TLV to JSON converter

This adds a TLV to JSON converter to provide a means to deal with TLV in
a more human-readable format. This does not rely on any schema
information and instead, just dumps out raw TLV present in cluster
payloads into a JSON format.

This relies on the jsoncpp third party library.

Testing
Added a unit-test.

src/lib/support/jsontlv/BUILD.gn

@@ -0,0 +1,24 @@
+# Copyright (c) 2020 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build_overrides/chip.gni")
+
+static_library("jsontlv") {
+  public_deps = [
+    "${chip_root}/src/lib/core",
+    "${chip_root}/third_party/jsoncpp",
+  ]
+
+  sources = [ "TlvJson.cpp" ]
+}

src/lib/support/jsontlv/TlvJson.cpp

@@ -0,0 +1,242 @@
+/*
+ *
+ *    Copyright (c) 2020 Project CHIP Authors
+ *    Copyright (c) 2013-2017 Nest Labs, Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+#include "lib/support/CHIPMemString.h"
+#include "lib/support/ScopedBuffer.h"
+#include <lib/core/DataModelTypes.h>
+#include <lib/support/Base64.h>
+#include <lib/support/jsontlv/TlvJson.h>
+
+namespace {
+/*
+ * Encapsulates the different types of keys permissible.
+ *
+ * Root Key = Key with a name of 'value'. This is the top-most key in a given JSON object generated from TLV.
+ * Struct Field = Key containing the 32-bit field ID of an item in a struct.
+ * Array Item = Key containing the 16-bit list index of an item in a list.
+ *
+ * In the latter two modes, the actual field ID/list index is encapsulated within the 'key' member.
+ *
+ */
+struct KeyContext
+{
+    enum KeyType
+    {
+        kRoot,
+        kStructField,
+        kArrayItem
+    };
+
+    KeyContext() = default;
+
+    KeyContext(chip::FieldId fieldId)
+    {
+        keyType = kStructField;
+        key     = fieldId;
+    }
+
+    KeyContext(chip::ListIndex listIndex)
+    {
+        keyType = kArrayItem;
+        key     = listIndex;
+    }
+
+    KeyType keyType = kRoot;
+    unsigned int key    = 0;
+};
+} // namespace
+
+//
+// For now, let's put a bound of the maximum length of a byte/char string to be the size of an IPv6
+// MTU. While this is smaller than that of the limit defined in the data model specification,
+// strings by virtue of not being chunked are intrinsically limited in size to the size of the encompassing packet.
+//
+static constexpr uint16_t kMaxStringLen = 1280;
+
+namespace chip {
+
+/*
+ * This templated function inserts a key/value pair into the Json value object.
+ * The value is templated to be of type T and accepts any of the following primitive
+ * types:
+ *      bool, uint*_t, int*_t, char *, float, double.
+ *
+ * This method uses the provided key context to deduce the type of element being added.
+ *
+ */
+template <typename T>
+void InsertKeyValue(Json::Value & json, const KeyContext & keyContext, T val)
+{
+    //
+    // This needs to accomodate either the string 'value', or a 32-bit integer.
+    // The size of the largest 32-bit integer key represented as a string is 11 characters long.
+    // Tack on 1 byte for the null character.
+    //
+    char keyBuf[12];
+
+    if (keyContext.keyType == KeyContext::kRoot)
+    {
+        Platform::CopyString(keyBuf, sizeof(keyBuf), "value");
+        json[keyBuf] = val;
+    }
+    else if (keyContext.keyType == KeyContext::kStructField)
+    {
+        snprintf(keyBuf, sizeof(keyBuf), "%" PRIu32, keyContext.key);
+        json[keyBuf] = val;
+    }
+    else
+    {
+        json[keyContext.key] = val;
+    }
+}
+
+std::string JsonToString(Json::Value & json)
+{
+    Json::StyledWriter writer;
+    return writer.write(json);
+}
+
+CHIP_ERROR TlvToJson(TLV::TLVReader & reader, KeyContext context, Json::Value & parent)
+{
+    bool isStruct = false;
+
+    switch (reader.GetType())
+    {
+    case TLV::kTLVType_UnsignedInteger: {
+        uint64_t v;
+        ReturnErrorOnFailure(reader.Get(v));
+        InsertKeyValue(parent, context, v);
+        break;
+    }
+
+    case TLV::kTLVType_SignedInteger: {
+        int64_t v;
+        ReturnErrorOnFailure(reader.Get(v));
+        InsertKeyValue(parent, context, v);
+        break;
+    }
+
+    case TLV::kTLVType_Boolean: {
+        bool v;
+        ReturnErrorOnFailure(reader.Get(v));
+        InsertKeyValue(parent, context, v);
+        break;
+    }
+
+    case TLV::kTLVType_FloatingPointNumber: {
+        double v;
+        ReturnErrorOnFailure(reader.Get(v));
+        InsertKeyValue(parent, context, v);
+        break;
+    }
+
+    case TLV::kTLVType_ByteString: {
+        ByteSpan span;
+
+        ReturnErrorOnFailure(reader.Get(span));
+        VerifyOrReturnError(span.size() < kMaxStringLen, CHIP_ERROR_INVALID_TLV_ELEMENT);
+
+        Platform::ScopedMemoryBuffer<char> byteString;
+        byteString.Alloc(BASE64_ENCODED_LEN(span.size()) + 1);
+        VerifyOrReturnError(byteString.Get() != nullptr, CHIP_ERROR_NO_MEMORY);
+
+        auto encodedLen              = Base64Encode(span.data(), span.size(), byteString.Get());
+        byteString.Get()[encodedLen] = '\0';
+
+        InsertKeyValue(parent, context, byteString.Get());
+        break;
+    }
+
+    case TLV::kTLVType_UTF8String: {
+        CharSpan span;
+
+        ReturnErrorOnFailure(reader.Get(span));
+        VerifyOrReturnError(span.size() < kMaxStringLen, CHIP_ERROR_INVALID_TLV_ELEMENT);
+
+        Platform::ScopedMemoryString charString(span.data(), span.size());
+        InsertKeyValue(parent, context, charString.Get());
+        break;
+    }
+
+    case TLV::kTLVType_Null: {
+        InsertKeyValue(parent, context, Json::Value());
+        break;
+    }
+
+    case TLV::kTLVType_Structure:
+        isStruct = true;
+
+        //
+        // Fall-through to the case below since
+        // arrays and structs are handled similarly with
+        // just a small difference in terms of handling of field IDs vs.
+        // list indices of the elements in the respective collections.
+        //
+
+    case TLV::kTLVType_Array: {
+        TLV::TLVType containerType;
+
+        ReturnErrorOnFailure(reader.EnterContainer(containerType));
+
+        CHIP_ERROR err;
+        Json::Value value;
+        size_t listIndex = 0;
+
+        while ((err = reader.Next()) == CHIP_NO_ERROR)
+        {
+            if (isStruct)
+            {
+                VerifyOrReturnError(TLV::IsContextTag(reader.GetTag()), CHIP_ERROR_INVALID_TLV_TAG);
+                KeyContext context2(static_cast<chip::FieldId>(TLV::TagNumFromTag(reader.GetTag())));
+
+                //
+                // Recursively convert to JSON the encompassing item within the struct.
+                //
+                ReturnErrorOnFailure(TlvToJson(reader, context2, value));
+            }
+            else
+            {
+                KeyContext context2(static_cast<chip::ListIndex>(listIndex++));
+
+                //
+                // Recursively convert to JSON the encompassing item within the array.
+                //
+                ReturnErrorOnFailure(TlvToJson(reader, context2, value));
+            }
+        }
+
+        VerifyOrReturnError(err == CHIP_END_OF_TLV, err);
+        ReturnErrorOnFailure(reader.ExitContainer(containerType));
+        InsertKeyValue(parent, context, value);
+        break;
+    }
+
+    default:
+        return CHIP_ERROR_INVALID_TLV_ELEMENT;
+        break;
+    }
+
+    return CHIP_NO_ERROR;
+}
+
+CHIP_ERROR TlvToJson(TLV::TLVReader & reader, Json::Value & root)
+{
+    KeyContext context;
+    return TlvToJson(reader, context, root);
+}
+
+} // namespace chip

src/lib/support/jsontlv/TlvJson.h

@@ -0,0 +1,38 @@
+/*
+ *
+ *    Copyright (c) 2020 Project CHIP Authors
+ *    Copyright (c) 2013-2017 Nest Labs, Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#include <json/json.h>
+#include <lib/core/CHIPTLV.h>
+
+namespace chip {
+
+/*
+ * Given a TLVReader positioned at a particular cluster data payload, this function converts
+ * the TLV data into a JSON object representation.
+ *
+ * NOTE: This only accepts data model payloads for events/commands/attributes. It does not support
+ * arbitrary TLV conversion to JSON.
+ */
+CHIP_ERROR TlvToJson(TLV::TLVReader & reader, Json::Value & root);
+
+/*
+ * Converts a JSON object into string representation
+ */
+std::string JsonToString(Json::Value & json);
+
+} // namespace chip

src/lib/support/tests/BUILD.gn

@@ -46,6 +46,7 @@ chip_test_suite("tests") {
     "TestStringBuilder.cpp",
     "TestThreadOperationalDataset.cpp",
     "TestTimeUtils.cpp",
+    "TestTlvToJson.cpp",
     "TestVariant.cpp",
     "TestZclString.cpp",
   ]
@@ -68,6 +69,7 @@ chip_test_suite("tests") {
 
   public_deps = [
     "${chip_root}/src/lib/core",
+    "${chip_root}/src/lib/support/jsontlv",
     "${chip_root}/src/platform",
     "${nlunit_test_root}:nlunit-test",
   ]