Plan 04: Switch Default Parallelism to Threads

Objective

Switch the default test parallelism from fork-based to thread-based. This eliminates the PG fork segfault that forces PARALLEL_WORKERS=1 on single-file runs, and enables the FeedFetcherTest split (Plan 01) to actually parallelize across workers. Thread-based parallelism is already proven working in coverage mode (COVERAGE=1).

Context

@ test/test_helper.rb -- current parallelism configuration (forks by default, threads only for coverage)
@ .vbw-planning/phases/02-test-performance/02-RESEARCH.md -- research confirming thread parallelism works in coverage mode
@ test/test_prof.rb -- TestProf setup (thread-compatible)

Rationale: The current code uses parallelize(workers: worker_count) which defaults to fork-based parallelism. This causes PG segfaults on single-file runs and prevents the FeedFetcherTest split from distributing across workers (since forks copy the process and the PG connection). Thread-based parallelism is already proven (used with COVERAGE=1) and avoids these issues.

Dependency on Plan 01: Plan 01 splits FeedFetcherTest into 6+ classes. Without the split, thread parallelism still cannot distribute the 71-test monolith across workers. The split must complete first for the parallelism switch to realize its full benefit.

Risk: Thread safety of reset_configuration! -- The global setup block calls SourceMonitor.reset_configuration! before every test. With threads, multiple tests may call this simultaneously. Since reset_configuration! replaces the entire @configuration instance, and each test reads config after setup, this is safe as long as no test modifies config mid-test while another test is reading it. The research confirmed this is pure Ruby assignment (microseconds). If any flaky failures appear, we add a Mutex around the reset.

Tasks

Task 1: Switch parallelize to threads

In test/test_helper.rb, replace the parallelism block:

ruby

# BEFORE:
if ENV["COVERAGE"]
  parallelize(workers: 1, with: :threads)
else
  worker_count = ENV.fetch("SOURCE_MONITOR_TEST_WORKERS", :number_of_processors)
  worker_count = worker_count.to_i if worker_count.is_a?(String) && !worker_count.empty?
  worker_count = :number_of_processors if worker_count.respond_to?(:zero?) && worker_count.zero?
  parallelize(workers: worker_count)
end

ruby

# AFTER:
worker_count = if ENV["COVERAGE"]
  1
else
  count = ENV.fetch("SOURCE_MONITOR_TEST_WORKERS", :number_of_processors)
  count = count.to_i if count.is_a?(String) && !count.empty?
  count = :number_of_processors if count.respond_to?(:zero?) && count.zero?
  count
end
parallelize(workers: worker_count, with: :threads)

Key change: Always use with: :threads (not just for coverage). Worker count logic stays the same.

Task 2: Add thread-safety comment to reset_configuration

Add a comment in the setup block explaining thread safety:

ruby

setup do
  # Thread-safe: reset_configuration! replaces @configuration atomically.
  # Each test gets a fresh config object. No concurrent mutation risk since
  # tests read config only after their own setup completes.
  SourceMonitor.reset_configuration!
end

Task 3: Verify single-file runs work without PARALLEL_WORKERS=1

The main benefit of thread-based parallelism: single-file runs no longer segfault.

bash

# These should now work WITHOUT PARALLEL_WORKERS=1
bin/rails test test/lib/source_monitor/fetching/feed_fetcher_success_test.rb
bin/rails test test/models/source_monitor/source_test.rb
bin/rails test test/controllers/source_monitor/sources_controller_test.rb

Task 4: Full suite verification

bash

# Full suite with thread parallelism
bin/rails test

# Verify worker count is respected
SOURCE_MONITOR_TEST_WORKERS=4 bin/rails test

# Lint
bin/rubocop test/test_helper.rb

Ensure all 1031+ tests pass with zero failures. Watch for flaky tests that might indicate thread-safety issues. If any test fails intermittently, check if it modifies global state (module-level variables, class variables, or singletons) and fix the isolation.

Files

Action	Path
MODIFY	`test/test_helper.rb`

Verification

bash

# Single-file run (no PARALLEL_WORKERS=1 needed)
bin/rails test test/models/source_monitor/source_test.rb

# Full suite
bin/rails test

# Lint
bin/rubocop test/test_helper.rb

Success Criteria

grep "with: :threads" test/test_helper.rb shows the threads configuration
bin/rails test passes all 1031+ tests
Single-file runs work without PARALLEL_WORKERS=1 workaround
No flaky test failures in 2 consecutive full suite runs
Full suite completes in <70s locally (down from 133s)