fix(#76): report request-scoped data freshness

internal/web/freshness.go

@@ -0,0 +1,56 @@
+package web
+
+import (
+	"net/http"
+
+	"github.com/AmanTahiliani/box-box/internal/api"
+)
+
+const (
+	dataSourceHeader    = "X-BoxBox-Data-Source"
+	dataFreshnessHeader = "X-BoxBox-Data-Freshness"
+)
+
+// markOpenF1Response publishes request-scoped success provenance. Callers must
+// pass the scoped client used for this response, never Server.client.
+func markOpenF1Response(w http.ResponseWriter, client *api.OpenF1Client) {
+	markOpenF1AggregateResponse(w, client, false)
+}
+
+func markOpenF1AggregateResponse(w http.ResponseWriter, client *api.OpenF1Client, partial bool) {
+	w.Header().Set(dataSourceHeader, "openf1")
+	if client != nil && client.LastResponseWasStale() {
+		w.Header().Set(dataFreshnessHeader, "stale")
+		return
+	}
+	if partial {
+		w.Header().Set(dataFreshnessHeader, "partial")
+		return
+	}
+	w.Header().Set(dataFreshnessHeader, "fresh")
+}
+
+func markDataResponse(w http.ResponseWriter, source, freshness string) {
+	w.Header().Set(dataSourceHeader, source)
+	w.Header().Set(dataFreshnessHeader, freshness)
+}
+
+func markLocalResponse(w http.ResponseWriter, partial bool) {
+	w.Header().Set(dataSourceHeader, "local")
+	if partial {
+		w.Header().Set(dataFreshnessHeader, "partial")
+		return
+	}
+	w.Header().Set(dataFreshnessHeader, "local")
+}
+
+func markMixedResponse(w http.ResponseWriter, client *api.OpenF1Client, partial bool) {
+	w.Header().Set(dataSourceHeader, "mixed")
+	if client != nil && client.LastResponseWasStale() {
+		w.Header().Set(dataFreshnessHeader, "stale")
+	} else if partial {
+		w.Header().Set(dataFreshnessHeader, "partial")
+	} else {
+		w.Header().Set(dataFreshnessHeader, "local")
+	}
+}