parser: Add zero_on_float_to_int logic and improve json parsing.

Author: Dwordcito

include/flatbuffers/idl.h

@@ -787,6 +787,10 @@ struct IDLOptions {
   /******************************* Python gRPC ********************************/
   bool grpc_python_typed_handlers;
 
+  // If set, when a float value is received for an integer field, set the value
+  // to 0 to avoid precision loss.
+  bool zero_on_float_to_int;
+
   IDLOptions()
       : gen_jvmstatic(false),
         use_flexbuffers(false),
@@ -861,7 +865,8 @@ struct IDLOptions {
         set_empty_vectors_to_null(true),
         grpc_filename_suffix(".fb"),
         grpc_use_system_headers(true),
-        grpc_python_typed_handlers(false) {}
+        grpc_python_typed_handlers(false),
+        zero_on_float_to_int(false) {}
 };
 
 // This encapsulates where the parser is in the current source file.

src/idl_parser.cpp

@@ -2189,6 +2189,24 @@ CheckedError Parser::ParseSingleValue(const std::string *name, Value &e,
   } else {
     // Try a float number.
     TRY_ECHECK(kTokenFloatConstant, IsFloat(in_type), BASE_TYPE_FLOAT);
+
+    // Special handling for integer types that might come as floats from JSON
+    // This happens when large integers are parsed as doubles by JSON parsers
+    // Set the value to 0 to avoid precision issues
+    if (!match && token_ == kTokenFloatConstant && IsInteger(in_type) &&
+        opts.zero_on_float_to_int) {
+      double float_val = std::stod(attribute_);
+      // For ulong fields that receive float values, set to 0
+      Warning("Float value " + std::to_string(float_val) + " received for <" +
+              std::string(TypeName(in_type)) +
+              "> field, setting to 0 to avoid precision loss " +
+              std::to_string(IsFloat(in_type)) + " " +
+              std::to_string(IsInteger(in_type)) + " " +
+              std::to_string(token_));
+      attribute_ = "0";
+      TRY_ECHECK(kTokenFloatConstant, IsInteger(in_type), BASE_TYPE_INT);
+    }
+
     // Integer token can init any scalar (integer of float).
     FORCE_ECHECK(kTokenIntegerConstant, IsScalar(in_type), BASE_TYPE_INT);
   }

tests/parser_test.cpp

@@ -928,5 +928,20 @@ void FieldIdentifierTest() {
 #endif
 }
 
+void ZeroOnFloatToIntTest() {
+  flatbuffers::IDLOptions opts;
+  {
+    opts.zero_on_float_to_int = true;
+    flatbuffers::Parser parser(opts);
+    TEST_EQ(true, parser.Parse("table T{ i: int = 1.0; }"));
+  }
+
+  {
+    opts.zero_on_float_to_int = false;
+    flatbuffers::Parser parser(opts);
+    TEST_EQ(false, parser.Parse("table T{ i: int = 1.0; }"));
+  }
+}
+
 }  // namespace tests
 }  // namespace flatbuffers

tests/parser_test.h

@@ -26,6 +26,7 @@ void ValidSameNameDifferentNamespaceTest();
 void WarningsAsErrorsTest();
 void StringVectorDefaultsTest();
 void FieldIdentifierTest();
+void ZeroOnFloatToIntTest();
 
 }  // namespace tests
 }  // namespace flatbuffers

tests/test.cpp

@@ -1729,6 +1729,7 @@ int FlatBufferTests(const std::string &tests_data_path) {
   EmbeddedSchemaAccess();
   Offset64Tests();
   UnionUnderlyingTypeTest();
+  ZeroOnFloatToIntTest();
   return 0;
 }
 }  // namespace