Cache bounded Discord avatars and coalesce downloads

src/sharing/avatar-cache.ts

@@ -0,0 +1,61 @@
+import { loadImage } from '@napi-rs/canvas';
+
+async function boundedBytes(response: Response) {
+  const reader = response.body?.getReader();
+  if (!reader) return null;
+  const chunks: Uint8Array[] = []; let size = 0;
+  try {
+    while (true) {
+      const { value, done } = await reader.read();
+      if (done) return Buffer.concat(chunks);
+      size += value.byteLength;
+      if (size > 1024 * 1024) { await reader.cancel(); return null; }
+      chunks.push(value);
+    }
+  } finally { reader.releaseLock(); }
+}
+
+type Avatar = Awaited<ReturnType<typeof loadImage>>;
+type Options = {
+  request?: typeof fetch;
+  now?: () => number;
+  maxEntries?: number;
+  maxPending?: number;
+  successTtlMs?: number;
+  failureTtlMs?: number;
+};
+/** Hash-based Discord URLs invalidate naturally; bound both cached images and downloads. */
+export function createAvatarCache({ request = ((url, init) => fetch(url, init)) as typeof fetch, now = Date.now, maxEntries = 128, maxPending = 4, successTtlMs = 3600000, failureTtlMs = 30000 }: Options = {}) {
+  const cache = new Map<string, { image: Avatar | null; expires: number }>();
+  const pending = new Map<string, Promise<Avatar | null>>();
+  return async (url: string): Promise<Avatar | null> => {
+    const cached = cache.get(url);
+    if (cached && cached.expires > now()) {
+      cache.delete(url); cache.set(url, cached);
+      return cached.image;
+    }
+    cache.delete(url);
+    const existing = pending.get(url);
+    if (existing) return existing;
+    if (pending.size >= maxPending) return null;
+    const job = (async () => {
+      let image: Avatar | null = null;
+      try {
+        const response = await request(url, { redirect: 'error', signal: AbortSignal.timeout(3000) });
+        if (response.ok) {
+          const bytes = await boundedBytes(response);
+          if (bytes) {
+            const decoded = await loadImage(Buffer.from(bytes));
+            if (decoded.width <= 256 && decoded.height <= 256) image = decoded;
+          }
+        }
+      } catch { /* A brief negative cache keeps an unavailable CDN out of the hot path. */ }
+      cache.set(url, { image, expires: now() + (image ? successTtlMs : failureTtlMs) });
+      while (cache.size > maxEntries) cache.delete(cache.keys().next().value!);
+      return image;
+    })().finally(() => { pending.delete(url); });
+    pending.set(url, job);
+    return job;
+  };
+}
+export const cachedAvatar = createAvatarCache();