Isolate test side effects

Tests must not leak global or shared state across runs, and must not depend on execution order. Before mutating anything shared with the rest of the process — signal listeners (SIGTERM/SIGINT/beforeExit), module-level caches, environment variables, or mocks — snapshot the prior state in beforeEach/setup and restore it in afterEach/finally, even when the test throws. When adding listeners for a test (e.g. process.once('SIGTERM', ...)), remove any pre-existing listeners first so only the test’s own handlers observe the emitted event, then restore the originals afterward.

Mocks must be reset between tests rather than relying on call order: use vi.resetAllMocks()/explicit re-implementation in beforeEach so a suite is correct under randomized test ordering, not just the order it happens to run in locally.

Avoid writing files or other artifacts to the repository checkout during a test run; commit required fixtures instead (e.g. config/test data files under fixtures/) so a test run leaves the working tree clean.

Before relying on an existing shared test harness for isolation (e.g. an in-memory DB reset between files), verify the harness actually provides the isolation needed rather than adding redundant setup/teardown, but err toward explicit local isolation when a test touches global, process-wide state.

Example:

it('cleans up on SIGTERM without ending the process', async () => {
  const saved = process.rawListeners('SIGTERM');
  process.removeAllListeners('SIGTERM');
  try {
    // ...test body registering and emitting SIGTERM...
  } finally {
    process.removeAllListeners('SIGTERM');
    saved.forEach((listener) => process.on('SIGTERM', listener));
  }
});