refactor(gtfs): optimize situation IDs fetch by eliminating duplicate DB queries

[3rabiii]

Context

This Pr was identified that GetSituationIDsForTrip was performing redundant database work:

• It queried the database for Trip and Route details to get the AgencyID.
• It then called GetAlertsForTrip, which performed the exact same database queries internally.

Changes

• Refactor: Extracted the alert filtering logic into a new method GetAlertsByIDs in internal/gtfs/realtime.go. This method accepts pre-fetched IDs, avoiding the need for internal DB lookups.
• Optimization: Updated GetSituationIDsForTrip in internal/restapi/trips_helper.go to fetch the Trip/Route/Agency data once and pass it to GetAlertsByIDs.
• Fix: Updated BuildTripStatus to explicitly initialize SituationIDs as an empty slice [] (instead of nil) when no alerts are found. This ensures the JSON response remains consistent ("situationIds": [] instead of null), preventing test failures in TestTripsForLocationHandler.

Impact

Reduces database load by eliminating duplicate queries per trip status request while maintaining API contract stability.

@aaronbrethorst
#fixes : #394

[aaronbrethorst]

Hey Adel, thanks for identifying the duplicate DB queries in GetSituationIDsForTrip — the optimization of fetching trip/route/agency data once and passing it through is the right idea. The extraction of GetAlertsByIDs as a reusable method is clean, and the GtfsDB != nil safety check is a nice defensive addition. Before we can merge this, I will need you to make a couple changes:

Critical

1. Situation IDs lost their agency prefix (trips_helper.go:787-789)

   The old code produced situation IDs in the standard OneBusAway {agencyID}_{alertID} format:

   situationIDs = append(situationIDs, utils.FormCombinedID(agencyID, alert.ID))

   The new code appends raw alert IDs without the agency prefix:

   situationIDs = append(situationIDs, alert.ID)

   This is an API contract regression — all OneBusAway IDs use the {agencyID}_{codeID} format. Clients parsing situation IDs will break. The fix is to restore the FormCombinedID logic:

   situationIDs := []string{}
   for _, alert := range alerts {
       if alert.ID != "" {
           if agencyID != "" {
               situationIDs = append(situationIDs, utils.FormCombinedID(agencyID, alert.ID))
           } else {
               situationIDs = append(situationIDs, alert.ID)
           }
       }
   }

2. Missing empty ID check (trips_helper.go:788)

   The old code guarded with if alert.ID != "" before appending. The new code appends unconditionally, which could add empty strings to the situation IDs array. Please restore the alert.ID != "" check.

Important

3. IMPORTANT: Caller must hold manager.RLock() comment is misplaced (realtime.go:108)

   The RLock comment was moved from GetAlertsForTrip to GetAlertsByIDs, but GetAlertsByIDs doesn't need the outer manager lock — it only accesses manager.realTimeAlerts, which is protected by the internal realTimeMutex.RLock() it acquires itself.

   The comment should remain on GetAlertsForTrip (which does DB queries via GetTrip/GetRoute) and on GetSituationIDsForTrip (which does the same DB queries). Fix:

   // GetAlertsByIDs returns alerts matching the given trip, route, or agency IDs.
   func (manager *Manager) GetAlertsByIDs(tripID, routeID, agencyID string) []gtfs.Alert {
   
   // GetAlertsForTrip returns alerts matching the trip, its route, or agency.
   // IMPORTANT: Caller must hold manager.RLock() before calling this method.
   func (manager *Manager) GetAlertsForTrip(ctx context.Context, tripID string) []gtfs.Alert {

Thanks again, and I look forward to merging this change.

[3rabiii]

@aaronbrethorst
Ready for review!

[aaronbrethorst]

Adel, nice work addressing the feedback from the first round. The core optimization is solid — fetching trip/route/agency data once instead of twice per call is the right call, and GetAlertsByIDs as a reusable building block is well-structured. All three previously requested changes have been properly resolved:

1. Agency prefix restored — FormCombinedID logic is back in GetSituationIDsForTrip (trips_helper.go:792-795)
2. Empty ID check restored — the alert.ID == "" guard with early continue is clean (trips_helper.go:789-791)
3. RLock comment correctly placed — present on GetAlertsForTrip, absent from GetAlertsByIDs

I verified that tests pass and lint is clean. This is ready to merge with a couple of optional suggestions for a follow-up.

Suggestions

These are non-blocking — take them or leave them.

1. Add a doc comment on GetAlertsByIDs (realtime.go:108)

   Go convention is to document exported methods. A one-liner would be sufficient:

   // GetAlertsByIDs returns alerts matching the given trip, route, or agency IDs.
   func (manager *Manager) GetAlertsByIDs(tripID, routeID, agencyID string) []gtfs.Alert {

2. Consider logging DB errors in GetSituationIDsForTrip (trips_helper.go:774-783)

   The old code returned nil when GetTrip failed — no alerts at all. The new code gracefully degrades to trip-only alert matching, which is better behavior. But database errors (connection failures, context timeouts) are now silently swallowed. A slog.Warn for non-sql.ErrNoRows errors would help operators detect database degradation without changing the graceful-degradation behavior. This is a nice-to-have, not a requirement.