Fix SimpleCov coverage for parallel runs

[samford]

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I used Claude Opus 5 (High) to identify the issue and implement the fixes. I reviewed the changes (having read the upstream SimpleCov documentation and changelog), made some tweaks, and compared brew tests --coverage output to confirm the fix (comparing the output for 0.22.0, the main branch, and this PR branch).

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We recently updated SimpleCov from 0.22.0 to 1.0.1 but 1.0 involves some changes to how coverage data is merged/finalized when run in parallel and our preexisting setup doesn't quite work as expected. We addressed API deprecations when upgrading and brew tests --coverage works but it produces incomplete coverage data (i.e., the percentages are much lower than expected).

This appears to be due to a couple of issues:

• In SimpleCov 1.0, finalize_merge false causes SimpleCov.result to return early, so workers will return their result without waiting for and merging results from other workers. We use it explicitly in .simplecov but it would also be set by default for parallel runs when a custom coverage_dir is set, as in our case. I think the expectation is that we would use SimpleCov.collate but our logic in spec_helper.rb doesn't.
• We use result.format! if ParallelTests.last_process? in the SimpleCov.at_exit logic in spec_helper.rb but SimpleCov 1.0 switched to using first_process? (to align with parallel_tests), so that no longer works as expected.

This addresses the issue by using finalize_merge SimpleCov.final_result_process? and removing the now-unnecessary SimpleCov.at_exit logic in spec_helper.rb, as the default behavior in 1.0 already does what we want. finalize_merge true technically works but it's notably slower for no added benefit (i.e., only one worker needs to merge and format the results).

This also removes the parallel = false if args.coverage? && files.length < Hardware::CPU.cores workaround in dev-cmd/tests.rb, as the related issues have been resolved in 1.0.

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I tested this by running brew tests --coverage using the commit before the SimpleCov 1.0 upgrade (09ad50f, which uses 0.22.0) and comparing that to the output on main (using 1.0.1). After implementing these changes, I ran it again and compared the previous 0.22.0 output to the output from this PR branch and confirmed that the coverage was generally what we would expect. I noticed a few small differences in the coverage but that may be attributable to how SimpleCov behaves in 1.0 (though that's something we can address in a follow-up PR, if needed).

[Copilot]

Pull request overview

This pull request updates Homebrew’s test coverage setup to work correctly with SimpleCov 1.0.x when RSpec tests are executed in parallel, ensuring coverage results are merged/finalized by the appropriate process and removing no-longer-needed workaround logic.

Changes:

• Adjust SimpleCov configuration to set finalize_merge dynamically using SimpleCov.final_result_process? for parallel runs.
• Remove custom SimpleCov.at_exit handling from test/spec_helper.rb that attempted to gate formatting to a single parallel worker.
• Remove the --coverage parallel-disable workaround in dev-cmd/tests.rb for small test sets.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/.simplecov	Updates SimpleCov parallel merge finalization logic for correct coverage aggregation in 1.0.x.
Library/Homebrew/test/spec_helper.rb	Removes now-unnecessary parallel-only SimpleCov.at_exit formatting guard.
Library/Homebrew/dev-cmd/tests.rb	Removes coverage-related parallel execution workaround in the brew tests command.

Comments suppressed due to low confidence (1)

Library/Homebrew/dev-cmd/tests.rb:106

• This change removes the previous guard that forced parallel = false for coverage runs with fewer spec files than CPU cores. Given this is a behavior change in brew tests (affecting whether we invoke parallel_rspec vs rspec under --coverage), it would be good to add/adjust an RSpec example in test/dev-cmd/tests_spec.rb that exercises #run with --coverage and a small file set and asserts we still run parallel_rspec (or, if intended otherwise, codifies the new expected behavior).

          parallel_rspec_log_name = "parallel_runtime_rspec"
          parallel_rspec_log_name = "#{parallel_rspec_log_name}.generic" if args.generic?
          parallel_rspec_log_name = "#{parallel_rspec_log_name}.online" if args.online?
          parallel_rspec_log_name = "#{parallel_rspec_log_name}.log"

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[p-linnane]

Nice work @samford!

[MikeMcQuaid]

Thanks, love a fix that's mostly deleting!