ci: Run RuboCop serially to avoid JRuby thread-parallel failures

[kinyoklion]

Run RuboCop serially, mirroring launchdarkly/openfeature-ruby-server#34, because --parallel is broken under JRuby as of RuboCop 1.90.0.

• The shared check composite action runs on jruby-9.4, and the gemspec's rubocop "~> 1.76" floats to 1.90.0 (released 2026-08-24), so this repo is exposed to the same failure
• Parallelism buys little here, and the CRuby jobs only lose a negligible speedup

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

launchdarkly/openfeature-ruby-server#34

Implementation details

Root cause (from the investigation in openfeature-ruby-server#34)

Under JRuby the parallel gem cannot fork, so it runs work in threads instead of worker processes. RuboCop 1.90 began preserving cop instances across files, and RuboCop::Cop::Base documents the assumption that each --parallel worker is its own process with its own cop instances. With threads that assumption breaks: reused cop instances see concurrent processed_source values, producing internal cop errors and phantom offenses with mismatched locations. The mechanism is a reading of the RuboCop source, not an upstream-confirmed diagnosis.

Verified there (JRuby 10.0.4/10.0.6/10.1.1 with RuboCop 1.90.0: --parallel fails, serial clean; RuboCop 1.89.0: both clean), so no JRuby upgrade avoids it and pinning RuboCop only defers it.

Scope

Only .github/actions/check/action.yml changes; that action is used by ci.yml, build-gem.yml, release-please.yml, and the manual publish workflows, so all of them pick up the serial run.

Alternatives considered

• Pin rubocop to ~> 1.89.0: hides the problem and freezes lint at an old version.
• Skip RuboCop on JRuby: works, but drops JRuby-specific lint coverage for no benefit over running serially.

Link to Devin session: https://app.devin.ai/sessions/feaff386987f4b3db39becb95bce9fa4
Open in Devin Desktop: https://app.devin.ai/desktop/session/feaff386987f4b3db39becb95bce9fa4?variant=devin
Requested by: @kinyoklion

---

Note

Overview
The shared Quality control checks composite action (.github/actions/check/action.yml) now runs bundle exec rubocop without --parallel, so RuboCop executes serially on every workflow that uses this action (CI, gem build, release, etc.).

This avoids JRuby CI failures introduced with RuboCop 1.90, where thread-based “parallel” workers reuse cop instances and trigger internal errors and bogus offenses; serial execution matches the fix used in launchdarkly/openfeature-ruby-server#34.

^{Reviewed by Cursor Bugbot for commit 8b0080f. Bugbot is set up for automated code reviews on this repo. Configure here.}

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

@cursor review