Ractor-shareable JSON::Coder by etiennebarrie · Pull Request #897 · ruby/json

etiennebarrie · 2025-11-18T16:36:15Z

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 · 2025-11-18T16:45:41Z

CI needs some fixing but otherwise 👍

eregon · 2025-11-18T20:12:17Z

    },
    0, 0,
-    RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED,
+    RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_FROZEN_SHAREABLE,


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.

BTW bool parsing_name; seems unused.

Good catch: ab5efca

@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.

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

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 · 2025-11-19T14:25:20Z

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 · 2025-11-20T11:38:50Z

I can't repro the errors with JRuby

Weird, I can't either.

eregon · 2025-11-20T14:31:44Z

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 · 2025-11-20T15:06:11Z

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 · 2025-11-20T15:11:51Z

Maybe it’s not worth testing this closely anyways?

etiennebarrie · 2025-11-20T15:31:34Z

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 · 2025-11-20T17:01:32Z

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 · 2025-11-20T17:04:57Z

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.

etiennebarrie · 2025-11-21T15:58:56Z

I've added back the test for JSON::Parser::Config and skipped it on JRuby.
I also moved the duplicated macro for RUBY_TYPED_FROZEN_SHAREABLE compatibility to ext/json.h.

eregon reviewed Nov 18, 2025

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Comment thread lib/json/common.rb Outdated

eregon reviewed Nov 18, 2025

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etiennebarrie force-pushed the ractor-shareable-json-coder branch 5 times, most recently from 77c03ae to 7ea7a05 Compare November 19, 2025 14:06

eregon reviewed Nov 19, 2025

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Comment thread ext/json/ext/generator/generator.c Outdated

etiennebarrie force-pushed the ractor-shareable-json-coder branch from 7ea7a05 to 76ed0f4 Compare November 19, 2025 14:49

etiennebarrie force-pushed the ractor-shareable-json-coder branch from 0c34822 to 7ca198c Compare November 20, 2025 15:32

Ractor-shareable JSON::Coder

1df7523

etiennebarrie force-pushed the ractor-shareable-json-coder branch from 7ca198c to 1df7523 Compare November 21, 2025 10:51

etiennebarrie added 2 commits November 21, 2025 11:53

Skip test failing with JRuby in CI

b7e1734

Move RUBY_TYPED_FROZEN_SHAREABLE macro to json.h

1219ce0

etiennebarrie force-pushed the ractor-shareable-json-coder branch from bfcd535 to 1219ce0 Compare November 21, 2025 15:58

byroot merged commit 2a4ebe8 into ruby:master Nov 21, 2025
37 checks passed

	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;

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