fix: add AbortSignal-based server lifecycle cleanup

[eventualbuddha]

Overview

Generated with Claude Code

• detectDevices adds attach/detach listeners to the global usb singleton but never removes them, causing MaxListenersExceededWarning in tests that call start() multiple times.
• Updated detectDevices to return a cleanup function.
• Added an optional signal: AbortSignal to each app's StartOptions. Aborting the signal shuts down everything start() created: removes USB listeners and closes the HTTP server.
• Updated all four apps (central-scan, scan, admin, mark-scan) with consistent abort behavior.

Demo Video or Screenshot

Before: pnpm test:coverage in central-scan backend emits:

(node) MaxListenersExceededWarning: Possible EventEmitter memory leak detected.
11 attach listeners added to [EventEmitter].

After: no warning. Tests use AbortController for clean shutdown:

const shutdownController = new AbortController();
const server = start({ ..., signal: shutdownController.signal });
// ... test ...
shutdownController.abort(); // cleans up listeners + closes server

Testing Plan

• cd apps/central-scan/backend && pnpm test:run — all 110 tests pass, no MaxListenersExceededWarning
• cd apps/scan/backend && pnpm test:run src/server.test.ts — 5 tests pass
• cd libs/backend && pnpm test:run src/detect_devices.test.ts — 2 tests pass (includes cleanup verification)

Checklist

• I have prefixed my PR title with "VxDesign: ", "VxPollBook: ", or "HWTA: " if my change is specific to one of those products.
• I have added logging where appropriate for any new user actions.
• I have added the "user-facing-change" label to this PR, if relevant, to automate an announcement in #machine-product-updates.

[Copilot]

Pull request overview

This PR updates the shared backend USB device detection helper to support listener cleanup, and wires that cleanup into each app server’s lifecycle to prevent MaxListenersExceededWarning when servers are started repeatedly in tests.

Changes:

• Change detectDevices to return a cleanup function that unregisters usb attach/detach listeners.
• Register that cleanup function on each backend server’s close event (central-scan, scan, admin, mark-scan).
• Adjust central-scan backend server unit tests to return an EventEmitter from the mocked listen() so server.on('close', …) can be registered.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
libs/backend/src/detect_devices.ts	Make USB listener registration reversible via returned cleanup function.
apps/scan/backend/src/server.ts	Capture server from listen() and register cleanup on close.
apps/mark-scan/backend/src/server.ts	Register device-listener cleanup on server close.
apps/central-scan/backend/src/server.ts	Register device-listener cleanup on server close.
apps/central-scan/backend/src/server.test.ts	Update listen() mocks to return an EventEmitter-like server.
apps/admin/backend/src/server.ts	Register device-listener cleanup on server close.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

[Copilot]

detectDevices now returns a cleanup function, but this test ignores it. That leaves USB listeners attached across tests and can reintroduce MaxListenersExceededWarning (and cross-test interference) within this suite. Capture the returned cleanup function and call it at the end of the test (or in an afterEach).

Suggested change

	detectDevices({ logger });
	testDetectDevices(logger, expect);
	const cleanup = detectDevices({ logger });
	testDetectDevices(logger, expect);
	cleanup();

[Copilot]

Cleanup only runs on AbortSignal abort, but this server is also closed directly in tests (e.g. server?.close() in apps/admin/backend/src/server.test.ts). Closing the server currently does not remove the USB listeners, so listeners will still accumulate across test cases. Consider registering stopDetectingDevices on server's 'close' event (once) and/or wrapping server.close so cleanup happens regardless of how shutdown is triggered. Also handle the case where signal is already aborted before the listener is attached (otherwise cleanup never runs).

[Copilot]

Cleanup only runs on AbortSignal abort, but this server is closed directly in multiple tests via server.close(). That shutdown path currently does not remove the USB listeners, so they will still accumulate across tests. Register stopDetectingDevices on the server's 'close' event (once) and/or ensure server.close() triggers the same cleanup logic. Also consider handling an already-aborted signal so cleanup runs immediately.

[Copilot]

Cleanup only runs on AbortSignal abort, but this server is also closed directly in tests (e.g. server.close() in apps/central-scan/backend/src/auth.test.ts). Closing the server currently does not remove the USB listeners, so they will continue to accumulate across test cases. Register stopDetectingDevices on the server's 'close' event (once) and/or ensure server.close() triggers the same cleanup logic. Also handle an already-aborted signal so cleanup runs deterministically.

[Copilot]

StartOptions.signal is declared as required, but start accepts Partial<StartOptions> and uses signal?.addEventListener, which implies it is optional (and other apps’ StartOptions make signal optional). Making this field required is inconsistent and can mislead callers; consider making signal optional here for consistency.

Suggested change

	signal: AbortSignal;
	signal?: AbortSignal;