Is JSON::Coder intended to be thread safe ?

[daveferrykami]

In JRuby we hit a race condition: ActiveSupport uses a single shared JSON::Coder
instance and calls #dump on it from multiple threads.

ActiveSupport creates one shared Coder:
https://github.com/rails/rails/blob/v8.1.3.1/activesupport/lib/active_support/json/encoding.rb#L152

…and reuses it on every encode:
https://github.com/rails/rails/blob/v8.1.3.1/activesupport/lib/active_support/json/encoding.rb#L188

The error we got, on a flat object was

JSON::NestingError: nesting of 100 is too deep. Did you try to serialize objects with circular references?

Question: is a single JSON::Coder instance intended to be safe to call
concurrently from multiple threads (i.e. is #dump/#generate on one Coder
intended to be thread-safe)?

Versions: json 2.21.2 (java), JRuby 10.0.5.0, activesupport 8.1.3.1.

I have a small threaded reproduction I can add if useful.

[headius]

I also looked into this a little bit and could find no documentation one way or another about using a single Coder instance concurrently across threads.

I believe this issue affects both the Java code and the C code, but the C code might only expose it if some object implements part of its dumping in Ruby.