feat(kv-ir): Add a function signature C++ concept for the callback called when a new projected schema-tree node is read.

components/core/CMakeLists.txt

@@ -433,6 +433,7 @@ set(SOURCE_FILES_unitTest
         src/clp/ffi/ir_stream/protocol_constants.hpp
         src/clp/ffi/ir_stream/Serializer.cpp
         src/clp/ffi/ir_stream/Serializer.hpp
+        src/clp/ffi/ir_stream/search/NewProjectedSchemaTreeNodeCallbackReq.hpp
         src/clp/ffi/ir_stream/utils.cpp
         src/clp/ffi/ir_stream/utils.hpp
         src/clp/ffi/KeyValuePairLogEvent.cpp

components/core/src/clp/ffi/ir_stream/search/NewProjectedSchemaTreeNodeCallbackReq.hpp

@@ -0,0 +1,33 @@
+#ifndef CLP_FFI_IR_STREAM_SEARCH_NEWPROJECTEDSCHEMATREENODECALLBACKREQ_HPP
+#define CLP_FFI_IR_STREAM_SEARCH_NEWPROJECTEDSCHEMATREENODECALLBACKREQ_HPP
+
+#include <string_view>
+#include <type_traits>
+
+#include <outcome/single-header/outcome.hpp>
+
+#include "../../SchemaTree.hpp"
+
+namespace clp::ffi::ir_stream::search {
+/**
+ * Requirements for a callback that's called whenever:
+ *
+ * 1. a new schema-tree node is deserialized from the IR stream; and
+ * 2. that node corresponds to one of the projected key paths in the query.
+ *
+ * @tparam NewProjectedSchemaTreeNodeCallbackType
+ * @param arg_0 Whether the schema-tree node is for an auto-generated kv-pair.
+ * @param arg_1 The ID of the schema-tree node.
+ * @param arg_2 The projected key path.
+ * @return A void result on success or a user-defined error code indicating the failure.
+ */
+template <typename NewProjectedSchemaTreeNodeCallbackType>
+concept NewProjectedSchemaTreeNodeCallbackReq = std::is_invocable_r_v<
+        outcome_v2::std_result<void>,
+        NewProjectedSchemaTreeNodeCallbackType,
+        bool,
+        SchemaTree::Node::id_t,
+        std::string_view>;
+}  // namespace clp::ffi::ir_stream::search
+
+#endif  // CLP_FFI_IR_STREAM_SEARCH_NEWPROJECTEDSCHEMATREENODECALLBACKREQ_HPP