Try every event database file on delete and say which was left (closes #275)

DeleteDB returned on the first event database file it failed to remove, so with WAL sidecars present a failure could be reported as "the file was left behind" after the events themselves were already gone. It now tries the database file and both sidecars, and its error says which case happened and names each file left: the database file is still on disk, or it is gone (the events are lost) and only a -wal or -shm sidecar remains. The webhook delete handler logs a different message for each and never says the events survive when they do not. Tests cover a normal delete with both sidecars, and a failed removal of the database file or a sidecar, through DeleteDB and the handler.

Model: opus-5-5

internal/handlers/source_management.go

@@ -723,6 +723,17 @@ func (h *Handlers) HandleSourceDelete() http.HandlerFunc {
 	}
 }
 
+// The messages deleteWebhookResources logs when a file of the event
+// database cannot be removed: the database file itself, or only a
+// sidecar once the database file is gone.
+const (
+	eventDBLeftMsg = "webhook deleted, but its event database file is " +
+		"still on disk; remove it by hand"
+	sidecarLeftMsg = "webhook deleted and its events are gone, but a " +
+		"-wal or -shm sidecar of its event database is " +
+		"still on disk; remove it by hand"
+)
+
 // deleteWebhookResources soft-deletes config and hard-deletes
 // the per-webhook event database.
 func (h *Handlers) deleteWebhookResources(
@@ -762,13 +773,18 @@ func (h *Handlers) deleteWebhookResources(
 	err = h.dbMgr.DeleteDB(webhook.ID)
 	if err != nil {
 		// The configuration is committed, so the webhook is gone,
-		// but its event database file is still on disk with
+		// but a file of its event database is still on disk with
 		// nothing referencing it. Report the failure rather than
 		// redirecting as though everything succeeded: the file
 		// needs removing by hand, and the logged error names it.
-		h.serverError(
-			w, r, "failed to delete webhook event database", err,
-		)
+		// When only a sidecar is left, the events are already
+		// gone, and the message must not suggest they survive.
+		msg := eventDBLeftMsg
+		if errors.Is(err, database.ErrSidecarNotRemoved) {
+			msg = sidecarLeftMsg
+		}
+
+		h.serverError(w, r, msg, err)
 
 		return
 	}