Barrier Concurrency Testing

Deterministic race condition testing -- no flaky timing-based tests.

Instead of setTimeout (flaky) or sleep (slow), use barriers to pause execution at exact interleave points. The test controls when each concurrent operation proceeds, making race condition tests deterministic and reproducible on every run.

When to use: Testing concurrent operations, flush conflicts, parallel mutations, race windows between read and write, lock contention scenarios, any code where timing affects correctness.

When not to use: Sequential-only code, simple unit tests, UI components, read-only operations, code with no concurrency concerns.

Rationalizations (Do Not Skip)

Rationalization	Why It's Wrong	Required Action
"setTimeout is good enough"	Timing-based tests are inherently flaky -- they pass 99 times, fail on CI	Use barriers for deterministic control
"It passed 10 times, it's fine"	Heisenbugs hide in timing windows that haven't been hit yet	Barrier at every interleave point
"We don't have race conditions"	Any concurrent code has race windows	Write a barrier test to prove it
"Barriers are too complex"	15 lines of setup prevents hours of debugging flaky failures	Copy the pattern from references

---

Included Utilities

// Barrier infrastructure + invariant assertions + test data generators
import {
  createBarrier,
  createTrackedBarrier,
  releaseAllBarriers,
  assertPreservesConcurrentItems,
  assertPreservesOnFailure,
  assertSequenceContinuity,
  assertLastSequenceCorrect,
  assertRetryCountReset,
  createTestItems,
} from './test-fixtures.ts';

Violation Rules

inadequate_barrier_coverage

Race conditions MUST have barrier tests at each interleave point. If there are N interleave points in the code, there must be N barrier test cases.
Severity: must-fail

flaky_timing_test

NEVER use setTimeout, sleep, or arbitrary delays for concurrency testing. Use barriers for deterministic control.
Severity: must-fail

---

Quick Reference

Scenario	Pattern	Example
Block before transaction	barrier.wait() in transaction start	Verify concurrent writes preserved
Block during I/O	deferred.promise as mock return	Verify queue depth during processing
Multiple interleave points	Multiple barriers, release in sequence	Test all orderings of concurrent ops
Cleanup	afterEach(() => releaseAllBarriers())	Prevent test hangs

See patterns.md for full code examples, deferred promise alternative, and framework adaptation guide.