Workaround for broken cache expiry handling (#47)

Deno's cache API does not respect expiry dates.
This meant that feeds were cached much longer than intended.

This PR implments a manual cache handling instead of relying on
Deno's implementation. Relevant issue: https://github.com/denoland/deno/issues/25795

lib/utils.ts

@@ -27,6 +27,7 @@ export function responseXML(body: string, status: Status) {
 export const withExpiry = <T>(response: Response, ttlInSeconds: number) => {
   const clonedResponse = response.clone();
   clonedResponse.headers.set("Cache-Control", `max-age=${ttlInSeconds}`);
+  clonedResponse.headers.set("Expires", new Date(Date.now() + ttlInSeconds * 1000).toUTCString());
   return clonedResponse;
 };
 

routes/api/feeds/[seriesId].ts

@@ -5,6 +5,23 @@ import { responseJSON, responseXML, withExpiry } from "../../../lib/utils.ts";
 
 const FEED_CACHE_KEY = "feeds-cache";
 
+// Deno's cache API does not respect these headers.
+// Until it does, manual expiry handling is needed.
+// When it's working, this function would not be needed.
+// See: https://github.com/denoland/deno/issues/25795
+function isExpired(headers: Headers) {
+  const expiresHeader = headers.get("Expires");
+  if (!expiresHeader) {
+    console.log("No Expires header found, treating as expired");
+    return true;
+  }
+
+  const now = new Date();
+  const expiryDate = new Date(expiresHeader);
+  const expired = now > expiryDate;
+  return expired;
+}
+
 export const handler = async (
   request: Request,
   ctx: FreshContext,
@@ -13,11 +30,13 @@ export const handler = async (
 
   const cache = await caches.open(FEED_CACHE_KEY);
   const cachedResponse = await cache.match(request);
-  if (cachedResponse) {
+
+  if (cachedResponse && !isExpired(cachedResponse.headers)) {
     console.log(`Cache hit for feed ${seriesId}`);
     return cachedResponse;
   }
 
+  console.log(`Fetching feed for ${seriesId}`);
   const series = await caching.getSeries({ id: seriesId });
   if (!series) {
     return responseJSON({ message: "Series not found" }, STATUS_CODE.NotFound);
@@ -28,7 +47,7 @@ export const handler = async (
   const response = withExpiry(
     responseXML(feed, STATUS_CODE.OK),
     // 2 hours
-    60 * 60 * 2,
+    2 * 60 * 60,
   );
 
   await cache.put(request, response.clone());