Event log: show only the events with a failed or a pending delivery (closes #390)

The event log gains three links above the list: All, Failed (N) and
Pending (N), carried in a `show` query parameter; any other value shows
every event. Failed lists the events with a failed delivery, Pending
those with one pending or retrying, each once, under the full list's
50-row limit and order. A filtered list finds the matching deliveries
through `idx_deliveries_status`, looks up their events by ID and sorts
them, and reads the rows of only the events shown; its counts read the
deliveries alone. The line beside the heading says what a filter
counts, and Replay returns to the list it was pressed in. The README
and the comments naming the query parameters the service reads include
`show`.

Model: opus-5-5

internal/handlers/source_management.go

@@ -1110,6 +1110,15 @@ func (h *Handlers) ownedWebhook(
 	return webhook, true
 }
 
+// The event log's show query parameter and its two values: the events
+// with a failed delivery, and those with a delivery still pending or
+// retrying.
+const (
+	showParam   = "show"
+	showFailed  = "failed"
+	showPending = "pending"
+)
+
 // HandleSourceLogs shows the request/response logs for a
 // webhook.
 func (h *Handlers) HandleSourceLogs() http.HandlerFunc {
@@ -1129,17 +1138,36 @@ func (h *Handlers) HandleSourceLogs() http.HandlerFunc {
 			return
 		}
 
+		// Any other value of show lists every event, as no value
+		// does.
+		show := r.URL.Query().Get(showParam)
+
+		statuses := eventLogStatuses(show)
+		if statuses == nil {
+			show = ""
+		}
+
 		evts, total, ok := h.loadEventsWithDeliveries(
-			w, r, webhook, targets,
+			w, r, webhook, targets, statuses,
 		)
 		if !ok {
 			return
 		}
 
+		failed, pending, err := h.countFailedAndPendingEvents(webhook.ID)
+		if err != nil {
+			h.serverError(w, r, "failed to count events", err)
+
+			return
+		}
+
 		data := map[string]any{
-			tmplKeyWebhook: &webhook,
-			"Events":       evts,
-			"TotalEvents":  total,
+			tmplKeyWebhook:  &webhook,
+			"Events":        evts,
+			"TotalEvents":   total,
+			"Show":          show,
+			"FailedEvents":  failed,
+			"PendingEvents": pending,
 		}
 
 		h.renderTemplate(w, r, "source_logs.html", data)
@@ -1201,9 +1229,10 @@ func (h *Handlers) loadTargetMap(
 
 // loadEventsWithDeliveries loads the recentEventLimit newest events
 // and their deliveries from the per-webhook database, and the total
-// number of events stored. Events come back as capped projections
-// rather than database.Event rows: see eventLogColumns for why the
-// cut happens in SQL.
+// number of events stored. Given delivery statuses, both cover only
+// the events with a delivery in one of them. Events come back as
+// capped projections rather than database.Event rows: see
+// eventLogColumns for why the cut happens in SQL.
 //
 // The bool reports whether the load succeeded. It is false
 // once this has answered the request with an error, and the
@@ -1213,6 +1242,7 @@ func (h *Handlers) loadEventsWithDeliveries(
 	r *http.Request,
 	webhook database.Webhook,
 	targetMap map[string]eventLogTarget,
+	statuses []database.DeliveryStatus,
 ) ([]EventLogView, int64, bool) {
 	if !h.dbMgr.DBExists(webhook.ID) {
 		return nil, 0, true
@@ -1227,7 +1257,14 @@ func (h *Handlers) loadEventsWithDeliveries(
 		return nil, 0, false
 	}
 
-	rows, totalEvents := loadEventLogRows(webhookDB, webhook.ID)
+	rows, totalEvents, err := loadEventLogRows(
+		webhookDB, webhook.ID, statuses,
+	)
+	if err != nil {
+		h.serverError(w, r, "failed to load events", err)
+
+		return nil, 0, false
+	}
 
 	result, ok := h.eventLogViews(
 		w, r, webhookDB, webhook.ID, rows, targetMap,
@@ -1321,25 +1358,142 @@ func (h *Handlers) eventLogViews(
 
 // loadEventLogRows reads the event log projection of the
 // recentEventLimit newest events, newest first, and the total number
-// of events stored.
+// of events stored, both narrowed by statuses as eventsWithStatus
+// narrows them.
 func loadEventLogRows(
-	webhookDB *gorm.DB, webhookID string,
-) ([]eventLogRow, int64) {
-	var totalEvents int64
-
-	webhookDB.Model(&database.Event{}).Where(
-		"webhook_id = ?", webhookID,
-	).Count(&totalEvents)
+	webhookDB *gorm.DB,
+	webhookID string,
+	statuses []database.DeliveryStatus,
+) ([]eventLogRow, int64, error) {
+	totalEvents, err := countEventsWithStatus(
+		webhookDB, webhookID, statuses,
+	)
+	if err != nil {
+		return nil, 0, err
+	}
 
 	var rows []eventLogRow
 
-	webhookDB.Model(&database.Event{}).Select(
+	err = eventsWithStatus(webhookDB, webhookID, statuses).Select(
 		eventLogColumns, maxRenderedBodyBytes, maxRenderedBodyBytes,
-	).Where(
-		"webhook_id = ?", webhookID,
-	).Order("created_at DESC").Limit(recentEventLimit).Find(&rows)
+	).Order("created_at DESC").Limit(recentEventLimit).Find(&rows).Error
 
-	return rows, totalEvents
+	return rows, totalEvents, err
+}
+
+// eventLogStatuses returns the delivery statuses the event log's show
+// value lists events by, or nil for one that lists every event.
+func eventLogStatuses(show string) []database.DeliveryStatus {
+	switch show {
+	case showFailed:
+		return []database.DeliveryStatus{database.DeliveryStatusFailed}
+	case showPending:
+		return []database.DeliveryStatus{
+			database.DeliveryStatusPending,
+			database.DeliveryStatusRetrying,
+		}
+	default:
+		return nil
+	}
+}
+
+// eventsWithStatus selects the webhook's events, or, given statuses,
+// the recentEventLimit newest of those with at least one delivery in
+// one of them.
+//
+// Given statuses, its cost follows the matching deliveries. SQLite
+// never reorders a CROSS JOIN, so it reads each join's left side
+// first: the distinct event IDs of the matching deliveries, through
+// idx_deliveries_status; then each of those events by ID, sorted to
+// keep the newest; then the rows of only the events kept, so no other
+// event's body is read. With a plain "id IN (matching deliveries)"
+// condition instead, SQLite, which keeps no statistics on these
+// tables, walks every event newest first.
+func eventsWithStatus(
+	webhookDB *gorm.DB,
+	webhookID string,
+	statuses []database.DeliveryStatus,
+) *gorm.DB {
+	if statuses == nil {
+		return webhookDB.Model(&database.Event{}).Where(
+			"webhook_id = ?", webhookID,
+		)
+	}
+
+	matching := webhookDB.Model(&database.Delivery{}).
+		Distinct("event_id").Where("status IN ?", statuses)
+
+	newest := webhookDB.Table("(?) AS matching", matching).
+		Joins("CROSS JOIN events ON events.id = matching.event_id").
+		Where(
+			"events.webhook_id = ? AND events.deleted_at IS NULL",
+			webhookID,
+		).
+		Order("events.created_at DESC").Limit(recentEventLimit).
+		Select("events.id AS event_id")
+
+	return webhookDB.Table("(?) AS newest", newest).
+		Joins("CROSS JOIN events ON events.id = newest.event_id")
+}
+
+// countEventsWithStatus counts the webhook's events with at least one
+// delivery in one of the statuses, or every event when statuses is
+// nil. Given statuses, it counts the distinct events of the matching
+// deliveries and reads nothing but those deliveries, through
+// idx_deliveries_status, where counting the events would read every
+// event row. That is the same number, because retention deletes an
+// event's deliveries with it.
+func countEventsWithStatus(
+	webhookDB *gorm.DB,
+	webhookID string,
+	statuses []database.DeliveryStatus,
+) (int64, error) {
+	var count int64
+
+	if statuses == nil {
+		err := webhookDB.Model(&database.Event{}).Where(
+			"webhook_id = ?", webhookID,
+		).Count(&count).Error
+
+		return count, err
+	}
+
+	err := webhookDB.Model(&database.Delivery{}).Distinct("event_id").
+		Where("status IN ?", statuses).Count(&count).Error
+
+	return count, err
+}
+
+// countFailedAndPendingEvents returns how many of the webhook's events
+// the event log lists when it shows only those with a failed delivery,
+// and when it shows only those with a delivery pending or retrying.
+func (h *Handlers) countFailedAndPendingEvents(
+	webhookID string,
+) (int64, int64, error) {
+	if !h.dbMgr.DBExists(webhookID) {
+		return 0, 0, nil
+	}
+
+	webhookDB, err := h.dbMgr.GetDB(webhookID)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	failed, err := countEventsWithStatus(
+		webhookDB, webhookID, eventLogStatuses(showFailed),
+	)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	pending, err := countEventsWithStatus(
+		webhookDB, webhookID, eventLogStatuses(showPending),
+	)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	return failed, pending, nil
 }
 
 // resubmitCounts reports, for each of the page's events, how many