# Work Summary — 2026-08-04 23:18

## Task
Build test coverage for `domains/amazon`, the mock Amazon event processor - explicitly framed by the user as wanting tests around the *stateful background-processing* part specifically, both for long-term regression protection and to surface design insights, not just a coverage checkbox.

## What the code actually does
Traced the full pipeline before writing anything, since `domains/amazon/mock.go` is the consumer end of a chain that starts in SQL:

1. A row is inserted into `mock.raw_shop_events` (platform-agnostic staging table; this is what the "simulate a sale/refund/inventory change" UI writes to, via `domains/accounts/accounts.go`).
2. A Postgres trigger (`amazon_store_events`, `WHEN NEW.platform = 'amazon'`, defined across migrations `000026`/`000028`) copies the row into `mock.shop_amazon_events` and fires `pg_notify('mock_shop_amazon_event_inserted', null)`.
3. `(*Mocks).ProcessEvents` - the code under test - registers `LISTEN mock_shop_amazon_event_inserted` on a dedicated pooled connection, then loops: drain all unprocessed rows from `mock.shop_amazon_events` (paginated 100 at a time via `processUnprocessedEvents`), then block on whichever comes first: a notification, an error, context cancellation, or a 1-minute timeout (poll fallback). Each processed event marks the row `processed=true` and fires an async, fire-and-forget notification to an app-level `MockEventListener` (wired to SSE in `main.go`, so the UI updates live).

## Test approach
Rather than seeding `mock.shop_amazon_events` directly, tests insert into `mock.raw_shop_events` (the real entry point) and let the trigger do its job - so the tests exercise the exact same DB-side path production traffic does, not a hand-rolled approximation of it.

New file: `domains/amazon/mock_test.go`.
- `notifySpy`: a `MockEventListener` that records calls and exposes a channel-based `waitForCount`, since the production code notifies from an un-awaited goroutine - there's no way to assert on it without a synchronization point.
- `TestProcessUnprocessedEvents_ProcessesAllEventsAcrossBatches`: inserts 150 events (the batch loop's `LIMIT` is 100) and confirms every single one gets processed and notified - this is a real correctness test of the pagination loop, not just "does one row work."
- `TestProcessUnprocessedEvents_NoListenerConfigured`: confirms processing doesn't depend on / crash without a listener being set.
- `TestProcessEvents_ReactsToNotification`: the main event - runs the actual `ProcessEvents` loop in a goroutine, inserts a real event, and waits for the full reactive path (LISTEN → trigger → NOTIFY → WaitForNotification → reprocess → listener callback) to complete, then cancels and confirms clean shutdown.
- `TestProcessEvents_ShutsDownOnContextCancel`: isolates lifecycle/shutdown behavior from NOTIFY timing entirely, so a shutdown regression doesn't hide behind notification flakiness or vice versa.

## Verification
- `go build ./...` / `go vet ./...` clean.
- `go test ./domains/amazon/... -v`: all 4 pass; also ran 10x in a row (`TestProcessEvents_ReactsToNotification` specifically, since it's the timing-sensitive one) with consistent ~0.25s runs, no flakes.
- `go test ./domains/amazon/... -race`: clean, no data races detected under this exercise.
- `make test` and `make test-against-dev-db`: full suite green both ways; confirmed zero leftover rows in the dev DB afterward.

## Design insights surfaced while building these tests
This is the part the user specifically asked for - not fixed, just documented, since these are real behavior/architecture calls, not bugs:

1. **No "now listening" readiness signal.** `listenForNotifications` issues `LISTEN` synchronously, but from outside `ProcessEvents` there's no way to know when that's happened - a caller (or a test) that inserts an event immediately after starting `ProcessEvents` in a goroutine is racing against Go's scheduler getting around to running it. The test above works around this with a flat 200ms sleep before inserting, which is reliable in practice (10/10 clean runs) but is inherently a "hope the goroutine got scheduled by then" workaround, not a real guarantee. If this loop is ever driven by something less forgiving than a local dev machine (heavier load, slower CI), a small readiness channel returned from `listenForNotifications` (closed once `LISTEN` succeeds) would remove the guesswork entirely, for tests and for any other caller that cares about "is it actually listening yet."

2. **Fire-and-forget listener notification.** `processEvent` spawns `go func() { listener.Notify(ctx, e) }()` and only logs a failure - the event is marked `processed = true` in the DB regardless of whether the SSE listener actually received it. That's a reasonable tradeoff (a flaky UI push shouldn't block or retry core event processing), but it does mean there's currently no compensating mechanism if a notification is dropped - the UI just silently misses that one live update, permanently, with no re-send. Worth a deliberate decision on whether that's acceptable long-term, since it isn't caught by anything short of a user noticing stale data.

3. **The 1-minute poll fallback is a hardcoded constant** (`time.After(time.Minute)` inline in `ProcessEvents`), not a field/parameter. That's fine for production but means the fallback-poll path itself is essentially untestable without either waiting a full minute per test run or refactoring the interval to be injectable - the tests above only exercise the NOTIFY-driven reactive path, not the poll fallback, for exactly this reason. If the fallback path's correctness ever needs its own regression test, that constant would need to become configurable first.

4. **A LISTEN-connection error is fatal to the whole application, not just this subsystem.** If `WaitForNotification` ever errors for a reason other than context cancellation (dropped connection, Postgres restart, pool churn), `listenForNotifications`'s goroutine sends that error on `errCh`, `ProcessEvents` returns it, and in `main.go`, `runApp`'s top-level `select` treats `eventErrCh` firing as cause to call `shutdown()` - which cancels the shared context and tears down the HTTP server too. There's no reconnect/retry loop around the LISTEN connection specifically. In other words: a transient hiccup on one background Postgres connection currently brings down the entire server, not just the mock Amazon event processor. This is probably the single most consequential finding here if this pattern gets reused for the other nine `mock_shop_*_event_inserted` channels already defined in the migrations (`big_cartel`, `ebay`, `ecwid`, `etsy`, `shopify`, `square_online`, `squarespace`, `tiktok`, `walmart_marketplace`, `wix`, `woo_commerce`, `zoho` all have the same trigger+notify shape already migrated, just no Go-side processor yet) - multiplying this fragility by twelve without addressing it first would mean any one platform's listen-connection blip can take the whole app down.

## Follow-ups / not done here
- None of the four insights above were acted on - flagging for a decision, same pattern as the `RefreshAccessToken` bug from the previous session. #4 in particular seems worth prioritizing before this pattern is replicated across the other mock platforms, given the "reports potentially later" plan implies more processors like this one are coming.
- `domains/reports` still has no coverage (explicitly deferred by the user to later).