From afed0a0b341f143c60221b7e0f76b59ce18fabfc Mon Sep 17 00:00:00 2001
From: Alex Gleason <alex@alexgleason.me>
Date: Sat, 13 Apr 2024 14:33:59 -0500
Subject: [PATCH] cache: serve stale response

---
 src/app.ts              |  2 +-
 src/middleware/cache.ts | 23 ++++++++++++++++++-----
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/app.ts b/src/app.ts
index 3f5804f..ad920c7 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -134,7 +134,7 @@ app.get('/users/:username', actorController);
 
 app.get('/nodeinfo/:version', nodeInfoSchemaController);
 
-app.get('/api/v1/instance', cache({ expires: Time.seconds(3) }), instanceController);
+app.get('/api/v1/instance', cache({ expires: Time.minutes(5) }), instanceController);
 
 app.get('/api/v1/apps/verify_credentials', appCredentialsController);
 app.post('/api/v1/apps', createAppController);
diff --git a/src/middleware/cache.ts b/src/middleware/cache.ts
index c897645..8e80005 100644
--- a/src/middleware/cache.ts
+++ b/src/middleware/cache.ts
@@ -12,19 +12,32 @@ export const cache = (opts: CacheOpts): MiddlewareHandler => {
   let expires = Date.now() + opts.expires;
 
   return async (c, next) => {
-    if (!response || (Date.now() > expires)) {
-      debug('Building cache for page', c.req.url);
-      expires = Date.now() + opts.expires;
+    const now = Date.now();
+    const expired = now > expires;
 
+    async function updateCache() {
       await next();
-
       const res = c.res.clone();
       if (res.status < 500) {
         response = res;
       }
-    } else {
+      return res;
+    }
+
+    if (response && !expired) {
       debug('Serving page from cache', c.req.url);
       return response.clone();
+    } else {
+      expires = Date.now() + opts.expires;
+      if (response && expired) {
+        debug('Serving stale cache, rebuilding', c.req.url);
+        const stale = response.clone();
+        updateCache();
+        await new Promise((resolve) => setTimeout(resolve, 0));
+        return stale;
+      }
+      debug('Building cache for page', c.req.url);
+      return await updateCache();
     }
   };
 };