feat: Expose non-io_buf API for fb_parser (#4683)

Expose non-io_buf API for fb_parser

This creates an entry-point for the parsing logic operating over a buffer pointer rather than io_buf; previously this was only used for unit testing, but this will, in the future, be used by consumers of the library, e.g. RLClientLib.

Added unit tests for this behaviour found a couple of bugs in the implementation of the FB parser as well as FB parser unit test validation logic:
* Fix logic issues with detecting presence/absence of feature names and hashes in example data, leading to an access violation crash
* Fix issues with properly parsing ExampleCollection buffers, around reporting doneness and advancing when parsing inner MultiExample objects.
* Fix tests to include coverage for incoming flatbuffers with/without names/hashes
* Fix validation logic to avoid crashing when feature names/hashes are missing

vowpalwabbit/core/CMakeLists.txt

@@ -420,7 +420,7 @@ if(VW_FEAT_CSV)
 endif()
 
 if(VW_FEAT_FLATBUFFERS)
-  target_link_libraries(vw_core PRIVATE vw_fb_parser)
+  target_link_libraries(vw_core PUBLIC vw_fb_parser)
 endif()
 
 # Handle generated header

vowpalwabbit/fb_parser/CMakeLists.txt

@@ -40,7 +40,9 @@ set(vw_fb_parser_test_sources
       tests/prototype_label.h
       tests/prototype_namespace.h
 
-      tests/flatbuffer_parser_test.cc
+      tests/affordance_validation_tests.cc
+      tests/read_span_tests.cc
+      tests/flatbuffer_parser_tests.cc
 )
 
 message(STATUS "vw_fb_parser_test_sources: ${vw_fb_parser_test_sources}")

vowpalwabbit/fb_parser/include/vw/fb_parser/parse_example_flatbuffer.h

@@ -5,32 +5,37 @@
 #pragma once
 
 #include "vw/core/api_status.h"
+#include "vw/core/example.h"
 #include "vw/core/multi_ex.h"
 #include "vw/core/shared_data.h"
 #include "vw/core/vw_fwd.h"
 #include "vw/fb_parser/generated/example_generated.h"
 
 namespace VW
 {
+
 class api_status;
+
 namespace parsers
 {
 namespace flatbuffer
 {
 int flatbuffer_to_examples(VW::workspace* all, io_buf& buf, VW::multi_ex& examples);
+bool read_span_flatbuffer(
+    VW::workspace* all, const uint8_t* span, size_t length, example_factory_t example_factory, VW::multi_ex& examples);
 
 class parser
 {
 public:
   parser() = default;
   const VW::parsers::flatbuffer::ExampleRoot* data();
-  int parse_examples(VW::workspace* all, io_buf& buf, VW::multi_ex& examples, uint8_t* buffer_pointer = nullptr,
+  int parse_examples(VW::workspace* all, io_buf& buf, VW::multi_ex& examples, const uint8_t* buffer_pointer = nullptr,
       VW::experimental::api_status* status = nullptr);
 
 private:
   size_t _num_example_roots = 0;
   const VW::parsers::flatbuffer::ExampleRoot* _data;
-  uint8_t* _flatbuffer_pointer;
+  const uint8_t* _flatbuffer_pointer;
   flatbuffers::uoffset_t _object_size = 0;
   bool _active_collection = false;
   uint32_t _example_index = 0;
@@ -40,7 +45,7 @@ class parser
   uint32_t _labeled_action = 0;
   uint64_t _c_hash = 0;
 
-  int parse(io_buf& buf, uint8_t* buffer_pointer = nullptr, VW::experimental::api_status* status = nullptr);
+  int parse(io_buf& buf, const uint8_t* buffer_pointer = nullptr, VW::experimental::api_status* status = nullptr);
   int process_collection_item(
       VW::workspace* all, VW::multi_ex& examples, VW::experimental::api_status* status = nullptr);
   int parse_example(VW::workspace* all, example* ae, const Example* eg, VW::experimental::api_status* status = nullptr);