Ractor-shareable JSON::Coder

[etiennebarrie]

I want to be able to reuse an instance of JSON::Coder across Ractors. We need to add RUBY_TYPED_FROZEN_SHAREABLE to the parser config typed data definition and freeze the instance variables, after which it's possible to use a frozen coder across Ractors.

[byroot]

CI needs some fixing but otherwise 👍

[eregon]

This seems correct because all the fields in

json/ext/json/ext/parser/parser.c

Lines 331 to 342 in 7b1fb27

	typedef struct JSON_ParserStruct {
	VALUE on_load_proc;
	VALUE decimal_class;
	ID decimal_method_id;
	enum duplicate_key_action on_duplicate_key;
	int max_nesting;
	bool allow_nan;
	bool allow_trailing_comma;
	bool parsing_name;
	bool symbolize_names;
	bool freeze;
	} JSON_ParserConfig;

are only written in parser_config_init().
However we need to make sure that's not called multiple times, so there should be a rb_check_frozen() at the beginning of parser_config_init().

BTW bool parsing_name; seems unused.

[byroot]

BTW bool parsing_name; seems unused.

Good catch: ab5efca

[etiennebarrie]

@eregon Thanks for the review! I added a call to rb_check_frozen() in cParserConfig_initialize.

>> JSON::Coder.new.freeze.instance_variable_get(:@parser_config).send(:initialize, {})
(irb):1:in 'JSON::Ext::ParserConfig#initialize': can't modify frozen JSON::Ext::ParserConfig: #<JSON::Ext::ParserConfig:0x0000000125000d40> (FrozenError)

I wasn't sure whether to add a test for that, it looked like this:

    coder = JSON::Coder.new.freeze
    parser_config = coder.instance_variable_get(:@parser_config)
    assert_instance_of JSON::Parser::Config, parser_config
    assert_raise FrozenError do
      parser_config.send(:initialize, {})
    end

BTW I just realized that the Generator::State object is already frozen in initialize, I'll actually do the same for the Parser::Config.

[etiennebarrie]

We'll also need to make sure a frozen Ext::Generator::State is not modified:

>> state = JSON::Coder.new.instance_variable_get(:@state)
=> #<JSON::Ext::Generator::State:0x000000012557c7d8>
>> state.frozen?
=> true
>> state.max_nesting
=> 100
>> state.max_nesting=1
=> 1
>> state.max_nesting
=> 1

[eregon]

Good catch, that's already marked as RUBY_TYPED_FROZEN_SHAREABLE but it's not correct since it can still be mutated. So I guess we need rb_check_frozen() in all methods that can mutate a Ext::Generator::State then.
For Ext::Generator::State fields which don't contain a shareable value (e.g. strings) it would break the Ractor invariant (unshareable objects must always stay in a single Ractor) and probably segfault. And it shouldn't be visible as mutable anyway if marked as RUBY_TYPED_FROZEN_SHAREABLE.

[etiennebarrie]

I can't repro the errors with JRuby on my machine. I don't have the same JDK but still…

$ ruby -v
jruby 9.4.14.0 (3.1.7) 2025-08-28 ddda6d5992 OpenJDK 64-Bit Server VM 24.0.1 on 24.0.1 +jit [arm64-darwin]

vs. https://github.com/ruby/json/actions/runs/19504076937/job/55825471457#step:3:39

/Users/runner/.rubies/jruby-9.4.14.0/bin/ruby --version
  jruby 9.4.14.0 (3.1.7) 2025-08-28 ddda6d5992 OpenJDK 64-Bit Server VM 21.0.9+10-LTS on 21.0.9+10-LTS +jit [arm64-darwin]

[byroot]

I can't repro the errors with JRuby

Weird, I can't either.

[eregon]

It seems somewhat transient as it doesn't happen on all platforms.
I wonder if maybe sometimes the wrong json jar gets picked and the one from stdlib is used instead?

[etiennebarrie]

The path seems correct:

/Users/runner/work/json/json/lib/json/ext/generator/state.rb
/Users/runner/work/json/json/lib/json/ext/generator.jar

https://github.com/ruby/json/actions/runs/19541236496/job/55947865782#step:5:24

[byroot]

Maybe it’s not worth testing this closely anyways?

[etiennebarrie]

Yeah at first I didn't have the test at all but then when I found out that the existing State#freeze was subtly incorrect, I felt like it was useful. But the important test is the high-level ractor test. I'll clean that up.

[eregon]

I would suggest to keep the test (it was a pretty subtle and important bug) and skip it on JRuby, it sounds like a JRuby bug to me, maybe something @headius could investigate when he has some time.

[eregon]

Re-reading the diff, the test for JSON::State is kept but the one for JSON::Parser::Config is removed.
I think it's important to add it back, because without it, RUBY_TYPED_FROZEN_SHAREABLE is quite unsafe, it could regress (missing frozen check) and e.g. cause segfaults.