Json : Add --size-prefixed option to flatc (#4645)

to be able to convert to json size prefixed buffers.

include/flatbuffers/flatbuffers.h

@@ -1664,13 +1664,14 @@ const T *GetTemporaryPointer(FlatBufferBuilder &fbb, Offset<T> offset) {
 /// This function is UNDEFINED for FlatBuffers whose schema does not include
 /// a file_identifier (likely points at padding or the start of a the root
 /// vtable).
-inline const char *GetBufferIdentifier(const void *buf) {
-  return reinterpret_cast<const char *>(buf) + sizeof(uoffset_t);
+inline const char *GetBufferIdentifier(const void *buf, bool size_prefixed = false) {
+  return reinterpret_cast<const char *>(buf) +
+         ((size_prefixed) ? 2 * sizeof(uoffset_t) : sizeof(uoffset_t));
 }
 
 // Helper to see if the identifier in a buffer has the expected value.
-inline bool BufferHasIdentifier(const void *buf, const char *identifier) {
-  return strncmp(GetBufferIdentifier(buf), identifier,
+inline bool BufferHasIdentifier(const void *buf, const char *identifier, bool size_prefixed = false) {
+  return strncmp(GetBufferIdentifier(buf, size_prefixed), identifier,
                  FlatBufferBuilder::kFileIdentifierLength) == 0;
 }
 

include/flatbuffers/idl.h

@@ -387,6 +387,7 @@ struct IDLOptions {
   std::string go_namespace;
   bool reexport_ts_modules;
   bool protobuf_ascii_alike;
+  bool size_prefixed;
 
   // Possible options for the more general generator below.
   enum Language {
@@ -442,6 +443,7 @@ struct IDLOptions {
         skip_flatbuffers_import(false),
         reexport_ts_modules(true),
         protobuf_ascii_alike(false),
+        size_prefixed(false),
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
         lang_to_generate(0) {}

src/flatc.cpp

@@ -100,6 +100,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "                     (default is \"github.com/google/flatbuffers/go\")\n"
     "  --raw-binary       Allow binaries without file_indentifier to be read.\n"
     "                     This may crash flatc given a mismatched schema.\n"
+    "  --size-prefixed    Input binaries are size prefixed buffers.\n"
     "  --proto            Input is a .proto, translate to .fbs.\n"
     "  --oneof-union      Translate .proto oneofs to flatbuffer unions.\n"
     "  --grpc             Generate GRPC interfaces for the specified languages\n"
@@ -233,6 +234,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.one_file = true;
       } else if (arg == "--raw-binary") {
         raw_binary = true;
+      } else if (arg == "--size-prefixed") {
+        opts.size_prefixed = true;
       } else if (arg == "--") {  // Separator between text and binary inputs.
         binary_files_from = filenames.size();
       } else if (arg == "--proto") {
@@ -325,7 +328,7 @@ int FlatCompiler::Compile(int argc, const char **argv) {
                 "\" matches the schema, use --raw-binary to read this file"
                 " anyway.");
         } else if (!flatbuffers::BufferHasIdentifier(
-                       contents.c_str(), parser->file_identifier_.c_str())) {
+                       contents.c_str(), parser->file_identifier_.c_str(), opts.size_prefixed)) {
           Error("binary \"" + filename +
                 "\" does not have expected file_identifier \"" +
                 parser->file_identifier_ +

src/idl_gen_text.cpp

@@ -263,8 +263,9 @@ bool GenerateText(const Parser &parser, const void *flatbuffer,
   std::string &text = *_text;
   assert(parser.root_struct_def_);  // call SetRootType()
   text.reserve(1024);               // Reduce amount of inevitable reallocs.
-  if (!GenStruct(*parser.root_struct_def_, GetRoot<Table>(flatbuffer), 0,
-                 parser.opts, _text)) {
+  auto root = parser.opts.size_prefixed ?
+      GetSizePrefixedRoot<Table>(flatbuffer) : GetRoot<Table>(flatbuffer);
+  if (!GenStruct(*parser.root_struct_def_, root, 0, parser.opts, _text)) {
     return false;
   }
   text += NewLine(parser.opts);

src/idl_parser.cpp

@@ -2412,9 +2412,15 @@ CheckedError Parser::DoParse(const char *source, const char **include_paths,
       }
       uoffset_t toff;
       ECHECK(ParseTable(*root_struct_def_, nullptr, &toff));
-      builder_.Finish(Offset<Table>(toff), file_identifier_.length()
-                                               ? file_identifier_.c_str()
-                                               : nullptr);
+      if (opts.size_prefixed) {
+        builder_.FinishSizePrefixed(Offset<Table>(toff), file_identifier_.length()
+                                                             ? file_identifier_.c_str()
+                                                             : nullptr);
+      } else {
+        builder_.Finish(Offset<Table>(toff), file_identifier_.length()
+                                                 ? file_identifier_.c_str()
+                                                 : nullptr);
+      }
     } else if (IsIdent("enum")) {
       ECHECK(ParseEnum(false, nullptr));
     } else if (IsIdent("union")) {
@@ -2521,7 +2527,11 @@ void Parser::Serialize() {
       builder_.CreateString(file_identifier_),
       builder_.CreateString(file_extension_),
       root_struct_def_ ? root_struct_def_->serialized_location : 0);
-  builder_.Finish(schema_offset, reflection::SchemaIdentifier());
+  if (opts.size_prefixed) {
+    builder_.FinishSizePrefixed(schema_offset, reflection::SchemaIdentifier());
+  } else {
+    builder_.Finish(schema_offset, reflection::SchemaIdentifier());
+  }
 }
 
 Offset<reflection::Object> StructDef::Serialize(FlatBufferBuilder *builder,