and read a TLV encoding to a chain of PacketBuffers. Use progressively larger
-    // and larger amounts of space in the first buffer to force the encoding/decoding to overlap the
+    // and larger amounts of space in the first buffer to force the encoding to overlap the
     // end of the buffer and the beginning of the next.
     for (; reserve < maxDataLen; reserve++)
     {
         buf->SetStart(buf->Start() + reserve);
 
-        writer.Init(buf.Retain(), /* useChainedBuffers = */ true);
-        writer.ImplicitProfileId = TestProfile_2;
+        {
+            System::PacketBufferTLVWriter writer;
+            // Scope for writer because we want it to go out of scope before we
+            // mess with the chain after writing is done.
+            writer.Init(buf.Retain(), /* useChainedBuffers = */ true);
+            writer.ImplicitProfileId = TestProfile_2;
 
-        WriteEncoding1(inSuite, writer);
+            WriteEncoding1(inSuite, writer);
+        }
 
         TestBufferContents(inSuite, buf, Encoding1, sizeof(Encoding1));
 
-        reader.Init(buf.Retain(), /* useChainedBuffers = */ true);
+        // Compact the buffer, since we don't allow reading from chained
+        // buffers.
+        buf->CompactHead();
+
+        reader.Init(buf.Retain());
         reader.ImplicitProfileId = TestProfile_2;
 
         ReadEncoding1(inSuite, reader);
 
-        buf = System::PacketBufferHandle::New(System::PacketBuffer::kMaxSizeWithoutReserve, 0);
+        buf = System::PacketBufferHandle::New(sizeof(Encoding1), 0);
     }
 }
 

src/system/TLVPacketBufferBackingStore.h

@@ -87,25 +87,24 @@ class TLVPacketBufferBackingStore : public chip::TLV::TLVBackingStore
     bool mUseChainedBuffers;
 };
 
-class DLL_EXPORT PacketBufferTLVReader : public chip::TLV::TLVReader
+class DLL_EXPORT PacketBufferTLVReader : public TLV::ContiguousBufferTLVReader
 {
 public:
     /**
      * Initializes a TLVReader object to read from a PacketBuffer.
      *
-     * @param[in]    buffer  A handle to PacketBuffer, to be used as backing store for a TLV class.
-     * @param[in]    useChainedBuffers
-     *                       If true, advance to the next buffer in the chain once all data
-     *                       in the current buffer has been consumed.
+     * @param[in]    buffer  A handle to PacketBuffer, to be used as backing
+     *                       store for a TLV class.  If the buffer is chained,
+     *                       only the head of the chain will be used.
      */
-    void Init(chip::System::PacketBufferHandle && buffer, bool useChainedBuffers = false)
+    void Init(chip::System::PacketBufferHandle && buffer)
     {
-        mBackingStore.Init(std::move(buffer), useChainedBuffers);
-        chip::TLV::TLVReader::Init(mBackingStore);
+        mBuffer = std::move(buffer);
+        TLV::ContiguousBufferTLVReader::Init(mBuffer->Start(), mBuffer->DataLength());
     }
 
 private:
-    TLVPacketBufferBackingStore mBackingStore;
+    PacketBufferHandle mBuffer;
 };
 
 class DLL_EXPORT PacketBufferTLVWriter : public chip::TLV::TLVWriter